Diborane anharmonic vibrational frequencies and Intensities: Experiment and theory

The vibrational dynamics of diborane have been extensively studied both theoretically and experimentally ever since the bridge structure of diborane was established in the 1950s. Numerous infrared and several Raman spectroscopic studies have followed in the ensuing years at ever increasing levels of spectral resolution. In parallel, ab initio computations of the underlying potential energy surface have progressed as well as the methods to calculate the anharmonic vibration dynamics beyond the double harmonic approximation. Nevertheless, even 70 years after the bridge structure of diborane was established, there are still significant discrepancies between experiment and theory for the fundamental vibrational frequencies of diborane. In this work we use parahydrogen (pH₂) matrix isolation infrared spectroscopy to characterize six fundamental vibrations of B₂H₆ and B₂D₆ and compare them with results from configuration-selective vibrational configuration interaction theory. The calculated frequencies and intensities are in very good agreement with the pH₂ matrix isolation spectra, even several combination bands are well reproduced. We believe that the reason discrepancies have existed for so long is related to the large amount of anharmonicity that is associated with the bridge BH stretching modes. However, the calculated frequencies and intensities reported here for the vibrational modes of all three boron isotopologues of B₂H₆ and B₂D₆ are within ± 2.00 cm⁻¹ and ± 1.44 cm⁻¹, respectively, of the experimental frequencies and therefore a refined vibrational assignment of diborane has been achieved.