NMR

2.  INTRODUCTION – NMR SPECTROSCOPY.
 INSTRUMENTATION
 SOLVENT REQUIREMENT IN NMR.
 RELAXATION PROCESS IN NMR.
 CHEMICAL SHIFT.
 FACTORS AFFECTING CHEMICAL SHIFT.
 NMR SIGNALS IN VARIOUS COMPOUNDS.

3. • This spectroscopic technique gives us information about the number and types of atoms in a
molecule.
• Nuclear magnetic spectroscopy is a powerful analytical technique used to organize organic
molecules by identifying carbon hydrogen frameworks within molecules.
 Principle :
• The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. If
an external magnetic field is applied, an energy transfer is possible between the base energy to a
higher energy level.

5.  A NMR spectrophotometer consists of following components :
• A Magnet
• Sample and Sample holder
• Radio frequency generator
• Detector
• Reader
 WORKING : In NMR spectrometer, the sample is dissolved
in CDCL3 and placed in magnetic field. Then radio frequency
generator irradiates the sample with short pulse of radiation
causing resonance. When nuclei fall back to their lower energy
state, detector measures the energy released and the spectrum
is recorded. The super conducting magnet in modern NMR
spectrometers have coils that are cooled in liquid Helium and
conduct electricity.

7.  The solvents used in NMR spectroscopy should be chemically inert, magnetically isotropic, devoid of hydrogen
atom and should dissolve the sample to a reasonable extent.
• Eg : CCl4, CS2, CDCl3, DMSO etc.
 When selecting a solvent for running Nuclear Magnetic Resonance Spectroscopy (NMR) analysis, typically a
deuterated solvent is used in order to minimize background signals and provide a lock signal to compensate for
drifts in the magnetic field.
 NMR solvents are distinctly different from other spectroscopic solvents as the majority of hydrogen nuclei are
replaced with deuterium so as to minimize the interference due to protons.
 Though deuterium also has a nuclear spin, it does not operate on the same frequency as protons in the given
magnetic field. Therefore, it serves the purpose of NMR spectroscopy.

8. • It is crucial to remember that the price increases with the degree of deuteration. Deuterated chloroform, is most
commonly used because of its low price. Plus, the NMR peak is observed with the minimal chemical shift.
• A reference standard such as tetramethylsilane is commonly added (around 0.03%) to most commercially
available solvent grades to serve as zero ppm chemical shift reference for NMR studies.
• However, there are several other factors to consider for NMR solvent selection:
1. Solubility
2. Interfering peaks
3. Price
4. Isotopic purity
5. Ease of NMR sample recovery.
• Common spectroscopic solvents are available commercially in different degrees of deuteration. Apart from
CDCl3 other deuterated solvents in common use are:

9. • Deuterated water-(D_2O)
• DMSO (There is no ???? NMR peak, making this one of the best solvents)
• Methanol
• Methylene chloride
• Pyridine
• Acetic acid
• Acetone
For Example :
1. CDCL3 :
This solvent is the deuterated form of chloroform ,since it is relatively in expensive with isotopic purity, dissolves
many compounds, and is easy to evaporate after analysis for NMR sample recovery.
Since no solvent is 100% deuterated there will always be an observed ¹H peak for the solvent, which may interfere
with the compound of interest.
2. DMSO :
Compounds like DMSO have benefits to use as an NMR solvent as they are relatively inexpensive and can
solubilize many compounds that are difficult to solubilize.

10. Relaxation Process In
NMR
 Relaxation is the process by which the spins in the
sample come to equilibrium with the surroundings.
 The rate of relaxation determines how fast an
experiment can be repeated. The rate of relaxation
is influenced by the physical properties of the
molecule and the sample.

11.  An understanding of relaxation processes is important for the proper measurement and interpretation of
NMR spectra.
 There are three important considerations.
• 1. The very small energy difference between α and β states of a nuclear spin orientation in a magnetic
field results in a very small excess population of nuclei in the ground vs the excited states. For many
nuclei, relaxation is a very slow process. It is thus very easy to saturate an NMR transitions, with the
resultant loss in signal quality, and failure to obtain correct peak areas.
• 2. NMR lines are extraordinarily sharp, and close compared to higher energy spectroscopic methods.
When relaxation is very fast, NMR lines are broad, J-coupling may not be resolved or the signal may
even be difficult or impossible to detect.
• 3. The success of many multipulse experiments, especially 2D and 3D spectra, depends crucially on
proper consideration of relaxation times.

12. NMR Relaxation is of two types :
1. Spin-Lattice or Longitudinal Relaxation.
2. Spin-spin or Transverse Relaxation.

13. • Relaxation process occurs along z-axis.
• Transfer of the energy to the lattice or the solvent material.
• Coupling of the nuclei magnetic field with the magnetic field of the ensemble of the vibrational and rotational
motion of the lattice or the solvent.
• Results in a minimal temperature increase in sample.
• Relaxation time (T1) → Exponential decay.
o Spin-Lattice or Longitudinal Relaxation

14. • Relaxation process in the X-Y plane.
• Exchange of energy between excited nucleus and low energy state nucleus.
• Randomization of spins or magnetic moment in X-Y plane.
• Related to NMR peak line-width.
• Relaxation time T2.
• T2 may be equal to T1.
• No energy change Mx = My = M0 [1- e(- t/T2]
o Spin-spin or Transverse Relaxation

15.  The shift in the position of the NMR region resulting from the shielding and deshielding by electrons is called
chemical shift.
 When a proton is present inside the magnetic field more close to an electro positive atom more applied magnetic
field is required to cause excitation. This effect is called shielding effect.
 When a proton is present outside the magnetic field close to a electronegative atom less applied magnetic field is
required to cause excitation . This effect is called deshielding effect.
 Greater the electron density around the proton greater will be the induced secondary magnetic field [ local
diamagnetic effect].
 Currents induced by fixed magnetic field result in secondary fields which can either enhance or decrease the field
to given a proton responds.

16. • Under the influence of the magnetic field electrons bonding the protons tends to process
around the nucleus in a plain perpendicular to magnetic field
• The position of the peaks in an NMR spectrum relative to the reference peak is expressed in terms of the
chemical shift
ᵟ = H0(reference) - H0(sample) X 106PPM
H0(reference)

17. • The value of H0 for the reference is usually greater than H0 for the sample, so subtraction in the
direction indicated gives a positive ᵟ.In terms of frequency unit ᵟ takes the form
ᵟ = ѵ(sample)- ѵ(reference) X 106ppm
ѵ(reference)
• Chemical shift is dimension less and expressed in parts per million (ppm ).
• Alternative system used is tau scale.
• τ=10-ᵟ scale the position of TMS signal is taken as 0.0 ppm
• Most chemical shifts value ranges from 0-10.
• A small value in tau represents low field absorption.
• High value represents high field absorption.
• Greater the deshielding of protons larger the value of delta.

18. Reasons for Chemical Shift
 Positive shielding: Resonance position moves upfield.
 Negative shielding: Resonance position moves downfield.

19.  In order to measure the magnitude of chemical shifts of
different kinds of protons,
 There must be some standard signal .
 0.5%Tetra methylsilane (TMS)(ch3)4si is used as reference or
standard compound.
 Chemical shift is represented by δ.
 Δ scale: 0 to 10 scale.
 TMS is taken as zero markers.
 Dimensionless expression; negative for most protons
Measurement of
Chemical Shift

20.  Accepted internal standard.
 TMS has 12 equivalent protons and gives an intense single
signal.
 Electro negativity of silicon is very low so the shielding of
equivalent protons in TMS is more than other compound so all
the signal arrives in a down field direction.
 Chemically inert
 Low boiling point
 So it can be easily removed by evaporation after the spectrum
has been recorded
 So the sample can easily recovered.
 TMS is not suitable in aqueous solution so DSS ( 2,2- dimethyl-
2silapentane-5 sulphate) used as reference.
 Protons in the methyl group of DSS gives a strong line.
TMS – As Internal
Standard

21. Factors Affecting
Chemical Shift
 1. Electronegativity and Inductive effect
 2. Anisotropic effect
 3. Vander waals Deshielding
 4. Hydrogen Bonding

22. 1. Electronegativity and Inductive effect.
 The proton is said to be deshielded if its attached with an electronegative atom/group. Greater the electro
negativity of atom greater is the deshielding caused to proton. If the deshielding is more, then δ value also more.
 Electronegative atoms like Halogens Oxygen and Nitrogen deshield the protons. There for the absorption occurs
downfield.
 The deshielding is directly proportional to the halogens oxygen or nitrogen.
 +I effect : An electron releasing group increase the electron density around the proton and give rise to its
shielding.
 -I effect : An electron withdrawing group is able to reduce electron density around the proton and Deshields the
proton.

23. 2. Anisotropic Effect (Space Effect)
 In this Shielding and deshielding can be determined by location of proton in space.
 It is also called as Space effect.
 Downfield or paramagnetic shift of protons attached to C = C, aldehydic or aromatic, proton is experienced by
the molecular magnetic field induced by an action of applied field Ho on pi electrons, this magnetic field
induced by pi electrons are directional or unsymmetrical and this directional measurement is called
ANISOTROPY.

24.  Anisotropic effect in Alkanes
o Alkanes do not possess the same degree of electron circulation as alkynes but do not exert nonlocal fields on
adjacent nuclei.
o The c-c σ bond shields a proton on its side more than its end.

25.  Anisotropy in Alkynes
• In a magnetic field, the pi-electrons of a carbon-carbon triple bond are induced to circulate, but in this case the
induced magnetic field opposes the applied magnetic field (B॰).
• Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is
shielded and the absorption is upfield.

26.  Anisotropy In Benzene
• In a magnetic field, the six electrons in a benzene ring circulate around the ring creating a ring current.
• The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity
of the protons.
• The protons feel a stronger magnetic field and thus are deshielded. A higher frequency is needed for
resonance.

27. 3. Vanderwaals Deshielding
 The electron cloud of a bulkier group will tend to repel the electron cloud surrounding the proton.
 Thus such a proton will be deshielded will resonate at slightly higher value of δ than expected in the absence
of this effect.
 The presence of bulky groups in a molecule cause deshielding due to weak VanderWaals force and give higher
δ value.

28. 4. Hydrogen Bonding
 Intra-molecular hydrogen bonding does not show any change in absorption due to change in concentration.
 While hydrogen atom involved in the intermolecular H-bonding shares its electrons with two electronegative
elements and as a result it itself deshielded and get higher δ value.
• E.g. Carboxylic acid dimer and β-diketones.

29.  NMR Signal in Various Compounds
 The number of NMR signals represent the number of protons in a molecule.

30. • For example,
let’s start with the simplest hydrocarbon; how many signals would you expect to see on the NMR
spectrum of methane?
Even though methane has four protons, they are all connected to the same atoms and have the same
neighbors on all sides – in other words, they are equivalent because they are in the same
environment.
Remember, equivalent protons give one NMR signal:

31. • It is the same with ethane (CH3-CH3); six protons – all equivalent, therefore one NMR signal:

32. For example:
• Bromoethane gives two NMR signals because the protons of the CH2 groups, being closer to the
bromine, are different from those in the CH3 group:

33. • Propane and butane give two signals.
• One because the protons of the CH2 group are different from those in the CH3 group, and the other,
because despite having four carbon atoms, the molecule is a combination of two identical CH2 and CH3
groups:

34. • Butane also gives 2 NMR signals.
• Because protons a are different from protons b. Each type gives one NMR signal.

35.  Introduction to spectroscopy-5th edition- NMR Spectroscopy
by Donald Pavia, Pg no: 215 – 262.
 Spectrometric Identification of Organic Compounds-6th
Edition-- Robert Silverstein, Pg No: 222 - 245.
 www.slideshare.net/chemicalshift
 https://www.sciencedirect.com/science/nuclear-magnetic-
resonance
 http://chem.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html
 Images – Internet search
REFERENC
ES

36. THANK YOU