Theoretical study of the highly vibrationally excited states of FHF−: Ab initio potential energy surface and hyperspherical formulation

A three-dimensional description of vibrationally highly excited linear molecules is formulated in hyperspherical coordinates, based on a successive adiabatic reduction scheme. The method is applied to the low-lying and highly excited vibrational states of FHF−, a prototype of symmetric bihalide anions, which has attracted spectroscopic interest due to its peculiar vibrational anharmonicity. Ab initio potential energy surfaces (PESs) which cover the ground-state potential well of FHF− and/or its dissociation to the F−+HF channel have been obtained by using the coupled electron pair approach (CEPA) method. An hyperspherical calculation using the ab initio PES of the sixth-order Simons–Parr–Finlan analytical form has correctly reproduced the experimental fundamental frequencies. Specifically, the vibrationally highly excited FHF− above the dissociation threshold is proposed as a candidate for transition state spectroscopy (TSS) of unimolecular dissociation reactions without barrier.