Experimental and computational studies on collective hydrogen dynamics in ammonia borane : Incoherent inelastic neutron scattering

Incoherent inelastic neutron scattering is used to probe the effects of dihydrogen bonding on the vibrational dynamics in the molecular crystal of ammonia borane. The thermal neutron energy loss spectra of B11 enriched ammonia borane isotopomers (B11H3NH3, B11D3NH3, and B11H3ND3) are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics. A harmonic vibrational analysis on NH3BH3 clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50–500 cm−1). Some assignment of modes to simple harmonic motion, e.g., NH3 and BH3 torsion in the molecular crystal is possible, and it is confirmed that the lowest modes are dominated by collective motion. We show that the vibrational dynamics as modeled with ab initio molecular dynamics provides a more complete description of anharmonic and collective dynamics in the low frequency region of the inela...