Methyl group internal rotation dynamics: Overtone study of gaseous methylpyridine-2-alpha d(2) and-3-alpha d(2)

Conventional infrared absorption and Raman spectroscopy have been used to r ecord the vapor phase spectra of methylpyridine-2-alphad(2),6-d(1), methylp yridine-2-alphad(2), and methylpyridine-3-alphad(2),-d(4) in the Delta upsi lon (CH) = 1-4 regions. The spectra are analyzed with a theoretical model t hat takes into account, in the adiabatic approximation, the coupling betwee n the internal rotation of the methyl group and the methyl CH stretching vi bration. The principal parameters used in this model have been determined b y ab initio calculations at the HF/6-31G** level of theory. A good agreemen t between experimental and calculated spectra is found. This indicates that this coupling is at the origin of the majority of the observed spectral pr ofiles. A comparison of these results with those previously obtained for si milar methylated molecules reveals that the change in type and size of the barrier to internal methyl rotation is at the origin of significant spectra l differences. These changes are particularly important for methylpyridine- 2-alphad(2), revealing that the methyl group experiences increasingly diffe rent internal dynamics with increasing energy. These spectral changes can b e well explained by the deformation of the effective internal rotation pote ntial in the vibrational excited states. The overtone spectra of the aryl C D stretching of methylpyridine-3-alphad(2),-d(4) have also been studied.