This presentation describes about the preparation, properties, bonding modes, classification and applications of metal Dioxygen Complexes. Also explains the MO diagram of molecular oxygen.
For UG students of All Engineering Branches (Mechanical Engg., Chemical Engg., Instrumentation Engg., Food Technology) and PG students of Chemistry, Physics, Biochemistry, Pharmacy
The link of the video lecture at YouTube is
https://www.youtube.com/watch?v=t3QDG8ZIX-8
It is also called as Co-ordination polymerisation. Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl4, ZnBr3 etc, with an organometallic compound like triethyl-aluminium or trimethyl-aluminium, stereospecific polymerisation can be carried out. Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst.
It is also called as Co-ordination polymerisation. Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl4, ZnBr3 etc, with an organometallic compound like triethyl-aluminium or trimethyl-aluminium, stereospecific polymerisation can be carried out. Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst.
Labile & inert and substitution reactions in octahedral complexesEinstein kannan
The first part includes a definition of labile and inert. lability and inertness on the basis of VB theory and CFT and also factors affecting inertness and lability of the complexes.
And also the second part includes Substitution Reactions in Octahedral Complexes like mechanisms and their evidence.
This presentation describes about the preparation, properties, bonding modes, classification and applications of metal Dioxygen Complexes. Also explains the MO diagram of molecular oxygen.
For UG students of All Engineering Branches (Mechanical Engg., Chemical Engg., Instrumentation Engg., Food Technology) and PG students of Chemistry, Physics, Biochemistry, Pharmacy
The link of the video lecture at YouTube is
https://www.youtube.com/watch?v=t3QDG8ZIX-8
It is also called as Co-ordination polymerisation. Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl4, ZnBr3 etc, with an organometallic compound like triethyl-aluminium or trimethyl-aluminium, stereospecific polymerisation can be carried out. Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst.
It is also called as Co-ordination polymerisation. Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl4, ZnBr3 etc, with an organometallic compound like triethyl-aluminium or trimethyl-aluminium, stereospecific polymerisation can be carried out. Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst.
Labile & inert and substitution reactions in octahedral complexesEinstein kannan
The first part includes a definition of labile and inert. lability and inertness on the basis of VB theory and CFT and also factors affecting inertness and lability of the complexes.
And also the second part includes Substitution Reactions in Octahedral Complexes like mechanisms and their evidence.
An overview of the use of the Marcus Theory to calculate the energies of transition states.
Contributed by: Elizabeth Greenhalgh, Amanda Bischoff, and Matthew Sigman, University of Utah, 2015
These are chemical shift reagents and solvent induced shifts have their application in resolving the NMR Spectra of complex structures by inducing shift with respect to reference compound. Thus useful in interpretation of structures of complex organic compounds.
What is the Rate Law for the Crystal Violet Reaction327-43.docxalanfhall8953
What is the Rate Law for the Crystal Violet Reaction?
3/27-4/3/2014
Section 10th
TA: Jinwei Zhang
Group 6
Daylin Morgan
Ahmed Alsharif
Shili Tong
Chang Chuan
Introduction
The main goal of this experiment was about observing the reaction between crystal violet and sodium hydroxide, CV+ + OH- -> CVOH. The objectives of this experiment are to monitor the absorbance of the crystal violet solution as a function of time, and determining the order of the reaction with respect to CV+. The pseudo rate constant k’ in this reaction is derived from the knowledge that OH- and the rate constant can be combined to a single constant. The Beer’s Law relationship between absorbance and concentration for CV+ is used to calculate the concentration in a solution given the Absorbance and a calculated constant. The half-life for this reaction with respect to CV+ and the rate law for this reactionare derived from total data.
Materials & Procedure
Materials
2.5*10-5M CV+(aq)
2.5*10-5 M OH-(aq)
Volumetric measuring equipment
Beaker
Cuvette
Stirring Rod
Spectrophotometer
Each student in a group will measure kinetics of the reaction at different concentration of sodium hydroxide. Before starting kinetic measurements, you should measure the spectrum of the crystal violet in the aqueous solution and establish the wavelength suitable for absorption measurements. Of course, this wavelength is the same for the whole class, so there is no reason to establish it once and once again.
Created 4 different solution of H2O and Crystal Violet at specific concentration. Measured the absorbance of each of these concentration using spectrophotometer and noted absorbance.
Graphed the absorbance vs. concentration, and found line of best-fit with an intercept of 0 to find value of m= εb.
Prepared spectrophotometer for kinetics measurements (separate instructions will be provided for each group).
Pour 10 mL of NaOH solution into 10 mL beaker. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
Pour 10 mL of 2.0 X 10-5 M crystal violet solution. CAUTION: Crystal violet is a biological stain. Avoid spilling it on your skin or clothing.
Initiated the reaction, simultaneously poured the 5-mL portions of crystal violet and sodium hydroxide into a 25-mL beaker and stirred the reaction mixture. Rinsed the cuvette with ~1-mL amounts of the reaction mixture and then filled it 3/4 full. Placed the cuvette in the cuvette slot of the spectrophotometer, and clicked "Collect" button. The program collected the absorbance data.
Analyzed the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet.
Results
Conc.(M)
A
0.000125
1.609
0.0001
1.244
0.000075
1.019
0.00005
0.53
*Measured with λmax= 600nm
Determining Beer-Lambert’s Law
A=εbC
Using the linear-fit line above εb was determined to equal 12695.
Determining the order wrt [CV+]:
To find molarity of CV+ used Beer-Lambert’s Law and value for εb.
UHPLC/UPLC: Ultra High Performance Liquid ChromatographyDarewin Mendonsa
Chromatography Techniques mainly include two basic sub-divisions: Separation Science and Analytical Science.
In 2004, separation science was revolutionized with the introduction of ‘Ultra High-Performance Liquid Chromatography which provides improved resolution, increased separation efficiency, shorter analysis time and lower operating costs.
It uses HPLC columns with a mean particle size less than 2μm and pressures up to 15,000 psi which drastically increases the number of theoretical plates of the column and results in enhanced column efficiency.
An overview of the use of the Marcus Theory to calculate the energies of transition states.
Contributed by: Elizabeth Greenhalgh, Amanda Bischoff, and Matthew Sigman, University of Utah, 2015
These are chemical shift reagents and solvent induced shifts have their application in resolving the NMR Spectra of complex structures by inducing shift with respect to reference compound. Thus useful in interpretation of structures of complex organic compounds.
What is the Rate Law for the Crystal Violet Reaction327-43.docxalanfhall8953
What is the Rate Law for the Crystal Violet Reaction?
3/27-4/3/2014
Section 10th
TA: Jinwei Zhang
Group 6
Daylin Morgan
Ahmed Alsharif
Shili Tong
Chang Chuan
Introduction
The main goal of this experiment was about observing the reaction between crystal violet and sodium hydroxide, CV+ + OH- -> CVOH. The objectives of this experiment are to monitor the absorbance of the crystal violet solution as a function of time, and determining the order of the reaction with respect to CV+. The pseudo rate constant k’ in this reaction is derived from the knowledge that OH- and the rate constant can be combined to a single constant. The Beer’s Law relationship between absorbance and concentration for CV+ is used to calculate the concentration in a solution given the Absorbance and a calculated constant. The half-life for this reaction with respect to CV+ and the rate law for this reactionare derived from total data.
Materials & Procedure
Materials
2.5*10-5M CV+(aq)
2.5*10-5 M OH-(aq)
Volumetric measuring equipment
Beaker
Cuvette
Stirring Rod
Spectrophotometer
Each student in a group will measure kinetics of the reaction at different concentration of sodium hydroxide. Before starting kinetic measurements, you should measure the spectrum of the crystal violet in the aqueous solution and establish the wavelength suitable for absorption measurements. Of course, this wavelength is the same for the whole class, so there is no reason to establish it once and once again.
Created 4 different solution of H2O and Crystal Violet at specific concentration. Measured the absorbance of each of these concentration using spectrophotometer and noted absorbance.
Graphed the absorbance vs. concentration, and found line of best-fit with an intercept of 0 to find value of m= εb.
Prepared spectrophotometer for kinetics measurements (separate instructions will be provided for each group).
Pour 10 mL of NaOH solution into 10 mL beaker. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
Pour 10 mL of 2.0 X 10-5 M crystal violet solution. CAUTION: Crystal violet is a biological stain. Avoid spilling it on your skin or clothing.
Initiated the reaction, simultaneously poured the 5-mL portions of crystal violet and sodium hydroxide into a 25-mL beaker and stirred the reaction mixture. Rinsed the cuvette with ~1-mL amounts of the reaction mixture and then filled it 3/4 full. Placed the cuvette in the cuvette slot of the spectrophotometer, and clicked "Collect" button. The program collected the absorbance data.
Analyzed the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet.
Results
Conc.(M)
A
0.000125
1.609
0.0001
1.244
0.000075
1.019
0.00005
0.53
*Measured with λmax= 600nm
Determining Beer-Lambert’s Law
A=εbC
Using the linear-fit line above εb was determined to equal 12695.
Determining the order wrt [CV+]:
To find molarity of CV+ used Beer-Lambert’s Law and value for εb.
UHPLC/UPLC: Ultra High Performance Liquid ChromatographyDarewin Mendonsa
Chromatography Techniques mainly include two basic sub-divisions: Separation Science and Analytical Science.
In 2004, separation science was revolutionized with the introduction of ‘Ultra High-Performance Liquid Chromatography which provides improved resolution, increased separation efficiency, shorter analysis time and lower operating costs.
It uses HPLC columns with a mean particle size less than 2μm and pressures up to 15,000 psi which drastically increases the number of theoretical plates of the column and results in enhanced column efficiency.
This is a PRESENTATION just to help students to easily understand one of the method of drug designing i.e. QSAR.. this is a combination of many slides and books..this is not my personal.
Gas chromatography- “It is a process of separating component(s) from the given crude drug by using a gaseous mobile phase.”
Principle- The principle of separation in GC is “partition.”
The mixture of components to be separated is converted to vapor and mixed with the gaseous mobile phase.
The component which is more soluble in the stationary phase travels slower and eluted later.
The component which is less soluble in the stationary phase travels faster and eluted out first.
No two components have the same partition coefficient conditions.
So the components are separated according to their partition coefficient.
The partition coefficient is “the ratio of solubility of a substance distributed between two immiscible liquids at a constant temperature.’
It involves a sample being vaporized and injected onto the head of the chromatographic column.
The sample is transported through the column by the flow of inert, gaseous mobile phase.
The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.
Two major types:
1. gas-solid chromatography: Here, the mobile phase is a gas while the stationary phase is a solid.
Used for separation of low molecular gases,
e.g., air components, H2S, CS2, CO2, rare gases, CO, and oxides of nitrogen.
2.Gas-liquid chromatography: The mobile phase is a gas while the stationary phase is a liquid retained on the surface as an inert solid by adsorption or chemical bonding.
Advantages-
Both qualitative and quantitative analyses are possible.
The instrument is simple, time of analysis is short.
High sensitivity.
The method is applicable to about 60% of organic compounds.
Very small sample sizes can be used.
Analysis can be highly accurate and precise.
Applications-
Quality control and analysis of drug products like antibiotics (penicillin), antivirals (amantadine), general anesthetics (chloroform, ether), sedatives/hypnotics (barbiturates), etc.
Assay of drugs – purity of a compound can be determined for drugs like :
Atropine sulfate
Clove oil
Stearic acid
In determining the levels of metabolites in body fluids like plasma, serum, urine, etc
Estimation of spoilage components, such as histamine and carbonyls, that cause rancidity.
Introduction, images of Arsenic, Industrial Uses and pollution sources, Speciation of Arsenic, Environmental levels and ecological effects, Biochemical effects, toxicology and toxicity, Treatment for Arsenic poisoning, Control measures.
Introduction
Critical discussion on heavy metals.
Target organs by heavy metal pollutantsIndustrial uses and pollution sources of Mercury.
Ef mercury
Biochemical effects, toxicology and toxicity of mercury
Biomethylation of mercury
Control of mercury pollutants
Treatment on mercury poisoning.
1.Weak forces of attraction
2.Concepts of Hydrogen bonding
3.Types of hydrogen bonding
4.Properties of hydrogen bond.
5.Methods of detection of hydrogen bond.
6.Importance of Hydrogen bonding.
7.Vander walls forces
a.Ion-dipole
b.Dipole-dipole
c.London forces.
8.Origin of hydrogen bonds.
9.Consequences of hydrogen bonding.
10.Ice has less density than water.
11.Intermolecular forces.
Scope, chemical industries, raw materials, Chemical production, raw materials, Pollution control, Human resources, Safety measures, R & D, objectives, Trade mark, copyright act, Patent act, pollution control
Ring n chain compounds
Silicates
Types of silicates
Principle of Silicate minerals
Soluble silicates
Amphiboles, Zeolites, Ultramarines,
Feldspars
Silicates in technology
Glass, quartz, micas
Inorganic Reaction mechanism
Stoichiometric classification
Trans effect
Electrostatic Polarization theory
π-bonding theory
Reactions without metal-ligand bond breaking
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
A brief information about the SCOP protein database used in bioinformatics.
The Structural Classification of Proteins (SCOP) database is a comprehensive and authoritative resource for the structural and evolutionary relationships of proteins. It provides a detailed and curated classification of protein structures, grouping them into families, superfamilies, and folds based on their structural and sequence similarities.
Richard's entangled aventures in wonderlandRichard Gill
Since the loophole-free Bell experiments of 2020 and the Nobel prizes in physics of 2022, critics of Bell's work have retreated to the fortress of super-determinism. Now, super-determinism is a derogatory word - it just means "determinism". Palmer, Hance and Hossenfelder argue that quantum mechanics and determinism are not incompatible, using a sophisticated mathematical construction based on a subtle thinning of allowed states and measurements in quantum mechanics, such that what is left appears to make Bell's argument fail, without altering the empirical predictions of quantum mechanics. I think however that it is a smoke screen, and the slogan "lost in math" comes to my mind. I will discuss some other recent disproofs of Bell's theorem using the language of causality based on causal graphs. Causal thinking is also central to law and justice. I will mention surprising connections to my work on serial killer nurse cases, in particular the Dutch case of Lucia de Berk and the current UK case of Lucy Letby.
The increased availability of biomedical data, particularly in the public domain, offers the opportunity to better understand human health and to develop effective therapeutics for a wide range of unmet medical needs. However, data scientists remain stymied by the fact that data remain hard to find and to productively reuse because data and their metadata i) are wholly inaccessible, ii) are in non-standard or incompatible representations, iii) do not conform to community standards, and iv) have unclear or highly restricted terms and conditions that preclude legitimate reuse. These limitations require a rethink on data can be made machine and AI-ready - the key motivation behind the FAIR Guiding Principles. Concurrently, while recent efforts have explored the use of deep learning to fuse disparate data into predictive models for a wide range of biomedical applications, these models often fail even when the correct answer is already known, and fail to explain individual predictions in terms that data scientists can appreciate. These limitations suggest that new methods to produce practical artificial intelligence are still needed.
In this talk, I will discuss our work in (1) building an integrative knowledge infrastructure to prepare FAIR and "AI-ready" data and services along with (2) neurosymbolic AI methods to improve the quality of predictions and to generate plausible explanations. Attention is given to standards, platforms, and methods to wrangle knowledge into simple, but effective semantic and latent representations, and to make these available into standards-compliant and discoverable interfaces that can be used in model building, validation, and explanation. Our work, and those of others in the field, creates a baseline for building trustworthy and easy to deploy AI models in biomedicine.
Bio
Dr. Michel Dumontier is the Distinguished Professor of Data Science at Maastricht University, founder and executive director of the Institute of Data Science, and co-founder of the FAIR (Findable, Accessible, Interoperable and Reusable) data principles. His research explores socio-technological approaches for responsible discovery science, which includes collaborative multi-modal knowledge graphs, privacy-preserving distributed data mining, and AI methods for drug discovery and personalized medicine. His work is supported through the Dutch National Research Agenda, the Netherlands Organisation for Scientific Research, Horizon Europe, the European Open Science Cloud, the US National Institutes of Health, and a Marie-Curie Innovative Training Network. He is the editor-in-chief for the journal Data Science and is internationally recognized for his contributions in bioinformatics, biomedical informatics, and semantic technologies including ontologies and linked data.
Cancer cell metabolism: special Reference to Lactate PathwayAADYARAJPANDEY1
Normal Cell Metabolism:
Cellular respiration describes the series of steps that cells use to break down sugar and other chemicals to get the energy we need to function.
Energy is stored in the bonds of glucose and when glucose is broken down, much of that energy is released.
Cell utilize energy in the form of ATP.
The first step of respiration is called glycolysis. In a series of steps, glycolysis breaks glucose into two smaller molecules - a chemical called pyruvate. A small amount of ATP is formed during this process.
Most healthy cells continue the breakdown in a second process, called the Kreb's cycle. The Kreb's cycle allows cells to “burn” the pyruvates made in glycolysis to get more ATP.
The last step in the breakdown of glucose is called oxidative phosphorylation (Ox-Phos).
It takes place in specialized cell structures called mitochondria. This process produces a large amount of ATP. Importantly, cells need oxygen to complete oxidative phosphorylation.
If a cell completes only glycolysis, only 2 molecules of ATP are made per glucose. However, if the cell completes the entire respiration process (glycolysis - Kreb's - oxidative phosphorylation), about 36 molecules of ATP are created, giving it much more energy to use.
IN CANCER CELL:
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
introduction to WARBERG PHENOMENA:
WARBURG EFFECT Usually, cancer cells are highly glycolytic (glucose addiction) and take up more glucose than do normal cells from outside.
Otto Heinrich Warburg (; 8 October 1883 – 1 August 1970) In 1931 was awarded the Nobel Prize in Physiology for his "discovery of the nature and mode of action of the respiratory enzyme.
WARNBURG EFFECT : cancer cells under aerobic (well-oxygenated) conditions to metabolize glucose to lactate (aerobic glycolysis) is known as the Warburg effect. Warburg made the observation that tumor slices consume glucose and secrete lactate at a higher rate than normal tissues.
(May 29th, 2024) Advancements in Intravital Microscopy- Insights for Preclini...Scintica Instrumentation
Intravital microscopy (IVM) is a powerful tool utilized to study cellular behavior over time and space in vivo. Much of our understanding of cell biology has been accomplished using various in vitro and ex vivo methods; however, these studies do not necessarily reflect the natural dynamics of biological processes. Unlike traditional cell culture or fixed tissue imaging, IVM allows for the ultra-fast high-resolution imaging of cellular processes over time and space and were studied in its natural environment. Real-time visualization of biological processes in the context of an intact organism helps maintain physiological relevance and provide insights into the progression of disease, response to treatments or developmental processes.
In this webinar we give an overview of advanced applications of the IVM system in preclinical research. IVIM technology is a provider of all-in-one intravital microscopy systems and solutions optimized for in vivo imaging of live animal models at sub-micron resolution. The system’s unique features and user-friendly software enables researchers to probe fast dynamic biological processes such as immune cell tracking, cell-cell interaction as well as vascularization and tumor metastasis with exceptional detail. This webinar will also give an overview of IVM being utilized in drug development, offering a view into the intricate interaction between drugs/nanoparticles and tissues in vivo and allows for the evaluation of therapeutic intervention in a variety of tissues and organs. This interdisciplinary collaboration continues to drive the advancements of novel therapeutic strategies.
(May 29th, 2024) Advancements in Intravital Microscopy- Insights for Preclini...
Tecniques determine Rate of Reaction pdf.pdf
1. Dr.Vikas A.Thakur,
Department of Chemistry
Rayat Shikshan Sanstha’s
K.B.P.College,Vashi(Autonomous)
TECHNIQUES FOR DETERMINATION
OF RATE OF REACTION
2. INTRODUCTION
➢ Generally, Kinetic study proceeds after reactant, Products and
Stoichiometry of the reaction have been satisfactorily characterized.
➢ In this part, techniques often used in Inorganic studies have discussed.
➢ Emphasizes made on their time range and general area of applicability.
➢ Any Experimental Kinetic method monitor somehow change of
concentration with time.
➢ Many studies are done under Pseudo-first order conditions.
➢ Then one must monitor the deficient reactant or product(s) because
other species undergo small changes in concentration.
➢ The Kinetic methods of choice often will be dictated by time scale of
the reaction.
➢ Detection methods will be determined by Spectroscopic properties of
species to be monitored.
2
3. INTRODUCTION
❑ Efficient use of materials can be a significant factor in the choice of
method because Kinetic study generally involves a number of runs at
different concentration and temperature.
❑ Detection methods for species should be as Specific as possible.
❑ Ideally, it should measure both reactant disappearance and Product
formation.
❑ Methods must not have any interference from other reactants and
should be applicable under a wide range of concentration conditions so
that the rate law can fully explored.
❑ There is often a practical trade off between Specificity, Sensitivity and
reaction time.
❑ E,g. NMR is quite specific but it is rather slow. It has relatively low
sensitivity unless system allows time for signal accumulation.
3
4. INTRODUCTION
Spectrophotometry in the UV and visible range often
has good sensitivity and speed but the specificity may
be poor due to broad absorption bands and
intermediates have Chromophoric(causing colour)
properties similar to reactant and/or products.
Vibrational Spectrophotometry can be better if IR
bands are sharp like in case of metal carbonyls.
But solvents must be chosen which provide
appropriate Spectral window.
Conductivity change can be very fast but it is rather
unspecific.
4
5. 1. Flow Methods
In these methods, the reagent solutions are brought together
by flowing them through a mixer.
From the mixer reaction solution emerges to be analysed.
The flow may be simply driven by gravity or by
mechanical pressure applied to syringes containing
reagents.
The minimum time scale depends on various factors such
as reagent flow rate, efficiency of mixer and response time
of analyser.
This general process has adapted in various ways to
minimize the amounts of reagents used, to optimize
detection sensitivity and shorten accessible reaction time.
5
6. 1(a). QUENCHED FLOW METHODS
❖ This method involves driving
the reagent solutions through a
Mixer and then have some
means of stopping(Quenching)
the reaction as solution emerges
from the Mixer.
❖ Reaction time can be controlled
by changing the length of
tubing between Mixer and
quencher.
❖ Calibration with a reaction at
known rate is necessary.
❖ The main trick is to find an
effective quenching methods.
6
7. 1(a). QUENCHED FLOW METHODS
This will depend on chemical reactions, addition of acid, base or
precipitating agent and rapid cooling.
The short time limit is ~ 20ms.But this depends on effectiveness
of quenching method.
➢ Advantages :
▪ Apparatus is simple and analysis of quenched solution can be
done without time constraints.
▪ The method is used especially for Isotope Exchange reaction
where subsequent analysis of isotopic content is a slow process.
➢ Disadvantages /Limitations:
▪ Sometimes analysis of many samples become tedious.
▪ Consumption of substantial amounts of reactants for each kinetic
run.
7
8. 1(b). STOPPED FLOW METHODS
o Method is most popular and
effective for reactions with half
times in the range ~ 10ms to ~ 60S.
o Several commercial instruments are
available.
o In a Typical instruments, the
reagent solutions are contained in
two drive syringes whose plungers
can be advanced by activating an
air pressure or electrical drive
system.
o This moves solution through a
Mixer into an Observation cell and
then to Stopping Syringe.
o A Mechanical stop on stopping
syringe or drive stops the flow and
triggers the observation and data
recording system.
8
9. 1(b). STOPPED FLOW METHODS
The standard system mixes equal volumes (~ 0.2 to 0.5 mL) of each
reagent solution and uses single-wavelength, single-beam UV visible
Spectrophotometry as a detection methods.
A Number of variations have been described using other detection
methods(Conductometry,Flurometry, NMR,ESR,EXAFS) and
multiwavelength at high pressure at subzero temp. and for multiple
mixing.
It is apply first for two reagents, then for third after some time interval.
The method is quite adaptable and widely applicable.
The time range limitations are determined on the short end by the dead
time of system(the time for mixing of reagents and transfer to
observation cell) and on long end by diffusion of reagents into
observation cell.
The experimental first order rate constant, Kexp can be corrected for
mixing time effect, Kmix to obtain the true Pseudo-first order rate
constant Ktrue.
9
10. 1(b). STOPPED FLOW METHODS
It is obtained from following relationship suggested by Dickson and
Margerum.
➢ (Ktrue)-1 = (Kexp)-1 – (Kmix)-1
The rearrangement of this expression we get
➢ Ktrue = Kexp(1 – Kexp/Kmix)-1 (1)
➢ Equation (1) is always shows that true rate constant is always larger
than experimental value.
• But the correction is insignificant if Kexp/Kmix << 1.
• To determine Kmix, measurements can be done on a well-characterized
system under Pseudo first order conditions with Ktrue = K1[R].
• Where K 1 is Known and R is the excess reagent whose concentration
can be varied to change Ktrue.
• The variation of Kexp with [R] is used to determine K.Mix.
❑ Margerum and co-workers have reported values of K.Mix for Durrum
and High-Tech Instruments of 1.7 x 103 and 2.9 x 103 S-1 respect.
10
11. 1(b). STOPPED FLOW METHODS
The deadline is due primarily to
physical separation of the mixer,
observation cell and on flood
velocity.
Typical deadline are in the 1 to 5
ms range. It can be determined by
extrapolation at the true time zero
as shown by dashed lines in fig.
The actual time t is related to
experimentally recorded time, texp
and deadtime,td by
t = texp + td (2)
For Spectrophotometric
detection,fig.3 illustrates
relationship between these lines
and Ao
Pred ,the predicted
absorbance at true zero time and
Ao
obsd,the initial absorbance at the
11
12. 1(b). STOPPED FLOW METHODS
➢ The reaction has pseudo-first order rate constant
➢ Kexp = K1 [R] .So that dependence of the absorbance change is given by
❑ (A - A) = (A - Ao
Pred ) exp(-Kexp t ) (3)
Where
A -- is final absorbance and A--- is the absorbance at any time t.
Both sides of equation(3) can be multiplied by exp.(Kexpt td )
texp = t-td, we obtain.
(A - A)exp(Kexp td) = (A - Ao
Pred ) exp(-Kexp
t d )
= (A - Ao
Pred ) exp(-Kexp/texp) (4)
Substituting limiting condition that A = Ao
obs when texp = 0 into equation 4 and
rearranging be given as
(A – Ao
obs) = (A - Ao
Pred ) exp(-Kexp td) (5)
Equation(5) can be used to determine td from measurable quantities Kexp, Ao
obs
, Ao
Pred
Substitution of equation (2) and (5) into equation(3) gives
12
13. 1(b). STOPPED FLOW METHODS
❑ (A – A) = (A – AO
obs) exp(-Kexp
. texp) (6)
➢ Equation(6) shows that Kexp is independent of td and is determined
from dependence of (A - A) on texp.
The situation is more complex for studies under second-order
conditions because reagent conc. at true zero time must be known.
❖ Limitations :
The deadtime could be adjusted to give Ao
pred if true zero time value is
known.
❑ Meagner and Corabacher have suggested an empirical way of dealing
mixing problem.
➢ Reagent solution of similar composition in order to avoid spurious
effects due to inhomogeneous mixing.
➢ It is always advisable to do blank observations to ensure that no
apparent reaction is observed in absence of each reactant.
13
14. 1(b). STOPPED FLOW METHODS
❑ Applications:
➢ Numerous applications are in collection and analysis of experimental
rate constants are entirely straightforward.
➢ 1.Hydrolysis of Cobalt(III) carbonate chelates in acidic medium has
been model for carbonic anhydrase catalysed dehydration of CO2.
➢ The mechanism is shown in Scheme 1.
➢ For some systems the protonated species has been detected and Ka is
determined.
➢ The two steps in the reaction are chelate ring opening K1 and
decarboxylation K2 of monodentate bicarbonate complex.
➢ 2.Reaction with aqueous Iron(III) with various ligands often yield
highly coloured complex.
❑ The Classic example is deep red thiocyanate complex Fe(OH2)5(NCS)2+
➢ This system is ideal for study because of large absorbance change and
ready availability of reagents.
14
15. 1(b). STOPPED FLOW METHODS
❑Recent arguments suggested that substitution
mechanism is associative for Fe(OH2)6
3+ and
dissociative for Fe(OH2)5(OH)2+ .
A number of pressure dependence studies have done in
expectation that ΔV* values help to violate reactivity
arguments.
Fe(OH2)6
3+ + SCN- →K1 → Fe(OH2)5(NCS)2+ +
H2O
Fe(OH2)5(OH)2+ + SCN- →K2 → Fe(OH2)4
(OH)(NCS)+ +
H2O (7)
15
16. 1(c). CONTINUOUS FLOW METHODS
➢ Theoretically, Continuous flow preceded Stopped flow as a method for
studying moderately fast reactions.
➢ The reactant solutions are made to flow continuously through the Mixer
and observation Chambers and time dependence of the reaction can be
obtained by changing the flow rate or moving the observation point to
various distances from the Mixer.
➢ The apparatus can be quite simple but large amounts of reagents are
consumed.
➢ In a pulse continuous flow method, a continuous flow is established for
a short time can reduce reagent consumption to ~5ml.
➢ The fast jet mixtures have lowered accessible reaction half-time to the
10S range.
➢ The concentration of reagent monitored can be lowered if integration
observation is used in which flowing solution is viewed down the
length of observation cell.
16
17. 1(d). PULSE ACCELERATED FLOW
METHODS
In this method pulsed continuous flow has adapted in
which flow rate through the mixer and observation
chamber is varied during the course of one run.
❑Most recent application of this method is of Margerum
and his Co-workers.
❖Advantage:
Method can be used for half-times down to ~ 10 S
compared to ~ 10 mS from Stopped flow.
❖Limitations:
Because of complexity of analysis the method is
limited to first order reaction conditions.
17
18. 2. RELAXATION METHODS
➢ In these methods, a system at equilibrium is subjected to a perturbation and
kinetics of system relaxing to new equilibrium condition is followed.
➢ The perturbation(deviation) normally is a change in temperature, Pressure
or concentration of one of the reagents .
➢ The methods are known as temperature jump, Pressure jump and
concentration jumps respectively.
❑ Advantages :
➢ Perterbation especially of temp. and pressure can be applied very quickly
and reaction with half-times in the micro second range can be observed.
❑ Eigen and co-workers done Pioneering work greatly extends time scale for
solution kinetic studies.
❑ Limitations:
➢ Major limitation is that equilibrium position of the reaction must involve
significant concentration of both reactant and products.
➢ Hence relaxation methods are not applicable to essentially irreversible
reactions.
18
19. 2. RELAXATION METHODS
❑ Applications :
These methods are especially useful for Lowry-Bronsted acid-
base reactions in which the equilibrium position can be adjusted
simply by changing the pH of solution.
Method is useful for ligand substitution reactions that involve
Proton production or consumption.
❑ A Noteworthy feature of this method is the changes in
concentration caused by the perturbation should not be too large
so that Mathematical Analysis can be simplified.
This poses some limitations for the detection method.
This method must be fast but also sensitive enough to detect
these small concentration changes.
But it is possible to repeat the perturbation and improve signal to
noise ratio through signal averaging.
19
20. 2. RELAXATION METHODS
The standard Mathematical analysis can be illustrated for following system.
A + B Kf→ C (8)
Kr
After the perturbation, the system comes to new equilibrium with final
concentration [Ae], [Be] and [Ce] and these may be related to concentration at
any time through the concentration change variable Δ, so that
[A] = [Ae] + Δ
[B] = [Be] + Δ
[C] = [Ce] - Δ
Simple Differentiation shows that
d Δ/dt = d[A]/dt = d[B]/dt = -d[C]/dt
We can write the usual differential equation for system
d Δ/dt = d[A]/dt =-Kf[A] [B] + Kr[C]
= -Kf ( [ Ae] + Δ) ( [ Be] + Δ) +Kr([Ce]- Δ) (9)
20
21. 2. RELAXATION METHODS
Expansion and collection of terms given is
d Δ/dt = -Kf [ Ae] + [Be] Δ – Kr Δ)–Kf[Ae][Be]+Kr[Ce]- Kf(Δ) 2
(10)
At equilibrium Kf = [Ae] [Be] = Kr[Ce], and these terms are cancelled.
Next, the assumption is made that Δ is very small, so that the term in Δ2
can be neglected and Equation (10) can be simplified to
d Δ/dt = -{Kf [ Ae] + [ Be] + Kr} Δ =Δ/ (11)
❑ Where t is called as relaxation time.
If Expts. are done with varying positions of the eqm then Plot of -1
versus [Ae] + [Be] should have slope of Kf and an intercept of Kr.
This is different from normal Pseudo-first order system.
Experimental rate constant is kf + kr.
21
22. 2(a). TEMPERATURE JUMP
For a system at eqm, if the temperature is changed by ΔT, then eqm
concentration will change due to thermodynamic relationship between
eqm constant and Enthalpy change
❑ (ln K/ T)P = -ΔH0
rxn/ RT2
In early applications and commercial instruments, the temperature was
quickly changed, typically by ~ 50C by an electrical discharge.
Sample was contained between two electrodes and subjected to a
voltage of ~ 10 to 100 KV across the electrodes to produce the
discharge.
This requires that solution is electrically conducting and somewhat
invasive on sample.
It is observed that ion Polarization at charged electrodes can cause
spurious but reproducible signals.
22
23. 2(a). TEMPERATURE JUMP
❑ Hague and Martin usedT-Jump to
study complexation of aqueous
Manganese(II) by 2,2’ bipyridine
shown in below
Mn(OH2)6
2+ + bpy Kf
K→
Mn(OH2)4(bpy)2+ + 2 H2O (12)
The work was extended by Doss
andVan Eldik to determine
Pressure dependence.
From both studies, Kf = 2 x 10 5
M-1 s-1 and the pressure
dependence gave Δf* = -3 cm3M-1
This negative value is as evidence
for an Ia mechanism for
substitution of Mn(OH2)6
2+
23
24. 2(b). PRESSURE JUMP
The method requires a finite volume change for the reaction,
ΔV0 rxn so that eqm constant will change with Pressure due to
relationship.
(ln K/ P)T = -ΔV0
rxn/ RT
➢ The experiment is done by putting the sample under high
Pressure and suddenly reduced the pressure by piercing a
diaphragm.
➢ High Pressure equipments and observation cells are required.
❖ But perturbation seems less invasive on the sample thanT-jump by
electrical discharge.
❑ Recently,A P-Jump system with conductivity has been detected.
24
25. 2(b).CONCENTRATION JUMP
The system is perturbed by adding a small amount of one of
the species in the eqm reaction.
Generally, apparatus is much simpler thanT-Jump or P-Jump
methods.
But the perturbation cannot be done as quickly so that short
time limit is in the millisecond range.
25
26. 3. ELECTROCHEMICAL METHODS
Various Electrochemical methods such as Cyclic Voltametry, Polarography,
Chronoamperometry and Chronopotentiometry can be used to measure
homogeneous reaction rates.
In general, electrochemical observations can give information about
homogeneous reaction rates when an electrode reaction is coupled to
homogeneous chemical reaction.
The rate of latter becomes limiting rate for the process at the electrode.
Sometimes the chemical rate constant can be extracted fairly directly from
observation. Or
It may require curve matching of experimental and simulated curves computed
with various rate constants.
Size and composition of the electrode and diffusion coefficient of reagents
affects the kinetics of electrode reaction.
Hence these factors will influence observations and effective time range.
The field has a well developed nomenclature and symbolism.
26
27. 3. ELECTROCHEMICAL METHODS
One electron electrode reaction is designated by E and a chemical
reaction by C.
There are extensions of systems like E+E for two electron electrode
reaction, E- and E- for reduction and oxidation,C1 and C2 for first and
second order reactions and C1’ for Pseudo first order reactions.
The most widely used technique describe is CyclicVoltammetry due to
availability of appropriate instrumentation and number of increasing
applications with recent software to simulate cyclic voltammograms.
Such simulations are generally essential for determination of meaningful
kinetic parameters.
An ideal cyclic voltammogram, CV, and some terminology of this
technique is shown in fig.
The experiment is carried out by changing the voltage, E,of working
electrode at some constant sweep rate, v and measuring current, i.
27
28. 3. ELECTROCHEMICAL METHODS
Then the sweeep rate and reagent
concentrations are changed and changes in
cathodic and anodic peak potential,Epc and
Epa and peak current ip are analyzed.
Quantitative analysis requires knowledge of
rate(s) of heterogeneous electrode
reaction(s),reagent diffusion coefficients and
transfer coefficient.
If electrode reaction is reversible ,most of
these parameters can be determined from
CV Experiments.
The formal reduction potential E0’,differs
from standard reduction potential E0.
Because latter is obtained by extrapolation
to infinite dilution while former refers to
actual experimental conditions of ionic
strength and temp.
28
29. 3. ELECTROCHEMICAL METHODS
Potentials are often reported relative to some standard electrode such as
ferrocene/ferrocinium ion,saturated calomel,SCE or Ag/AgCl.
This must be taken into account in comparing results from different sources.
One restriction of these methods is that the medium must contain an inert
electrolyte to maintain electrical conductivity.
Typically,0.1M tetraalkyl ammonium salts of PF6
-,CF3SO3
- or ClO4
- are used.
Problems arises due to adsorption of reagents on electrode and uncertainties in
chemical characterization of the product of electrode reaction.
The experiment can give number of electrons, n,involved and the reduction
potential and then nature of electrochemically generated reagent is inferred by
chemical reasoning and analogy.
It is possible to couple the system to some Spectroscopic technique such as EPR or
IR Spectroscopy for further characterization.
29
30. 3. ELECTROCHEMICAL METHODS
The substitution inertness of Cr(III) and
lability of Cr(II) have studied by Hecht,
Schultz and Speiser to observe some ring
opening and ring-closing reactions of
ammino-carboxylate ligands.
The experiments used a stationary Hg drop
electrode in 1.0M Na2SO4 at pH 8.5 at
ambient temp.
Their observations are explained in Scheme
2 where aliphatic substituent R gives trans-
N,N geometry shown.
For such a system, initial CV reduction of
Cr(III) complex shows a broad cathodic
wave whose position shifts from about -1.4
to -1.6V as sweep rate is increased.
This is a typical irrversible reduction with
sluggish electrode kinetics and is assigned to
Kshl Process.
30
31. 4.NUCLEAR MAGNETIC RESONANCE METHOD
There is a wide variety of applications of NMR to problems in inorganic
kinetics.
The time scale depends on type of system and can vary from hours to
microseconds .
One great advantage of NMR is that temp. can be changed over a wide range
from about -2000C to +1500C without significant instrument modification.
Another advantage is molecular specificity of NMR signal which often permits
an assignment of the composition and structure of stable intermediates and
products.
The specificity is augmented by ability to detect a wide range of NMR active
nuclei;1H,13C, 19F and 31P are standard for most modern NMR instruments and
many metals have NMR active isotopes that can observed with appropriate
modifications.
❑ Unique feature of NMR is ability to measure rates of reactions in which there is
no net chemical change such as solvent exchange and ligand fluxionality.
31
32. 4.NUCLEAR MAGNETIC RESONANCE METHOD
❑ Limitation:
Major limitation of NMR is sensitivity and concentrations must be typically
about 0.01M unless signal averaging is possible.
However, the small sample size of 0.5 to 2 ml allows for modest materials
consumption.
Discussion of theory and quantitative analysis in this area often use lifetime of
a nucleus in a particular site as a kinetic interest.
❑ By conventional definition, Lifetime is concentration of nuclei in the site
divided by their rate of disappearance from the site.
To establish relationship between rate constant and the lifetime, it is necessary
to define clearly due to ambiguities of number and populations of sites.
E,g.Exchange of nuclei between a hydrated metal ion
M(OH2)n
Z+ and bulk solvent water can be represented by a reaction with whole
water molecule exchange or a just proton exchange.
32
33. 4.NUCLEAR MAGNETIC RESONANCE METHOD
M(OH2)n z+ + H2O →M(OH2)n-1(OH2)z++ H2O (13)
M(OH2)n z+ + H2O →M(OH2)n-1(OH2)(z-1)++H H2O + (14)
In first case there are two lifetimes, m for the water ligands and s for
the bulk solvent.
Ambiguity is in whether whole water molecule exchange with n
coordinated sites as in reaction(13) or proton exchange with 2n such
sites as in reaction(14).
If 17O NMR is used, then only whole molecule exchange will be
observed and definitions are straightforward and given by
m = n[M2+]/Rate s =[H2O ]/Rate Rate =k[M2+] x [H2O ]y (15)
If 1H NMR is used, population in each site is multiplied by 2 because
there are two hydrogen per water molecule and the lifetimes are defined
by
’m =2x n[M2+]/Rate ’s =2x[H2O ]/Rate Rate =k’[M2+] x [H2O ]y
(16)
33
34. 4.NUCLEAR MAGNETIC RESONANCE METHOD
If one believes that exchange involves a water molecule, then k in eqn(15) is the
rate constant for exchange of one water ligand.
But k’ in eqn(16) is for exchange of one H and since there are two H atoms per
17O, then k = k’/2.
But,if if H exchange occurs only by reaction (13) then k’is the rate constant as
defined in eqn. (16).
Therefore 17O NMR will give k for water molecule exchange
But the k’ from 1H NMR has an ambiguious assignment.
For other solvents such as acetonitrile, DMF and DMSO where independent
exchange of methyl protons is very unlikely the site population factor in
eqn(16) is often omitted and definition refer to whole solvent molecule
exchange.
NMR methods are separated into four categories in the order of decreasing
time scale of their applicability.
However latter is quite dependent on system. Different methods might be used
in different temperature ranges.
34
35. 4(a) SIGNAL MONITORING
This method refers to simple monitoring of changes with time of concentration
of reactants and products.
These are determined from integrated intensities of appropriate peaks in the
NMR spectrum. The short time scale is few minutes required for temp.
equilibrium and instrument help.
Long time is limited only by sample stability.
For pulsed Fourier transform instruments, it is important to remember the
repetition rate or relaxation delay must be 8 to 10 times longer than nuclear
relaxation time(s),T1 to obtain correct relative intensities.
TheT1 values for 1H and 13C nuclei can be in the 1- to 10-s range.
A special example of this type of application is measurement of exchange
reactions using appropriate isotope.
E,g. Exchange between free CO and CO ligands in metal carbonyls can be
measured using 13C- enriched CO.
Exchange between H2O and Oxo anions or water ligands can be measured in
suitably inert system using 17O enrichment.
35
36. 4(b) MAGNETIZATION TRANSFER
➢ This method is simple to qualitatively envisage and interpret.
A selective pulse (or DANTE series of pulses) is used to produce spin inversion
or saturation at one site.
After a variable waiting period, tm, a 900 pulse is used to generate normal
spectrum of system.
As exchange proceeds, the inverted nuclei appear in other sites and the
intensities of the sites involved in the exchange will decrease.
As tm is increased to stage where tm>T1, the natural nuclear relaxation process
tend to restore the intensity.
Intensity of sites involved in the exchange will increase due toT1 process.
The accessible range of exchange lifetimes,t, for this method is determined on
the short end by tm and on the long end byT1.
For typical spectrometers tm can be as short as ~ 0.01S andT1 for protons is
often ~1s.so that first order rate constant of ~ 100 to 1 s-1 can be determined.
36
37. 4(b) MAGNETIZATION TRANSFER
37
AsT1 is usually has a lower activation energy(-5 to 10 Kjmol-1)
than , it is often possible to adjust the temperature to meet the
requirement of this method that Ti .
Inversion of a multiplet due to spin-spin coupling can be achieved
with a single pulse ,broad enough to cover the multiplet for small
coupling constants or by pulses of different frequencies in the
DANTE sequence for large coupling constants.
It has been found that spin-spin coupling does not adversely affect
rate constant determination by this method.
Because of competition between exchange and t1 relaxation
processes, it is advantageous for quantitative analysis to measure
theT1 values independently under slow exchange conditions.
38. 4(c) TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY
It is called 2D EXSY method.
In the experiment, a 900 pulse is applied to rotate the magnetization from +z to
the –x axis.
After a time t1, which is called evolution or labelling time, a second 900 pulse
rotates the magnetization from the xy plane into xz plane.
A field gradient or homospoil pulse is applied to dephase the magnetization
along x axis.
After a further time tm, called as mixing time, a third 900 pulse rotates the
magnetization to the y axis and Free Induction Decay, FID is collected during
time t2.
The magnetization evolves in the xy plane during the two time periods t1 and t2.
For a single site, the angular rate of precession is , and maxima will occur
when Cos( t1) and Cos(t2) equal 1.
Then a three dimensional plot of intensity versus t1 and t2 will show a
maximum when this condition is satisfied for t1 and t2.
38
39. 4(c) TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY
39
The peak is often represented by intensity contours and is really a cone.
For a multisite system, nuclei in different environments have different
precessional rates, 1,but will give a maxima when Cos(1t1) and Cos(2t2)
equal 1.
It gives a peak along the diagonal of the t1-t2 plane when t1=t2.
When system is undergoing chemical exchange, magnetization can transfer
between the sites during mixing time.
This produces off-diagonal cross peaks in the final three dimensional plot of
spectra.
These cross peaks give a map of the sites that are undergoing exchange.
The evaluation of rate constants from the information is based on the intensities
or more properly the volumes of the cross peaks.
❑ This analysis is not trivial, especially for multi state system and requires special
care in the collection and processing the data to ensure that volumes of the
peaks are properly evaluated.
40. 4(c) TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY
40
➢ There are several other sources of cross peaks in 2D EXSY experiment.
➢ Dipolar coupling with nearby nuclei produces cross peaks as observed in
standard NOESY experiment.
➢ These can be identified because exchange usually has a larger
temperature dependence than dipolar coupling.
➢ Scalar coupling interferes with 2D EXSY by producing J cross peaks that
can be eliminated by phase cycling.
Choice of mixing time is crucial for this method because it is quite time
consuming to do studies by varying tm,t1 and t2.
Perrin and Dwyer have suggested optimum mixing time given by for
two-site(AB) system
tm(opt) = 1/T1
-1 + kAB + kBA
Thus, for a multi-site system with different exchange rates, there will
not be a single optimum value.
41. 4(c) TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY
41
2D EXSY method can be applied to
rearrangement of tris(dithiolene)
complexes of the general type
M(S2C2R1R2)3,where M isW or
Mo,R1 is Phenyl or substituted
phenyl and R2 is H or Phenyl.
The structures in solution are
believed to be trigonal prismatic.
Asymmetric substitution gives
possibility of cis and trans isomers.
The authors assignment are shown
by left-hand structures.
Low-temperature 1H NMR
spectrum has three peaks in the
R2=H region that were assigned to
H1,H2 and H3as shown in left.
42. 4(c) TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY
42
The peaks have approximately
1:2:1 intensity ratio, respec. Due to
~3:1 eqm mixture of trans : cis
isomer.
Result of 2D EXSY experiment on
W(S22H(p-CH3Oph))3 are
represented in fig.
In addition to diagonal peaks, cross
peaks are observed in all the
protons.
This indicates all the sites are
undergoing exchange with each
other.
Cross peaks for the H1-H3 protons
are weakest and give less certain
rate constants for this interchange.
43. 4(d) BANDSHAPE ANALYSIS
➢ This method was the first one used to show the applicability of NMR to
dynamic process.
➢ For a system with two sites, in slow exchange or low temp. limit,one can
observe normal spectrum with two peaks.
➢ If the temperature is raised and exchange starts to occur, the two peaks begin to
broader and come together.The peak will coalesce at some temp. Called as
coalesce temp.
➢ This depends on exchange rate and chemical shift separation between the peaks
in the absence of exchange.
➢ On further increase in temp. the signal changes to a single sharp resonance in
the fast exchange limit.
➢ For systems with several sites and hence several peaks in the slow exchange
limit, it is possible to observe the broadening and coalescence of exchanging
sites.
➢ Quantitative analysis normally involves calculating the spectrum for various
exchange models and rate constants .
43
44. 4(d) BANDSHAPE ANALYSIS
44
Then choosing model that best fits the observed spectra and gives rate
constants having normal temp. dependence.
Computer programme are available to generate the calculated spectra.
The time scale for this method depends on chemical shift
difference.,ΔV0 between exchanging sites.
At the coalescence temp., the rate constant is given by
K = ΔV0 /2 = 2.22 x ΔV0 (17 )
Below coalescence temp, in the slow-exchange region, two peaks are
observed that are broadened over their natural full linewidth at half-
height by v and k= 2 v in this region.
Above coalescence temp, in the fast-exchange region, only one peak is
observed with a linewidth of v and k= 4pi v02/v. in this region.
For a typical values for 1H NMR of v = 5Hz and v0= 100Hz,K can be
in range from ~30 to -3x104 s-1.
45. 4(d) BANDSHAPE ANALYSIS
45
Since ΔV0 depends directly on magnetic field strength,the range can be
extended to larger k by working at higher fields.
For other nuclei like 13C,19F and 31P and for paramagnetic systems, ΔV0 can be
much larger and upper limit can be greatly extended by ~102 by bandshape
analysis.
➢ Limitations:
➢ One needs chemical shift and line width for non-exchanging systems.
➢ When possible this is done by cooling the sample to well below slow exchange
limit, but temperature dependence of the shifts and line width is rarely
determined and are treated as constants in the analysis.
➢ Exchange pathways are not always clearly delineated by bandshape analysis,
especially in multisite systems.
➢ A model is chosen and fitted to the data but initial choice is somewhat
subjective and some pathways may be missed.
➢ A problem can be arises in the data collection on pulsed Fourier transform
instruments.
46. 4(d) BANDSHAPE ANALYSIS
46
Applications :
A typical application of bandshape analysis to inorganic mechanism
problem is the recent study of Raymond and co-workers on
fluxionality of tris-catecholate complex of Ga(III), where ligands
are 2,3-dihydroxy-N,N’-substituted-terephthalamides.
Under slow-exchange conditions the 1H NMR at 300 MHz of the
isopropyl derivatives shows two methyl resonances due to
chirality.
In D2O, as temperature is raised these two peaks merge and
coalesce at -570C.Further increase in temp. produces the expected
sharpening to one methyl signal.
Similar studies were reported on asymmetrical amide ligand with
a benzyl group on one nitrogen and tertiary butyl on the other.
47. 4(e) RELAXATION RATE MEASUREMENTS
This type is a specialised extention of bandshape analysis in which temp.
dependence of transverse nuclear relaxation time,T2,is used to measure
rate of exchange.
T2 can be determined from width of NMR peak or more accurately by
special pulse sequences.
The method is generally applied to simple systems with well-separated
peaks in the NMR spectrum.
It is specially useful for measurements of solvent exchange rates from
paramagnetic metal ions.
Swift and Connick first published the basic equations that are solution of
bloch equations modified for chemical exchange by Mcconnel.
Analogous eqns have been given for T1,for three-site problems and for
rotating frame relaxation time in such systems.
47
48. 4(e) RELAXATION RATE MEASUREMENTS
48
An ideal temperature dependence of
the relaxation rates is shown in fig.
The parameter plotted isTip-1 is the
difference between the relaxation rates
in the presence of exchanging species
and the rate for the pure solvent
divided by metal ion concentration.
In the high-temp.limit at the left of fig.
exchange is fast and relaxation is
controlled by the nuclear relaxation
rate in the inner coordination sphere of
the metal ion.T2m
-1.
As temp.lowered,exchange becomes
slower and relaxation is controlled by
dephasing of nuclear precession
frequency due to difference in chemical
shift between bulk and coordinated
nucleus Δm.
49. 4(e) RELAXATION RATE MEASUREMENTS
49
The slower is the exchange, more effective is dephasing so that measured
relaxation rate increases with decreasing temp. In the region called as non-
Arrhenius region.
At still lower temp.exchange becomes slow enough so that dephasing is
controlled by exchange lifetime, m, and the relaxation rate decreases with
decreasing temp.inArrhenius region.
Finally, at low temp.,inner sphere solvent exchange is so slow that only
relaxation due to outer-sphere interactions.,T20
-1 is observed.
The latter effect occurs actually at all temps.
At the maximum betweenArrhenius non-Arrhenius regions, m-1= Δm.
As Δm in the range of 103 to 105 s-1 for paramagnetic systems, gives some
indication of range of applicability.
In practice, a particular system often show only two or three of these specific
regions.
50. 4(e) RELAXATION RATE MEASUREMENTS
50
The problem is to fit this temp. dependence to known
functions primarily to determine ΔH* and ΔS* for exchange
process.
This also requires some knowledge or estimates of activation
energies Em and E0 forT2m andT20 respect.
Measurement of T-1 can be helpful in this regard as T1 is not
affected by dephasing and exchange is apparent only in
Arrhenius region as shown in fig.
Main difficulty is separating various factors that affect temp.
dependence of T2
-1 when limiting region are not well
defined.
51. 5.ELECTRON PARAMAGNETIC RESONANCE METHOD
➢ This is a powerful technique for the detection and monitoring of species
with unpaired electrons.
➢ This type of spectroscopy is designated by EPR,ESR,EMR(electron
Magnetic Resonance)
➢ EPR is quite sensitive with detection limits in the range of 10-6 M in
favourable cases. It is quite informative as to structure because of electron-
nuclear hyperfine coupling to metal and ligand nuclei.
➢ Disadvantages :
➢ Mainly many species with unpaired electrons do not give a leads to broad or
undetectable signals.
➢ Signals are more detactable in the crystalline or frozen glassy state.
➢ For the first row transition metals in their common oxidation state,solution
EPR is useful for complexes of V(IV),Mn(II) and Cu(II) while Cr(III) and
Fe)III) often give broad spectra in solution.
51
52. 5.ELECTRON PARAMAGNETIC RESONANCE METHOD
52
Most organic radicals give EPR signals that are quite useful for detection and
identification of such species as reaction Intermediates.
Most EPR spectra run at X-band frequency of 9.4GHz in the microwave region
of the electromagnetic spectrum.
Magnetic field of ~ 0.3 T is changed into give resonance condition for signal
detection.
Sample tube should be quartz to avoid impurity signals found in pyrex.
The concentrations of paramagnetic species should be <10-3 M to minimize
signal broadening due to intermolecular relaxation interactions.
EPR spectra are usually displayed as plots of the derivative of signal intensity
verses magnetic field.
Double integeration of such data is necessary to get proper integrated signal
intensities.
With proper calibration, the signal intensity gives direct measure of
concentration of EPR active species.
53. 5.ELECTRON PARAMAGNETIC RESONANCE METHOD
53
Pulse EPR method is more widely available.
A 900 pulse is typically in the range of 10 to 30ns and FID after
such a pulse can be used to measure the electron spin relaxation
time.
It can also be used to monitor the decay of radicals produced by
some fast method like flash photolysis or pulse radiolysis.
Most applications use EPR as a detection method for reactants and
products.
Various flow methods can be coupled with EPR to monitor time
dependence of EPR active species.
It is possible to use EPR line broadening to measure exchange
rates in time scale of ~ 10 -7 s.
54. 6.PULSE RADIOLYSIS METHOD
It is possible to quickly generate reactive species and solvated electrons by
passing a high-energy pulse of electrons through solution.
The pulse is typically 5 to 100 ns long with energies in the range of 2 to 20
MeV, depending on source apparatus.
High energy electrons initially are present in hot spots and thermalized species
are present after ~ 10 -7 s.
In water, the species and number produced per 100 ev of energy absorbed,in
brackets, are : eaq
-(2.65), *H(0.65), *OH(2.65), H2O2(0.72), H2(0.45)
In most applications, the initial radiolysis products are scavanged by additives to
remove undesired species or to produce a new reactive species.
E,G. Water saturated with N2O(~0.022 M under 1 atm of N2O) converts eaq-
to *OH in ~50ns by following reaction.
Eaq- + N2O + H2O ------- *OH + N2 + OH-
54
55. 6.PULSE RADIOLYSIS METHOD
55
Most studies in this area use spectrophotometric detection
and time scale can be from microseconds to seconds.
The products are produced at low concentration, it is often
possible to do multiple radiation pulses on the same sample.
The main problem is the lack of molecular specificity of the
spectrophotometric method so that the nature of reaction
and products are often inferred by analogy and by
concentration dependence of the reaction rate.
56. 7.FLASH PHOTOLYSIS METHOD
This technique is somewhat analogous to pulse radiolysis in that the
system is subjected to a short high-energy pulse.
Then subsequent events are monitored.
In Flash photolysis the pulse is usually by a laser beam of photons.
The immediate product is some photo excited state of absorbing
reactant(s).
Then subsequent events are monitored on the nanosecond or longer
time scale most commonly by Fourier Transform IR or UV-visible
spectrophotometry.
Flash photolysis is much cleaner than pulse radiolysis there is not the
multiplicity of initial reactants or need to add reagents to remove
undesired reactants.
In both the methods there is a problem of identifying the reactive
intermediates from often limited spectroscopic signatures they provide.
56
57. 7.FLASH PHOTOLYSIS METHOD
57
Many studies of the activation of C-H bonds by coordinatively
unsaturated species can be generated by flash photolysis.
Recent study by Harris and co-workers have observed flash
photolysis of Rh(Tp*)(CO)2 in pentane with a 295nm laser
pulp(Tp* = hydridotris(3,5-dimethylpyrazolyl)borate).
They observed reformation of Rh(Tp*)(CO)2 at 2054cm-1 with
= 70 ps and cooling of a vibrationally excited intermediate with
= 23 ps,to give a vibrational ground state absorbing at 1972cm-1
.
❑ The final product Rh(Tp*)(CO)(R)H, appears on a longer time
scale of ~ 500ns.
❖ Flash photolysis also provides access to electronic excited states
whose photochemistry, energy transfer and electron transfer
properties can be observed after the flash.
58. 8.Potentiometric Method
58
When a ligand is substituted by another ligand,
then it indicates that a new complex is formed.
The rate of such reaction can be measured by
using Potentiometer.
It measures the current that the electron carries
during the substitution reaction.
59. 9. Spectrophotometric Method
59
The rate of reaction can be determined by using a Spectrophotometer.
It is based on Beer-Lambert law/
A = ԑ Cl
Where,
A = Absorbance
C = Conc. of analyte
l = path length
Consider a reaction of
CuSO4 + EDTA ----[ Cu EDTA] Complex
The free Cu2+ ions are complexed by hexadentate EDTA ligands.
A number of solutions are prepared wherein the value of EDTA added is
measured for 0,1,2,3,4,5,6,and 7cm3.
The absorbance for each of the solution of Cu2+ EDTA are recorded.
From this the rate of reaction can be determined.
60. Beer-lambert law
60
For given material, the sample path length and concentration
of the sample are directly proportional to the absobance of
the light.
Q.Explain the following techniques for the
determination of rate of reaction in complexes.
62. References
62
1. D.Tzur and E. Kirowa-Eisner, Anal.Chim.Acta, 355,
85(1997).
2. D.A. Skoog, Principles of Instrumental Analysis.
3. D.A. Skoog and D. M.West, Principles of Instrumental
Analysis.
4. D.A. Skoog and J. J. Leary, Instrumental Analysis.
5. D. C. Harris, Quantitative Chemical Analysis.
6. G. D. Christian, Analytical Chemistry.