Spectroscopic methods in inorganic chemistry 2019 IR

Spectroscopic Methods in
Inorganic Chemistry
Dr.Chris
UP August 2019
IR Spectroscopy
1
Interactions light - molecules
We need to understand the different interactions
dependent on the wavelenght or energy of light:
① ② ③ ④ ⑤
Whatarethespectroscopic
methods1-4?
2
Animation
https://www.youtube.com/watch?v=0S_bt3JI150
3
Polarity and Dipole moments
Dipole moment can be calculated as the product of
the charge (abbreviated Q) times the distance
(abbreviated r) between the charges.
4
Printed with FinePrint trial version - purchase at www.fineprint.com

Why do CO2 and CCl4 do not have a dipole moment ?
5
Absorption of energy
uv/vis
IR
6
Raman vs. IR
7
WHAT INFLUENCES THE WAVENUMBER
OF A BOND VIBRATION ?
8

(1) Bond energy
https://www.youtube.com/watch?v=9HfJNnoRMPA
9
(2) Multiple bonds
10
(3) Atom weights and hybridization
11
(4) Types of motions
Stretch:
symmetric asymmetric
wagging twisting scissoring rocking
Bending:
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Infrared_
Spectroscopy/Infrared%3A_Theory
12

Which modes are stretching and bending ?
13
Examples
Identify
stretching and
bending
modes !
14
15
https://webbook.nist.gov/chemistry/name-ser/
16
Zinc Oxide
Zinc Hydroxide
Additional O-H stretches

17
Exercises: FTIR of ZnO NP
https://www.sciencedirect.com/science/article/pii/S0001868617301197
ZnO NP Preparation 1 – sol-gel method
J. of NanoStructure 5(2015) 395-401
18
Firstly, 10 ml of aqueous 1 M citric acid solution and 50 ml
of aqueous 0.1 M zinc acetate solution, prepared using
deionized water. The solution is kept in an ultrasonic bath
at room temperature for 2 hours to obtain a
homogeneous solution. The obtained solution were
heated using a hot plate on 60°C with a magnetic stirring
until forming the gel. Then the temperature was
increased to 180°C until preparation a dry gel. Finally, the
precursors were calcinated in an oven at 400, 500 and
600 οC for 4 hours.
What reactions take place here ?
19
UV spectrum of ZnO NP – what is the band gap energy ?
20
FTIR of sample 500 C calcinated
Which impurities can we detect ?

ZnO NP preparation 2 – MW combustion
J. of Chemistry 2013, 1-4
21 22
What can we say about the purity of the product ?
ZnO NP preparation 3 - by micro-emulsion
Intl Letters of Chemistry, Physics and Astronomy, 2013, 26-36
23
The stable reverse micelle microemulsion was prepared by mixing
a non-ionic surfactant, Triton X-100 [(C14H22O(C2H4O)n] Polyvinyl
pyrollidone (PVP) and 1:9 ratio of cyclohexane
and triple distilled water. The microemulsion was mixed rapidly
with continuous stirring for five minutes. ZnSO4·6H2O solution
(0.5 M) was added drop by drop to microemulsion with
continuous stirring. A sky blue color mixture was obtained. PVP
was used as a stabilizing agent. After half an hour of equilibration,
2.0 M hydrazine hydrate solution was added drop by drop with
continuous stirring at room temperature. The reverse micelles
were broken by adding THF. ZnO nanoparticles were subsequently
washed with ethanol and triple distilled water to remove residual
PVP and surfactant molecules. After washing ZnO nanoparticles
were dried in oven at 100.0 °C for 48 hours.
24
Addition of N2H4·H2O to the aqueous solutions of zinc sulphate
heptahydrate results to production of white precipitates of Zn
nanoparticles inside the miceller core. PVP act as stabilizer for
these Zn nanoparticles. The surfactant and PVP molecules
adhere to the surface of nanoparticles which serve as a
protective layer to prevent the further reaction. The Zn
nanoparticles are oxidized into ZnO nanoparticles in the
presence of atmospheric oxygen at 100 °C.
The reaction profile of formation ZnO nanoparticles can be
illustrated as:
(1) ZnSO4(aq) + N2H4 + 2H2O(l) → 2Zn(s) + 2(N2H5)SO4(aq) +
O2
(2) 2Zn(s) + O2 → 2ZnO(s)

25
ZnO NP preparation 4 – “green route”
Journal of Macromolecular Science, Part A: Pure and
Applied Chemistry (2014) 51, 941–947
26
A solution of zinc acetate (0.1 mol) and ammonium carbonate
(0.1 mol) in 100 mL of deionized water was added to a 10%
aqueous solution of Gum tragacanth (10 g Gum tragacanth in
100 mL deionized water) at room temperature. The resulting
solution was stirred at room temperature for 24 h. The ZnO
nanoparticles thus formed bound with Gum tragacanth which
was later precipitated by the addition of methanol, which also
facilitated the removal of ammonium acetate formed as a
byproduct of the reaction. The obtained precipitate was filtered,
washed with methanol several times, dried under vacuum for 4
h and then calcinated in a furnace at 600 C for 3 h to complete
removal of the remnant Gum tragacanth, which gave pure ZnO
nanoparticles.
27
ZnO NP preparation 5 – “green route”
Open Journal of Synthesis Theory and Applications,
2016, 5, 1-14
28

29 30
Preparation (1)
precipitation method
31
The aqueous solution was prepared by mixing zinc nitrate hexahydrate
and sodium hydroxide aqueous solutions. In a typical procedure, 2.28 g
of zinc nitrate hexahydrate was dissolved in 75 ml of deionized water
and then, 0.6 g of NaOH in 150 ml of deionized water was added
dropwise under magnetic stirring.
After the addition was completed, the stirring was continued for 30 min
and then cooled with cold water.
The precipitates were filtered and washed by pure water several times.
Then the obtained precipitates were dried at 60◦C for 24 h and
calcinated at 200◦C for 2 h.
https://www.ias.ac.in/article/fulltext/boms/038/04/1033-1038
Lab method
32
Prepare 2 solutions:
• 5 mmol Zn(Oac)2 = 1.10 g in 50 ml water
• 10 mmol NaOH = 0.4 g in 100 ml water
Add the NaOH sol. slowly to the Zinc acetate solution
under stirring. Continue stirring for 10 minutes.
Filtrate the mix over Buchner filter and dry in oven.
.

Characterization
33
Figure 2 shows the FTIR spectra of the
synthesized ZnO nanoparticles in the
range of 4000–400 cm−1.
The broadband at 3504 cm−1 is the
stretching vibration of O–H group.
The peak at 1386 cm−1 is due to the O–
H bending of water.
The peak at 447 cm−1 is a ributed to
the Zn–O stretching of vibration
Preparation (2)
sol-gel method
34
Zinc Oxide nanostructure was synthesized by using sol-gel method. In order to
prepare a sol, 2 g of Zinc Acetate Dihydrate and 8 g of Sodium Hydroxide were
weighted using a weighting balance. Then, 10 ml and 15 ml of distilled
water were measured by a measuring cylinder. After that, 2 g of zinc acetate
dihydrate was dissolved with a 15 ml of distilled water and 8 g of sodium
hydroxide was dissolved in a 10 ml of distilled water. The solutions were stirred
with a constant stirring for about five minutes each. After well mixed, sodium
hydroxide solution was poured to the solution containing zinc acetate with a
constant stirring by magnetic stirrer for about five minutes. Then, a burette
was filled with 100 ml of ethanol and tilt rate dropwise to the solution
containing both sodium hydroxide solution and zinc acetate.
After the reaction, white precipitate was formed..
https://core.ac.uk/download/pdf/82238669.pdf
Lab Method
35
Prepare 2 solutions:
• 1.0 g of Zn(OAc)2 in 7 ml H2O
• 4.0 g NaOH in 5 ml H2O
Add NaOH sol. to the Zn acetate solution slowly under stirring
Add 100 ml EtOH denat. slowly under stirring.
Let the mix stand for about 30 minutes.
ZnO should sediment out.
Decantate and disperse the precipitation in water by US.
Let product sediment and decantate -> collect and dry in oven
Preparation (3)
solid-state method
36
https://link.springer.com/article/10.1007/s00339-008-4533-z

Lab method
37
Weight 5 mmol Zn(Oac)2 = 1.10 g into mortar.
Grind it to a fine powder..
Add 10 mmol NaOH = 0.8 g into the mortar, add some drops i-
Propanol.
Grind the mix for about 20 minutes.
Wash the mix with water into a beaker, let sediment and
decantate. Put water again and decantate again, then wash
with Ethanol. Put the sediment on a petridish and let dry in
the oven.
IR / Raman Simulation
http://chemtube3d.com/Organic%20Structures%20and%20Bonding.html
38
IR Spectrum of “Rennie”
Example: identify products in Antacids
http://www.ptfarm.pl/pub/File/Acta_Poloniae/2000/2/083.pdf
(1) Carbonate Compound
39
Ref. spectra
Mg CO3
Ca CO3
O-HCa-O
Mg-O
40

(2) Hydroxy Compounds
IR of “Maalox (an)”
IR of “Alusal”
41
Ref. spectra
Al(OH)3
Mg(OH)2
Al(OH)3
Mg(OH)2
42
ORGANO-METAL COMPOUNDS
M(CO)X
43
CO bonding modes
https://books.google.co.th/books?id=oZeFG6QDNekC&pg=PA382&lpg=PA382&dq=M(CO)2L4&sourc
e=bl&ots=u9uyncbsDi&sig=Qr5CRFxT1cPpud5vnjs5PWgkkzc&hl=en&sa=X&ved=0ahUKEwjp84-
n5eXKAhVSkY4KHQ5QAiUQ6AEIIDAC#v=onepage&q&f=false 44

45
https://www.youtube.com/watch?v=_nZeXNpMYBo
46
The C–O stretching wavenumbers are shifted to lower values when
there are changes in the extent of back-bonding in the compound.
Removing positive charge from the metal causes the shift of
electrons from the metal to the CO π orbitals causes the CO
wavenumber values to decrease.
The highest excess of negative charge on the metal occurs in the
[V(CO)6 ]− complex and so more back-bonding occurs than in the
other complexes.
The next highest excess of electron density is in Cr(CO)6 , and then
[Mn(CO)6 ]+.
47
Electron density on the metal
Higher C-O
strength
Lower C-O strength
M=C=O character
48

Ligand donation effects
49
CO Substitution patterns
We compare cis- and trans-ML2(CO)2 complexes in IR:
What are the point groups ?
50
Tetrahedral Td Octahedral Oh
Linear: D∞h for A-B-A ( i )
C ∞h for A-B
http://en.wikibooks.org/wiki/Introduction_to_Mathematical_Physics/N_body_problem_in_quantum_mechanics/Molecules
51
Character Tables for cis and trans
52

Representations of 2 C-O groups
Which contains the irreducible representations :
Which contains the irreducible representations :
Conclusion: Number of IR peaks for cis and trans complex:
53 54
How do you distinguish whether the structure of
transition metal complex molecule
M(CO)4L2 is cis or trans by inspection of the CO
stretching region of the IR spectra?
-> determine the symmetry group:
M(CO)4L2
55
-> Check the character tables:
c2v
d4h
56

Reducing stretching motions
4 stretching vectors
4 0 0 2
4 0 0 0 0 0 0 4 2 0
57 58
Experimental
59
Metal-carbonyl compounds
60

Exercise
61
How many IR peaks do we expect from these
ML(CO)3 compounds ?
Step 1: determine the point groups of each molecule
Step 2: Get the characters for the 3 stretching vectors
in the character table of this point group
Step 3: find the irreducible representations that are included in
this rep -> how many have x, y or z ?
Tetrahedral Td Octahedral Oh
Linear: D∞h for A-B-A ( i )
C ∞h for A-B
http://en.wikibooks.org/wiki/Introduction_to_Mathematical_Physics/N_body_problem_in_quantum_mechanics/Molecules
62
63 64

65
M(II)hexamine complexes
N-HN-H
stretch
N-HN-H
bend
M-NH3M-NH3
bend
M-NH3M-NH3
rock
66Which compound has the weakest/strongest M-N bond ?
The spectra presented in Figure 5.6 show a trend in
the wavenumber shifts for the three hexamine
complexes; the N–H bands shift to lower wavenumbers
from Co to Cr to Ni.
This indicates that the N–H bond order (bond strength)
decreases as the metal–N bond order increases in the
stability order mentioned
67
ATR Method – attenuated total reflection
68

RAMAN
69
https://www.youtube.com/watch?v=yQ1MctWU9Mg
(https://www.youtube.com/watch?v=TMLnUmbLwUI)
70
3 possible transitions
71
UV/VIS
Induced dipoles
https://www.youtube.com/watch?v=1_IqMY6t6w0
Elastic scattering
Rayleigh
(https://www.youtube.com/watch?v=1Q45PpodjJY)
72

73 74
• no sample preparation
• non-destructive
• can be used with a microscope
75 76

77 78
Figliola, et. al. Organometallics 2014, 33, 4449
Compound 1
A = S
2061
2021
1976
1955
1878
Compound 2
A = Se
2054
2014
1970
1950
1875
We should decide which structure is more likely
based on IR:
5 peaks, higher frequencies for A = Sulfur
① or ② ?
79
Find the point group of both molecules:
①
②
(Practise with:
http://symmetry.otterbein.edu/challenge)
80

C2v
E C (z) (xz) (yz)
Linear f,
rotations
Quadratic
f
A1 1 1 1 1 z x, y, z
A2 1 1 -1 -1 R xy
B1 1 -1 1 -1 x, R xz
B2 1 -1 -1 1 y, R yz
 8 0 4 0
Molecule ① : 8 CO groups
Reduce  ¼ (8x1x1 + 0x1x1 + 4x1x1 + 0x1x1) = 3 A1
¼ (8x1x1 + 0x1x1 + 4x-1x1 + 0x-1x1) = 1 A2
¼ (8x1x1 + 0x-1x1 + 4x1x1 + 0x-1x1) = 3 B1
¼ (8x1x1 + 0x-1x1 + 4x-1x1 + 0x1x1) = 1 B2
The A2 can be ignored since it does not contain
x, y or z and is therefore not IR active.
This gives 7 IR active CO vibrations.
81
C2v
E C (z) (xz) (yz)
 6 0 2 0
Molecule ②: also Point Group C2v
Reduce  ¼ (6x1x1 + 0x1x1 + 2x1x1 + 0x1x1) = 2 A1
¼ (6x1x1 + 0x1x1 + 2x-1x1 + 0x-1x1) = 1 A2
¼ (6x1x1 + 0x-1x1 + 2x1x1 + 0x-1x1) = 2 B1
¼ (6x1x1 + 0x-1x1 + 2x-1x1 + 0x1x1) = 1 B2
The A2 can be ignored since it does not contain
x, y or z and is therefore not IR active.
This gives 5 IR active CO vibrations.
A1 1 1 1 1 z x, y, z
A2 1 1 -1 -1 R xy
B1 1 -1 1 -1 x, R xz
B2 1 -1 -1 1 y, R yz
82
Fe(CN)6 complexes
83
How can we explain the difference to Fe(III) ?
84

Problem solving
85 86
87

Spectroscopic methods in inorganic chemistry 2019 IR

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