The empirical harmonic potential function of diborane

A total of 114 spectroscopic data accumulated from 6 isotopic species of diborane have been used to permit a precise determination of the empirical harmonic potential function to be made for the first time. Of the 33 independent force constants, 30 can be determined with numerical significance. The physical significance of the values is probably best demonstrated by the rather close correspondence throughout with scaled ab initio force constants from two sources, only one rather large disagreement in magnitude being found. The potential function reproduces all known frequency, isotopic frequency shift, Coriolis ζ, and centrifugal distortion Δ data to high accuracy. It is used to calculate parameters which will be of value to further spectroscopic and structural studies, to enable accurate analyses to be made of a number of major Fermi resonance interactions present in the spectra, and to explain the anomalously small Coriolis interaction between the infrared active rocking and wagging fundamentals. The advantages of alternative symmetry coordinate definitions are considered, particularly in relation to making comparisons between diborane and ethylene.