Raman Spectroscopy, X-ray Diffraction, and Hydrogenation Thermochemistry of N,N,N,N-Tetramethylcyclotriborazane under Pressure

The novel hydrogen-rich BN materials Me2NHBH3 and c-N2B2H4Me4 have been studied by a combination of vibrational spectroscopy and single crystal X-ray diffraction over the pressure range 0-40 GPa. Assignments of Raman-active vibrational modes were made for c-N2B2H4Me4 on the basis of a combination of gas-phase predictions and previous assignments for similar compounds. The Raman spectrum of single crystals were found to have excellent signal-to-noise for pressures over the 0-40 GPa range, making it an ideal method for in situ analysis of high pressure reactions involving c-N2B2H4Me4. The enthalpy of the reaction c-N2B2H4Me4 + 2 H-2 -> 2 Me2NHBH3 was estimated to be 2.9 kcal/mol endothermic at ambient pressure. The corresponding pressure dependence of Delta G(rxn), was estimated from the P-V equations of state (EOS) measured for Me2NHBH3, c-N2B2H4Me4, and H-2 over the 0-12 GPa range. Using the EOS for fluid hydrogen, the reaction is estimated to have a favorable Delta Delta G(rxn) of 10 kcal/mol over the 0-2 GPa pressure range. Above 2 GPa, a positive pressure dependence of Delta G(rxn) is observed. On the basis of these experimental observations, we estimate the reaction thermochemistry to approach a thermoneutral equilibrium over the 0-2 GPa range. Above 2 GPa, the reaction volume becomes positive, causing this hydrogenation pathway to remain unfavorable over a pressure range extending to greater than 100 GPa at 298 K.