The influence of N-H center dot center dot center dot pi hydrogen bonding on the anharmonicity of the nu(N-H) mode and orientational dynamics of nearly continuously solvated indole

Fourier transform infrared spectra of indole in carbon tetrachloride soluti ons were recorded. The spectroscopic data indicate that even at relatively low concentrations, the solute forms molecularly associated pairs through N -H . . . pi hydrogen bonding. Both NIR data and AM1 calculations show that, in comparison with the monomeric v(N-H) mode, the anharmonicity of the v(N -H . . . pi) mode decreases, and that of the dimeric v(N-H) mode slightly i ncreases. The anharmonicity constants for these modes are calculated on the basis of the NIR data, and the changes of the v(N-H) potential upon N-H . . . pi hydrogen bonding are quantitatively studied. The orientational dynam ics of monomeric and associated indole species are studied within the frame work of the time correlation function formalism. The period of essentially free rotation in the condensed phase reduces from 2.3 ps for the monomeric indole to 0.4 ps for the proton-donor molecule within the dimer. (C) 2000 E lsevier Science B.V. All rights reserved.