Mark M. Law

Chemistry Department, University of Aberdeen Meston Walk, Aberdeen, AB24 3UE, Scotland, UK

The methyl halide molecules and their isotopomers have been the subjects of extensive spectroscopic investigations over many years. Here we review the most recent experimental and theoretical work and discuss the major challenges for future development of our understanding of the vibrational quantum states in these prototype systems.
The mid- and near-infrared overtone absorption spectra of the methyl halides are dominated by features arising from C-H stretching
excitation. However in all cases (including the deuterated derivatives) the C-H(D) stretching manifolds are strongly perturbed by Fermi resonances involving some or all of the CH3 bending overtones and combinations. These and other important resonances ensure that nearly all vibrational degrees of freedom are strongly coupled. Comparisons between the methyl halides are particularly interesting in view of the changes, with substitution of successively heavier halide atoms, in the densities of states and the patterns of vibrational couplings. Approximate computations of highly excited vibrational states in these molecules have included studies using the perturbation-resonance approach in which most terms in the complete Hamiltonian are treated by low order perturbation theory whilst resonant terms are taken into account exactly but using a substantially reduced basis. More accurate variational calculations have been attempted on the methyl halides for low internal energies. The prospects for extending such methods to high excitations are discussed.