Abstract. Room temperature (300 K) experiments have earlier established that the replacement of a fluorine atom in p-difluorobenzene (pDFB) with a methyl group to make p-fluorotoluene @FT) generates a qualitative difference in intramolecular vibrational redistribution (IVR) characteristics as seen in the S, states. Here we report S , S , fluorescence excitation and dispersed single vibronic level (SVL) fluorescence spectra that have been obtained for IVR comparisons in the cold (5 K) environment of supersonic expansions. As in the 300 K experiments, the cold beam S, vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S, level interactions first appears after pumping an S, pFT level near 800 cm-'. In contrast, congestion in pDFB spectra is still absent from levels with twice this energy. Attention is directed to the relative S, ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S, zero point levels and fluorescence excitation spectra are combined with published information about S, fundamentals to show that the S, vibrational level structures of the two molecules are as closely related as those of an isotopomer pair. It is argued that the small differences in S, fundamentals cannot be a principal contribution to the qualitative IVR differences.

Classical trajectory calculations for collisions between vibrationally hot pyrazine (E ) 40332 cm -1 ) and room-temperature CO are compared with recent diode laser experiments. Ab initio calculations were carried out to determine parameters for the pairwise Lennard-Jones intermolecular potential. The trajectory results appear to show good qualitative agreement with preliminary experimental results. The highest ∆E collisions occur when the CO happens to lie above the pyrazine plane just as the underlying CH wag executes an unusually high amplitude motion. Artificially doubling the length of the CO produces results for the angular momentum transfer to the CO molecule that look quite similar to those for angular momentum transfer to CO 2 in analogous pyrazine-CO 2 experiments. † Part of the special issue "William H. Miller Festschrift".

ABSTRACT:Reaction mechanisms between AlH ⌺ and HF molecule are theoretically investigated. Ab initio calculations demonstrate that there are two parallel reaction channels: one is an addition reaction to give H AlF via the three-membered ring 2 Ž . transition state TS and the other is a dehydrogenation reaction to give AlF q H via the 2 four-membered ring TS. The addition reaction is thermodynamically favorable and the dehydrogenation reaction is kinetically favorable. Thermodynamics and Eyring transition Ž . state theory TST with the Wigner correction are also used to compute the thermodynamic functions, the equilibrium constants, A factors, and the rate constants of these reaction channels at 200᎐1000 K. From the thermodynamics and TST calculations, it is valuable to point out that consideration on the entropy and thermal enthalpy is quite important in the study of chemical reactions on the basis of ab initio method.

ABSTRACT:The ground-state geometries, scaled quantum mechanical SQM quadratic force field, and infrared and Raman intensities of p-difluorobenzene and p-fluorotoluene have been determined from density functional theory, using the B3-LYP hybrid functional and the 6-31G U basis set. Based on these calculations, one b mode of 2 u p-difluorobenzene is reassigned. Possible reassignments of a few other, less certain, experimental bands are proposed.

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