The vapor phase far-infrared spectra of 1H-indene, benzo[b]furan, 1,3,2-benzodioxaborole, and indole: Evidence for planar skeletons and kinetic anharmonicity in the out-of-plane ring deformations

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Abstract
The far-infrared spectra of the vapors of 1H-indene, benzo[b]furan, 1, 3, 2-benzodioxaborole (catechol borane), and indole are reported in the region of 100–500 cm−1. The spectra of the first three compounds exhibit series of C-type Q branches for the three out-of-plane ring deformations, establishing planar skeletal configurations and indicating that the aromatic skeletons are not truly rigid. The lower of these two sequences are assigned as interlocking transitions arising from the deformation of the five-membered ring and the butterfly mode, and are analyzed by means of a two-dimensional Hamiltonian which includes for each mode a quadratic potential constant and a quadratic dependence of the reduced mass on each displacement coordinate, as well as a potential constant coupling the two modes.