High-Pressure Raman Spectroscopic Study of the Ammonia-Borane Complex. Evidence for the Dihydrogen Bond

The Raman spectra of the ammonia-borane complex, NH3BH3, have been investigated as a function of pressure up to 40 kbar. Vibrational modes involving the NH3 group show negative pressure dependences, supporting the existence of the dihydrogen bond, but the vibrations of the BH3 group have a positive dependence. Two transitions were observed in the solid phase under pressure, in contrast to the temperature behavior, where a single transition occurs. Factor group splitting occurs for the degenerate vibrations, and this allows the correct assignment of the observed vibrations.