This document provides an overview of spectroscopic methods in inorganic chemistry, focusing on infrared (IR) spectroscopy. It discusses key concepts like dipole moments, factors that influence bond vibrational frequencies, and using symmetry to determine vibration modes. Specific applications covered include identifying functional groups, distinguishing between cis and trans transition metal carbonyl complexes using IR spectra, and analyzing metal-ligand vibrations in complexes. The document also lists several online resources for IR spectral simulation, prediction, and reference databases.

1. Spectroscopic Methods in
Inorganic Chemistry
Dr.Chris
UP Feb 2016

2. Overview

3. (I) IR SPECTROSCOPY

4. Animation
https://www.youtube.com/watch?v=3RqEIr8NtMI

5. 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.

6. The dipole moment of HCl is 1.03 D, and the bond length is
127 pm. What is the percent ionic character of the HCl bond?
First we will assume that this molecule is 100% ionic. In this
case, the charges are separated by the bond length, and we
can calculate the dipole moment in this extreme case.
The actual dipole moment measured for this molecule is
1.03 D, so the molecule is not completely ionic.
http://wps.prenhall.com/wps/media/objects/4678/4791085/ch10_01.htm

7. Why do CO2 and CCl4 do not have a dipole moment ?

9. AM1 calculation
“ArgusLab” Freeware

10. Absorption of energy

11. 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. WHAT INFLUENCES THE
WAVENUMBER OF A BOND ?

13. (1) Bond energy
https://www.youtube.com/watch?v=9HfJNnoRMPA

14. (2) Multiple bonds

15. (3) Atom weights

16. VIBRATION MODES FROM
SYMMETRY

17. Normal modes of vibration
Triatomic
molecules

18. Character tables – c2v

19. Vibration modes
https://www.youtube.com/watch?v=QnnAInt4mlM

20. Examples
bendingstretch stretch

21. Which peaks belong
to stretching or
bending ?

22. Planar BF3 molecules
How many degrees of freedom ?
(bond vibrations without rotation and translation)
1
2

23. 3
4

24. 5

25. Answers
1
2

26. 3

27. 4
5

28. Final result for BF3

29. E’ A2’’ E’

30. Examples
Identify
stretching and
bending
modes !

31. Unknown anion

32. IR / Raman Simulation
www.molwave.com

33. ORGANO-METAL COMPOUNDS
M(CO)X

34. 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

35. The C–O stretching wavenumbers are shifted to lower values when
there are changes in the extent of backbonding 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
backbonding occurs than in the other complexes. The next highest
excess of electron density is in Cr(CO)6 , and then [Mn(CO)6 ]+.

36. Character of the M-CO bond
Slightly
antibonding HOMO

37. Electron density on the metal
Higher C-O
strength
Lower C-O strength
M=C=O character

38. Ligand donation effects

39. CO IR vibrations
We compare cis- and trans-ML2(CO)2 complexes in IR:
What are the point groups ?
39

40. 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
40

41. Character Tables for cis and trans
41

42. 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:
42

43. Metal-carbonyl compounds

44. M(II)hexamine complexes
N-H
stretch
N-H
bend
M-NH3
bend
M-NH3
rock

45. 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

46. FUNCTIONAL GROUPS

47. Find functional groups
https://www.youtube.com/watch?v=ItW6Mj2CQKc

48. https://www.youtube.com/watch?v=9HfJNnoRMPA

51. Inorganic Compounds

52. Spectra Database:
sdbs.db.aist.go.jp

54. http://www.chemicalbook.com/SpectrumEN_144-55-8_IR1.htm

55. Fe(CN)6 complexes

56. How can we explain the difference to Fe(III) ?

58. Raman

59. IR Spectrum of “Rennie”
Example: identify products in Antacids
http://www.ptfarm.pl/pub/File/Acta_Poloniae/2000/2/083.pdf
(1) Carbonate Compound

60. Ref. spectra
Mg CO3
Ca CO3
O-HCa-O
Mg-O

61. (2) Hydroxy Compounds
IR of “Maalox (an)”
IR of “Alusal”

62. Ref. spectra
Al(OH)3
Mg(OH)2
Al(OH)3
Mg(OH)2

63. RAMAN

64. https://www.youtube.com/watch?v=TMLnUmbLwUI

68. SUPPLEMENTS

69. 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:
CO substitution pattern

70. -> Check the character tables:
c2v
d4h

71. Reducing stretching motions
4 stretching vectors
4 0 0 2
4 0 0 0 0 0 0 4 2 0

73. Experimental

74. ATR Method – attenuated total reflection

75. http://cnx.org/contents/uieDnVBC@20.16:EFE
BTsu4@2/Attenuated-Total-Reflectance-F