PHOTOACOUSTIC SPECTROSCOPY OF NON-EQUIVALENT METHYL C-H OVERTONE VIBRATIONS

Author Institution: The Standard Oil Company (Ohio) Corporate, Research CenterRecently considerable interest has been focussed on the success of the local mode model for describing the highly excited vibrational overtones of molecules containing X-H bonds (X=C,O,N). The development of sensitive photothermal techniques has significantly enhanced the amount of available information regarding the overtone structures in the near-IR and visible spectral region. In this report, we present the observation of non-equivalent methyl C-H bonds in four types of anisotropic environments: (a) equatorial-axial C-H bonds (such as in iso-butane and t-butanol), (b) the adjacent lone-pair electron moiety (such as in dimethyl ether, dimethyl sulfide and methyl amines), (c) the adjacent C=C $\pi$-electron moiety (such as in propylene an 2-alkenes), and (d) the adjacent C=O $\pi$-electron (such as in acetaldehyde and acetone). In all cases, two distinct C-H stretching overtone bands are observed which correspond to two types of C-H bonds in the methyl group. A MO analysis which will be described in the next paper suggests that the lone-pair or $\pi$-orbital delocalization is responsible for the appearance of two methyl C-H bands at each overtone even if the methyl group is freely rotating