Overtone investigation of methyl-substituted pyridines

Conventional near-infrared absorption spectroscopy and intracavity laser ph otoacoustic spectroscopy have been used to record the room temperature vapo r phase overtone spectra of 4-methylpyridine-d(0), 3-methylpyridine-d(0), a nd 2-methylpyridine-d(0) in the Delta v(CH) = 2-6 regions. These methods ar e also used to record the hydrogen impurity spectra of 4-methylpyridine-d(7 ) and 3-methylpyridine-d(7) in the Delta v(CH) = 2-5 regions. Oscillator st rengths are calculated using an anharmonic oscillator local mode model and ab initio dipole moment functions. Ab initio geometries and intensity calcu lations, which contain no adjustable parameters, are used to assign spectra l peaks in the aryl regions. Peaks corresponding to two nonequivalent aryl local modes are assigned in the spectrum of 4-merhylpyridine. Peaks corresp onding to four nonequivalent aryl local modes are assigned in the spectra o f 3-methylpyridine and 2-methylpyridine. The effects of a change in barrier type and size to internal methyl rotation on the complicated methyl band p rofile are examined. The aryl regions of the hydrogen impurity spectra are similar to the undeuterated compounds, but a significant change is observed in the methyl regions.