Methylation effects in state resolved quenching of highly vibrationally excited azabenzenes (Evib∼38500 cm-1).: I.: Collisions with water

To investigate the role of molecular structure in collisions that quench highly vibrationally excited molecules, we have performed state resolved transient infrared absorption studies of energy gain in a number of rotational levels of H2O(000) resulting from collisions of water with vibrationally excited 2-methylpyridine (2-picoline) and 2,6-dimethylpyridine (2,6-lutidine) in a low-pressure gas-phase environment at 298 K. Vibrationally excited methylpyridines were prepared with similar to 38 500 cm(-1) of internal energy using 266 nm ultraviolet excitation to an S-1 electronic state followed by rapid radiationless decay to the S-0 electronic state. Collisions that populate rotationally excited states of H2O(000) were investigated with infrared absorption by monitoring the appearance of individual rotational states of H2O(000) with energies between 1000 and 2000 cm(-1). Rotational state distributions for recoiling water molecules were characterized by Boltzmann temperatures of T-rot=590 +/- 90 K for quenching of hot picoline and T-rot=490 +/- 80 K for lutidine quenching. Doppler-broadened transient absorption line profiles show that the scattered H2O(000) molecules have laboratory-frame translational energy distributions corresponding to T(trans)approximate to 600 K for deactivation of picoline and T(trans)approximate to 590 K for lutidine. Energy transfer rate constant measurements indicate that rotational excitation of H2O(000) with E-vib> 1000 cm(-1) occurs for one in 31 picoline/water collisions and one in 17 lutidine/water collisions. Comparison with earlier quenching studies on pyrazine [M. Fraelich, M. S. Elioff, and A. S. Mullin, J. Phys. Chem. 102, 9761 (1998)] and pyridine [M. S. Elioff, M. Fraelich, R. L. Sansom, and A. S. Mullin, J. Chem. Phys. 111, 3517 (1999)] indicate that, for the same initial internal energy in the hot donor, the extent of rotational excitation in water is diminished as the number of vibrational modes in the donor increases. The energy transfer probability for this pathway exhibits opposite behavior, with the larger donor molecules being more likely to excite the high energy rotations in water. These results are interpreted using a statistical description of the high energy donors and highlight the role of low frequency vibrational modes in the vibrationally hot donor molecules. A Fermi's golden rule approach is successful at explaining differences in the observed scattering dynamics for the various donor molecules. (C) 2001 American Institute of Physics.