Evolution of the dihydrogen bonding network in ammonia borane from INS and temperature dependent Raman spectra

Inelastic neutron scattering spectra were collected on a series of selectively deuterated 11B enriched ammonia borane samples at 15 K. The INS spectra are compared to temperature dependent, low frequency Raman spectra collected from 80 to 300 K. At the lowest temperatures there is excellent correspondence between the modes seen in the INS spectra and the low frequency Raman. The temperature dependent Raman studies permit following the frequency shift and linewidth changes of the low frequency modes through the orthorhombic to tetragonal phase transition. Using the observed isotopic frequency shifts and results from molecular simulation of the INS spectra some of the low frequency modes can be associated with torsional motions of the amine and borane groups while other modes appear to be associated with dihydrogen bonding. The observed temperature dependence of both these moieties are analyzed in view of previously measured quasielastic neutron measurements and high frequency Raman measurements over the same temperature region. These complimentary spectroscopic and simulation results provide insight to the evolution of dihydrogen bonding network and hydrogen dynamics in ammonia borane over a large temperature range.