Fourier transform spectroscopy of CH 3OH: rotation–torsion–vibration structure for the CH 3-rocking and OH-bending modes

Abstract

High-resolution Fourier transform spectra of CH 3OH have been investigated in the infrared region from 930 to 1450 cm −1 in order to map the torsion–rotation energy manifolds associated with the ν 7 in-plane CH 3 rock, the ν 11 out-of-plane CH 3 rock, and the ν 6 OH bend. Upper-state term values have been determined from the assigned spectral subbands, and have been fitted to power-series expansions to obtain substate origins and effective B-values for the three modes. The substate origins have been grouped into related families according to systematic trends observed in the torsion–vibration energy map, but there are substantial differences from the traditional torsional patterns. There appears to be significant torsion-mediated spectral mixing, and a variety of “forbidden” torsional combination subbands with |Δ υ t |>1 have been observed, where υ t denotes the torsional quantum number (equivalent to υ 12). For example, coupling of the ( υ 6, υ t )=(1,0) OH bend to nearby torsionally excited ( υ 7, υ t )=(1,1) CH 3-rock and ( υ 8, υ t )=(1,1) CO-stretch states introduces ( υ 6, υ t )=(1,0)←(0,1) subbands into the spectrum and makes the ν 7+ ν 12− ν 12 torsional hot band stronger than the ν 7 fundamental. The results suggest a picture of strong coupling among the OH-bending, CH 3-rocking, and CO-stretching modes that significantly modifies the traditional energy structure and raises interesting and provocative questions about the torsion–vibration identity of a number of the observed states.