Infrared spectrum of the H3N-HI complex in solid Ne, Ar, Ne/Ar, Kr, and N2.: Comparisons of matrix effects on hydrogen-bonded complexes

Ammonia and hydrogen iodide vapors from the thermal decomposition of NH4I were codeposited with excess neon at 5 K to form the H3N-HI complex. New 630, 1192, and 3435 cm(-1) infrared absorptions are assigned to the antisymmetric N-H-I stretching, symmetric NH3 bending, and antisymmetric NH3 stretching modes of the 1:1 complex. Vibrational assignments are supported by (NH3)-N-15, ND4I, and mixed H/D isotopic substitution. Complementary experiments were done with argon, neon/argon mixtures, krypton, and nitrogen to investigate the 1:1 complex in a range of matrix environments and to compare with previous work using the reagent gases. The above modes are shifted to 592, 1259, and 3400 cm(-1) in solid argon owing to an increased interaction with the more polarizable argon matrix atoms. Neon and 2% argon mixtures gave intermediate absorptions, which evolved to the pure argon values on annealing to allow diffusion. The strong nitrogen matrix infrared absorption at 1955 cm(-1) shifts approximately 4 cm(-1) with N-15-substitution, which suggests an N-H stretching mode and substantial proton transfer toward a more ionic complex 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 this polar H3N-HI hydrogen-bonded complex.