Anharmonicity of CH 3 deformation vibrations and Fermi resonance between the symmetrical CH 3 stretching mode and overtones of CH 3 deformation vibrations

Abstract

Experimental measurements in dilute CCl 4 solution have been made of the i.r. and Raman spectra of a range of compounds containing CH 3 X groups. In this paper attention is paid to the region of νCH 3 stretching fundamentals (3000–2700 cm −1) and the δCH 3 internal angle-bending fundamentals (1500–1200 cm −1), with particular reference to additional absorptions in the νCH 3 region which originate in overtone transitions of δCH 3 fundamentals. For the 2δ s overtones, graphical plots of (δ × 2 − 2δ)—where δ × 2 represents the calculated value of the overtone and 2δ the observed value—or of the intensity ratio I 2δ/I ν s , versus the frequency difference (ν 3 - 2δ) between the ν 3(CH 3) ‘fundamental’ and the 2δ ‘overtone’ absorptions, shows clearly that Fermi resonance occurs when high values of (δ × 2 − 2δ) are found. However, observed values for this anharmonicity parameter of from 10 to 20 cm −1 correspond to ‘normal’ anharmonicity with little contributions from Fermi resonance. By contrast, the 2δ a absorptions, although they in some cases have considerable intensity and often fall much closer to the position of the ‘ν s ’ absorption band, usually exhibit frequencies with ‘normal’ anharmonicity parameters in the range 10–25 cm −1. An apparent exception is the ‘2δ a ’ absorption of methyl fluoride, but this can be reassigned to a 2δ s absorption, because in this case δ a and δ s have very similar frequencies. In the case of the methyl halides comparison of the mean value for ν a (CH 3) (doubly-degenerate) and v s (CH 3) with the value observed for ν(CHD 2)—where Fermi resonance is absent—confirms that this phenomenon causes appreciable modification of the position of the ν s (CH 3) band only for CH 3F. Amongst other molecules studied, Fermi resonance between ν s and 2δ s is strong for the CH 3O groups of a range of esters and appreciable for CH 3·NCO, CH 3·NCS, methyl acetylene and toluene. More pronounced Fermi resonance effects occur for CH 3O and CH 3N groups in which O or N lone-pairs of electrons are not delocalised as they are in the esters.