Detailed features in the local mode overtone bands of ethane, neopentane, tetramethylbutane, and hexamethylbenzene

Abstract

The overtone spectra of ethane in the gas phase at room temperature and neopentane in the liquid phase at 0 °C are measured and analyzed in the region of the pure local mode overtones corresponding to Δv_CH=3, 4, and 5. Overtone spectra are also measured and analyzed for 2, 2, 3, 3‐tetramethylbutane in hexachlorobutadiene solution and for hexamethylbenzene in carbon tetrachloride solution, both at room temperature in the region of Δv_CH=3 and 4. For ethane, neopentane, and tetramethylbutane, the dominant contribution to the overtone band corresponds to a pure CH₃ local mode overtone. Combination peaks between different CH₃ local oscillators are characterized. Combination peaks are also identified involving a CH₃ local oscillator and a low frequency normal mode, with the transitions originating from both the ground vibrational state and a state containing one quantum of the low frequency mode. A computer decomposition of the overtone bands is carried out and a satisfactory fit is achieved in terms of Lorentzian peaks, independently of the degree of vibrational excitation or phase. Narrow bandwidths are observed for the pure local mode overtones of neopentane and the narrow linewidths are attributed to weak intermolecular interactions. Doublet structure in the pure local mode overtones of hexamethylbenzene is attributed to two types of configurationally inequivalent hydrogens.