τ-Clustering for molecules with large torsional splittings: the ν7+ν8 and 2ν8 combination and overtone bands of CH3OH

Line clustering into two groups of differing τ torsional symmetries is considered for parallel infrared vibrational bands of 3-fold internal rotor molecules with large torsional splittings, such as methanol. In bands involving large changes in torsional parameters, the τ-clustering produces a distinctive pair of Q-branch peaks in the low-resolution spectrum. The origin of the τ-clustering and the formation of the twin Q-branch peaks are discussed together with new subband assignments in Fourier transform spectra of the ν7+ν8 CH3-rock/CO-stretch combination and 2ν8 CO-stretching overtone bands of CH3OH, which are good representatives of large and small changes in torsional parameters, respectively.