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I.
Factors Affecting Group Frequencies
A. Introduction
1. Examples of characteristic frequensies
a) Table 8-1, p 176
(1) Force constant double & triple for double and triple bonds
b) Hooke’s Law for 2 atoms
(1) When bonds interact, no simple model
(a) Multiple CHs
(b) Bending vs stretching
II. Factors Affecting Group Frequencies
A. Symmetry
1. Center of symmetry – all vibrations that are symmetric with respect to center are
inactive in IR – no change in dipole moment
a) IR needs an oscillating dipole
b) If molecule has center of symmetry & a vibration symmetric with center has
no oscillating dipole
c) But an antisymmetric vibration will show up
d) Symmetric modes active in Raman
(1) Mutual exclusion rule
e) Trans-1,2-dichloroethene
H
H
C
f)
C
C
Cl
Cl
Cl
H
C
H
Cl
(1) C=C stretch 1580 in Raman
(2) C-Cl stretch
(a) Symmetric 840 in Raman
(b) Antisymmetric 895 in IR
(3) C-H
(a) 1200 in IR
(b) 1270 in Raman
H
H
C
C
C
H
H
g) Propyne
(1) 2150 in both IR & Raman
H
H
C
H
H
C
h) 2-butyne
(1) 2150 only in Raman
C
C
H
H
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H
H
C
H
C
C
C
H
i)
benzene
(1)
(2)
(3)
(4)
(5)
(6)
C
C
H
H
3n-6 = 30
high symmetry – 10 pairs – identical frequencies
leaving 20 different normal frequencies
reduce number of modes for change in dipole moment (IR)
change in polarizability
4 fundamentals for IR, 6 for Raman
Cl
H
C
C
C
Cl
H
C
C
C
Cl
H
j)
1,3,5-trichlorobenzene
(1) symmetry reduced, number IR & Raman increase
k) 1-chloro-2-bromobenzene symmetry removed, some modes too
Cl
H
H
C
Br
C
C
C
C
C
H
H
weak
l) larger molecules may have local areas of symmetry
m) –CH3 3n- 6 = 6 normal modes of vibration
(1) symmetric & degenerate asymmetric stretching and bending
(2) attached to molecule – 3 new modes
(a) torsional & degenerate pair of rocking modes (rotation in
the free methyl group)
(b) Me part of molecule with lower symmetry – degeneracies
removed leading to observation of doublets in some regions of
spectrum where –CH3 expected
n) –CH2- local symmetry group that has 2-fold axis & 2 planes of symmetry
(1) free –CH2- 3 modes
(a) symmetric stretch
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(b) antisymmetric stretch
(c) bending or scissors
2.
Symmetry Elements
a) Twofold axis
b) Threefold axis
c) Plane of symmetry
d) Center of symmetry
3. Point group – collection of all symmetry elements that a molecule possesses
a) Classifying symmetry of molecule
4. Used to understand symmetry of normal vibrations of a molecule and predict number
of frequencies expected in IR and Raman
a) N-fold axis of symmetry Cn
b) Plane of symmetry
c) Center of inversion or center of symmetry
d) N-Fold Rotation – Reflection Axis of Symmetry Sn
B. Mechanical Coupling of Vibration
1. 2 free identical diatomic molecules – identical frequencies
2. 2 bonds share common atom – seldom independent
a) vibration of 1 displaces the other atoms
b) in-phase & out-of-phase combinations of 2 diatomic vibrations
c) if widely separated – little effect
3. carbon dioxide
a) symmetrical in-phase 1340 – Raman
b) asymmetrical out-of-phase 2350 IR
c) diketone separated by 2 or more C
(1) 1715 – stretch
(2) tautomerization & hydrogen bonding
4.
H
C
C
H
acetylene
a) 3375 – raman, in-phase
b) 3280 – IR, out-of-phase
H
5.
c) diacetylene 3330 & 3295
NH – 2 for primary amine and amide
H
C
C
H
O
C
C
C
N
C
O
H
C
C
H
H
6.
7.
H
O
C
C
H
8.
Allene 1960 IR, 1070 Raman
H
C
O
O
H
C
C
H
H
C
H
H
H
Carbonyl – 2 for anhydride & imide
C-H – 2 for methylene and methyl
H
C
H
H
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9. 1,3-butadiene 1640 IR, 1600 Raman
10. Vibrations of 2 different diatomic groups not coupled unless the uncoupled
frequencies are similar (from mass and force constant effects)
H
H
S
C
C
H
N
H
H
C
C
H
H
H
11. C=S of thioamides and xanthates
a) k=650 Nm-1 =8.72amu =1120
b) C-N k=480 Nm-1 =6.46amu =1120
c) C-O k=510 Nm-1 =6.86amu =1120
12. C-C-O coupled asymmetric vibration for alcohols
a) 1034 for MeOH, 1053 for EtOH
13. Coupling of stretching and bending
a) Secondary amines C-N stretch & N-H bend
b) Secondary acyclic amides 1563-1515 couple NH bend and CN stretch
14. Always coupling between C-Cs adjacent
a) Not linear like allene
b) Therefore several bands in IR and Raman 1200-800
15. Requirements for successful coupling
a) Vibrations same symmetry species
b) Strong coupling – common atom
c) Interaction greatest when coupled groups absorb, independently, near same
frequencies
d) Bend and stretch couple if stretch bond form 1 side of angle
e) Common bond coupling of bending vibrations
f) Coupling negligible if groups separated by 1 or more C and vibrations
mutually perpendicular
C. Fermi Resonance – coupling of fundamental vibrations with overtone or combination
1. 3n-6 fundamental vibrations i=1
2. overtone i=2,3,…
3. combinations more than 1I is nonzero
4. accidental degeneracy – energy of overtone or combination very close to fundamental
5. carbon dioxide Raman 1340 ->actually 1286 &1388
a) fundamental bend = 666, 1st overtone = 1334
6. Fermi Resonance requires vibration levels be of same symmetry species (most
organic compounds have no symmetries) & interacting groups be located in molecule so
that mechanical coupling appreciable
7. Cyclopentanone doublet – fermi resonance with overtone of alpha CH2
8. Overtone or combination band weak but when Fermi resonance occurs, sharing of
intensity – overtone can be quite strong
a) O=C-NH 2 bands 3300 and 3205 for polyamides
(1) NH stretch fundamental and overtone of NH deformation mode
(near 1550) -> 2 bands
b) CHO 2900 & 2700 CH deformation 1400 and CH stretch 2800
D. Hydrogen bonding
1. Proton donors –CO2H, OH, NH, CONH (X-H) donor group
2. Proton acceptors O, N, X, C=C
3. Max strength when proton donor group and axis of lone pair orbital are collinear
4. Strength inversely proportional to distance between X & Y
5. Alters force constant of both
a) –H stretch lower frequency, increased intensity and band widening
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b) acceptor stretch frequency reduced but to lesser degree
c) X-H bend – shorter wavelength, frequency raised
6. Intermolecular – decrease with decreasing concentration
a) RCO2H – monomer, dimer, oligomer
b) Polymer molecules (neat solution)
7. Intramolecular – 5 or 6 membered ring, concentration no effect
a) Intramolecular in 2-chloroethanol stabilizes gauche at 3597
(1) Anti (trans) at 3623
8. Both inter and intra temperature dependent
9. Change in frequency between “free” OH and bonded OH measure of strength of Hbonding
a) Ring strain, molecular geometry, relative acidity and basicity of proton
donor and acceptor groups affect strength of bonding
b) 6-membered ring – intra
c) stabilized by resonance
E. Ring Strain
1. Cyclohexanone 1714, cyclopentanone 1746, cyclobutanone 1783
a) Exocyclic strain increase, frequency increase
b) Mechanical interaction with adjacent co-planar C-C
2. Methylene-cyclohexane 1649, pentane 1656, butane 1677
3. Cyclohexene 1650, cyclopentene 1615, cyclobutene 1565
a) Endocyclic decrease ring angle, frequency decreases
4. P=O, S=O, SO2 are little affected – not coplanar with the 2 attached single bonds
F. Electronic Effects
1. Inductive and resonance effects
a) High C=O stretch inductive RCOCl 1800
b) Low C=O stretch delocalized structure RCONH2 1650
(1) Resonance
c) Conjugation of double bond – lower double bond character and increase
bond order of C-C
(1) C=O stretching frequency lowered by 20-30 cm-1for alpha-beta
unsaturated
(2) Isoprene 1637, 1604
(3) 1,4-pentadiene 1644
G. Constitutional (structural) Isomerism
1. C3H7O2N
a) Amino acid alanine
b) Urethane ethyl carbamate
c) 1-nitropropane
2. o, m, p substituted benzene
a) CH out-of-plane deformation 850-700
3. Substituted pyridines, pyrimidines & other heterocyclic compounds
H. Stereoisomerism
1. Optical isomers –enantiomers
a) Arrangement of atoms relative to each other the same
2. Diastereomers – geometric or cis-trans
a) Cis & trans-1,2-dichloroethene
(1) Spectra different
(2) Trans usually more symmetry, fewer absorptions
(3) Highly symmetrical modes always produce a change in
polarizability – Raman
I. Conformational Isomerism
1. Barriers to rapid interconversion
a) Gauche, trans (anti), eclipsed
b) 1,2-dichloroethane
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(1) (trans - book) anti 1290
(2) gauche 1235
2. Stabilizing interaction – eclipsed more stable
a) Alpha-halo ketones, esters, acid halides and amides
(1) 2 carbonyl stretch bands
(a) higher frequencies – eclipsed interaction
(b) other at normal frequencies
3. axial-equitorial conformations of cyclohexane and cyclopentane
a) alpha-chlorocylopentanone & hexanone
(1) 2 distinct carbonyl peaks
(a) 1745 for equitorial cyclohexanone
(b) 1725 for axial
(c) ratio dependent on phase, T, solvent (followed in IR)
spectra
b) cyclohexanols
4. ortho-halogenated benzoic acids – 2 carbonyls due to cis & trans with respect to X
and C=O
5. vinyl ethers 1640-1620 & 1620-1610
a) rotational isomers about C-O
J. Tautomerism
1. Structural Isomers readily interconverted
a) Beta-diketones
(1) Dicarbonyl doublet near 1730
(2) Enol lowered 100-80 due to hydrogen bonding
(a) The C=O & C=C may overlap
K. Chemical and Environmental Effects on Group Frequencies
1. Lower T – bands sharper, better resolution (especially for solids)
2. Polar solvents – significant shift of group frequencies
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