A rotation–torsion–vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH 3-bending fundamentals of methanol

Abstract

A theoretical model has been developed to account for certain features of both newly observed and previously reported CH 3-bending subbands between 1450 and 1570 cm −1 in the high-resolution Fourier transform infrared spectrum of CH 3OH [Can. J. Phys. 79 (2001) 435]. The features include (i) an apparent inversion of the rotationless E– A torsional splitting with respect to the ground state, i.e., the A state located above the E state, (ii) a pronounced upward slope in the K-reduced torsion–vibration energy pattern for the subband origins, and (iii) unexpected A 1/ A 2 inversion of the K=2 A and K=3 A J-rotational levels that led to ambiguity in identifying the vibrational mode as ν 4 (A 1) or ν 10 (A 2) . The model is an effective internal coordinate Hamiltonian constructed in G 6 molecular symmetry with the CH 3-bends coupled to each other and to torsion and including a- and γ-type Coriolis coupling. With this model, 33 out of 36 experimental upper-state K-term values for newly assigned ν 4, ν 5 , and ν 10 subbands plus previous ν 4 subbands have together been fitted successfully, employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm −1. The P γ Coriolis term appears to be the leading cause of the upward shift in the K-reduced energies. When J-dependence is introduced via a rotational Hamiltonian including b- and c-type Coriolis terms in addition to molecular asymmetry, the observed A 1/ A 2 inversion of the K=2 A and 3 A rotational levels can also be reproduced. Predictions using the fitted K-rotation–torsion–vibration Hamiltonian show an interesting Coriolis-induced crossover and mixing of the ν 5 and ν 10 torsion–vibration energy patterns. These predictions played a role in identifying two of the new ν 5 subbands in the crossing region, thereby helping to validate the model.