Marie-Thèrèse Forel, Nelson Fuson, and Marie-Louise Josien, "Infrared Band Assignments for the 2900 cm−1 Region Methyl Group Vibrations in XC6H4COOCH3, XC6H4CH3, and XC6H4COCH3 Molecules," J. Opt. Soc. Am. 50, 1228-1231 (1960)
The spectra of monomethyl aromatic compounds in which the methyl group is attached either directly to the aromatic ring or to an oxygen or a carbonyl group carbon on a sidechain exhibit a 150 cm−1 wide absorption region centering around 2920 cm−1. This region consistantly contains four bands, which have been assigned, in decreasing wave number order, to the following methyl group vibrations: the totally asymmetric stretch; the axially asymmetric stretch which, however, preserves its symmetry with respect to the molecule’s symmetry plane; the totally symmetric stretch; and the second harmonic of the asymmetric deformation. The exact position of this absorption region depends upon the point of attachment of the methyl group. Within certain series of homologous molecules, such as the para substituted methyl benzoates, the position of the region is also noticeably sensitive to the nature of the substituent.
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Wave numbers ν (cm−1) and molar extinction coefficients ∊ν, (liter mole−1 cm−1) of absorption bands assigned to the methyl group vibrations of monosubstituted methyl benzoates X−C6H4COOCH3.
The absorption frequencies for this compound are taken from those given in reference 12 after correcting for a small calibration error.
The superposition of the chelated OH stretching vibration band did not permit measuring the extinction coefficients of this compound.
Weak bands partially overlapped; ∊ν difficult to evaluate in several cases.
Table II
Wave numbers ν(cm−1) and molar extinction coefficients ∊ν (liter mole−1 cm−1) of bands assigned to the methyl group vibrations of monosubstituted toluenes X−C6H4CH3.a
Omitted from this table are several weak bands which are not consistently present in the spectra of all members of the series and which were, therefore, not assigned to the methyl group.
See reference 19.
Table III
Wave numbers ν (cm−1) and molar extinction coefficients ∊ν (liter mole−1 cm−1) of absorption bands assigned to the methyl group vibrations of acteone CH3COCH3, acetophenone C6H5COCH3, and para substituted acetophenones XC6H4COCH3.
νa″
νs″
νs
2δa
δa+δs
2δs
ν
∊ν
ν
∊ν
ν
∊ν
ν
∊ν
ν
ν
CH3COCH3
3005
15
2961
12
2920
8
2847
2
2778
⋯
C6H5COCH3
3003
18
2964
10
2920
5
2859
4
2781
2704
XC6H4COCH3
X=NH2
3004
22
2963
12
2920
18
2850
4
2773
⋯
F
3004
11
2965
6
2921
4
2859
2
2795
2693
Cl
3005
10
2963
6
2920
3
2845
2
2775
2691
Br
3004
12
2961
7
2918
4
2842
1
2767
⋯
I
3005
11
2964
8
2920
6
2845
3
2765
2690
Average values and ranges
3004±1
2963±2
2920±2
2850±9
2780±15
2697±7
Tables (3)
Table I
Wave numbers ν (cm−1) and molar extinction coefficients ∊ν, (liter mole−1 cm−1) of absorption bands assigned to the methyl group vibrations of monosubstituted methyl benzoates X−C6H4COOCH3.
The absorption frequencies for this compound are taken from those given in reference 12 after correcting for a small calibration error.
The superposition of the chelated OH stretching vibration band did not permit measuring the extinction coefficients of this compound.
Weak bands partially overlapped; ∊ν difficult to evaluate in several cases.
Table II
Wave numbers ν(cm−1) and molar extinction coefficients ∊ν (liter mole−1 cm−1) of bands assigned to the methyl group vibrations of monosubstituted toluenes X−C6H4CH3.a
Omitted from this table are several weak bands which are not consistently present in the spectra of all members of the series and which were, therefore, not assigned to the methyl group.
See reference 19.
Table III
Wave numbers ν (cm−1) and molar extinction coefficients ∊ν (liter mole−1 cm−1) of absorption bands assigned to the methyl group vibrations of acteone CH3COCH3, acetophenone C6H5COCH3, and para substituted acetophenones XC6H4COCH3.