Knowledge Bank

Recently 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.