J . Phys. Chem. 1990, 94. 7106-7 110
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perturbation theory (MP2) calculation decreased the magnitude of the empirical bond additivity correction. Comparison of the rate constants computed by using the potential information from the HF/BAC-MP4 and MP2/BAC-MP4 methods showed little difference except at temperatures below 300 K where tunneling is more sensitive to details of the potential information. The good agreement obtained here is encouraging for the applicability of the more practical HF/BAC-MP4 to generate potential information for a variety of chemical reactions. As a further test of the methods employed here, the computed rates were compared with experiment for the reactions H + NH, ;2 H2 + N H 2 and D + ND, D2 ND,. For the reaction H + NH, Hz + NH2 (RI), theory and experiment are in excellent agreement over the entire experimental temperature range from 500 K to 1770 K. The computed reaction rates for the reaction H 2 + NH2 H + NH, (R2) are in very good agreement with experimental ones of Hack, Rouveirolles, and Wagner4 and Sutherland and M i ~ h a e lbut , ~ they disagree with those of Demissy and Lesclaux.' Reaction R2 is the reverse of R1 and the rates are related by detailed balance: therefore, the excellent agreement between theory and experiment for the former reaction at the lower temperatures is inconsistent with the larger discrepancies seen for the reverse reaction R2 in the same temperature range. The
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computed and experimental rates for the deuterated reaction D + ND3 D, ND2 are in good agreement over the entire experimental temperature range from 590 to 1220 K. The agreement at the lower temperatures for this reaction is not quite as good as for reaction R1: the theoretical results tend to overestimate the rates slightly. The overall good agreement of the computed rate constants using different levels of theory for the potential information and the good agreement between the computed and experimental rate constants have given more confidence in the theoretical methods utilized here. Besides the comparisons with these experimental results, extensions of the rates down to 200 K and up to 2400 K are provided for reactions R I , R2, and R3 and predictions are made for reaction R4.
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Acknowledgment. B.C.G. thanks Dr. J. V. Michael for helpful suggestions concerning the use of scaled HF frequencies to improve the agreement of the computed equilibrium constant with the experimental ones. The work at Chemical Dynamics Corporation was sponsored by SDIO/IST and managed by the Office of Naval Research under contract number N00014-87-C-0746. The work at NRL was supported by the Mechanics Division of the Office of Naval Research under contract number N0014-89-WX-24024.
Picosecond Time-Resolved Absorption and Emission Studies of the Singlet Excited States of Acenaphthyiene Anunay Samanta,*vt Chelladurai Devadoss, and Richard W. Fessenden* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: December 27, 1989; In Final Form: April 18, 1990)
Radiative and nonradiative processes in acenaphthylene have been examined by a combination of steady-state and picosecond time-resolved emission and transient absorption measurements. Fluorescence from a higher excited state (S,) is found to compete with the nonradiative decay of this state. A lifetime of
