Infrared spectra of the H3N-HBr complex in solid Ne, Ne/Ar, Ar, Kr, and N2.: Strong matrix effects on a hydrogen-bonded complex

Ammonia and hydrogen bromide vapors from the thermal decomposition of NH4Br were co-deposited with excess neon at 4-5 K to form the H3N-HBr complex. New 1208.8 and 1079.9 cm(-1) infrared absorptions are assigned to the H-Br stretching and symmetric NH3 bending modes of the 1:1 complex. Vibrational assignments are supported by (NH4Br)-N-15, ND4Br, (ND4Br)-N-15, and mixed H, D isotopic substitution. Complementary experiments were done with neon/argon mixtures, argon, krypton, and nitrogen to investigate the 1:1 complex in a range of matrix environments. The above modes are shifted to 729.3 and 1146.7 cm(-1) in solid argon, owing to an increased interaction with the matrix and the resulting increased proton sharing. The neon matrix spectrum suggests a strong hydrogen bond, slightly stronger than in the gas-phase complex, but not as strong as found in the argon and krypton matrix hosts, owing to increased solvation by the more polarizable matrix atoms and more proton transfer. The strong nitrogen matrix infrared absorption at 1392.2 cm(-1) shows a 5.2 cm(-1) N-15 shift, 1336.6 and 1134.8 cm(-1) mixed H, D and 1058.0 cm(-1) ND4Br counterparts, and is better described as a mixed N-H stretching, symmetric NH3 bending mode, which suggests even more proton transfer in the nitrogen host. These and earlier matrix isolation experiments show that the matrix environment markedly affects the hydrogen bonding interaction and the degree of proton transfer in the polar H3N-HBr complex.