Coriolis resonance in C_s-asymmetric tops

Abstract

The bands centred near 1430 and 1110 cm^-1 in the infrared spectrum of acetaldehyde exhibit strongly distorted K-structures, indicating Coriolis resonance between v ₅(a′) and v ₁₂(a″) in the first, and v ₈(a′) and v ₁₃(a″) in the second case. The K-structures of these bands have been analysed. The theory, which is very generally applicable to study Coriolis resonance phenomena in C _s -asymmetric tops, is formulated in terms of effective hamiltonians. Approximate solutions of the reduced problems involved are obtained by using perturbation theory. A least-squares analysis of sub-band frequencies yielded estimates for a number of molecular parameters. Among these, the vibrational frequencies are obtained as v ₅ ⁰ = 1429·91, v ₁₂ ⁰ = 1435·83, v ₈ ⁰ = 1114·04 and v ₁₃ ⁰ = 1107·30 cm^-1. A computer program has been written allowing the simulation of the Q-branch of a Coriolis resonance band involving an a′- and an a″-fundamental of a C _s -asymmetric top molecule. This program was used to verify hypothetical assignments of sub-bands and of characteristic central features, and to estimate, roughly, relevant intensity parameters. An extraordinary K-structure anomaly observed with the v ₅, v ₁₂ band is traced to a term similar to the r_t -type interaction in symmetric top molecules.