Pressure-induced complexation of NH(3)BH(3)-H(2).

High pressure Raman spectroscopy of NH(3)BH(3)-H(2) mixtures up to 60 GPa reveals unusual pressure-induced complexation and intermolecular interactions. Stretching modes of H(2) in the complex arise at 6.7 and 10 GPa, increasing in frequency with pressure of up to 60 GPa with different pressure coefficients, and at approximately 40 GPa, the lower frequency mode approaches vibron frequency of bulk H(2). Pressure-induced transformations in pure NH(3)BH(3) studied up to 60 GPa reveal a disorder-order transition at 1 GPa (phase II) and further transitions at 5 (phase III) and 10 GPa (phase IV). The spectra of both pure NH(3)BH(3) and the NH(3)BH(3)-H(2) complex provide evidence for strengthened of the N-H(delta+)...H(delta-)-B dihydrogen bonding linkages up to 50 GPa, beyond which they weaken. The dihydrogen bonding breaks down due to interactions with H(2) between 15 and 20 GPa in the NH(3)BH(3)-H(2) complex. The behavior of the nu(NH(3)) modes in the NH(3)BH(3)-H(2) complex indicates a dominant role of the NH(3) functional group in the observed interactions.