J . Phys. Chem. 1990, 94, 6341-6350 found to be close to the experimental values by a factor of 4. Similar theoretical treatment to describe the CH, + OH reaction obtained reasonable agreement.20 Consequently, the rate coefficient for the abstraction of fluorine atoms from the fluoroformyl radical by oxygen atoms is computed by using transition-state theory2' kBT - e - EQ* k(T) = o/bT h
QO~QFCO
(9)
where Qo, QFC0,and Q* are total partition functions for the reactants (0and FCO) and transition state ([O-FCO]'). These quantities can be evaluated by using statistical mechanical methods for the structures, the total energies, and the vibrational frequencies for the reactant and transition state. In our evaluation of the rate coefficient, UMP2/6-3 IC* results for the structure (Table I), vibrational frequencies (Table II), and total energies calculated in the PMP4SDTQ/6-31G*//UMP2/6-31G* level (Supplementary Table 1) were used. Rates are calculated from the classical transition state theory expression and corrections due to tunneling using Wigner22and &kartU potential procedure were incorporated. Rates calculated by using the Eckart potential have shown better consistency with experimental data in that they can usually reproduce the shape of the Arrhenius plot within experimental uncertainty. A summary of the calculation for three different temperatures (295, 700,and 1000 K) is shown in Table V. The reaction barrier, AE* (including thermal and zeropoint-energy corrections), and the enthalpy of activation, AH*, are listed along with the Eckart and Wigner tunneling corrections (19) Francisco, J. S.;Zhao, Y.J . Chem. Phys. 1990, 93, 276. (20) Gonzalez, C.; McDouall, J. J. W.; Schlegel, H. 9. J . Phys. Chem., in press. (21) Steinfeld, J. 1.; Francisco, J. S.;Hase, W. L. Chemical Kinetics and Dynamics; Prentice-Hall, Englewood Cliffs, NJ, 1989. (22) Wigner, E. P. 2.Phys. Chem. 1932, 819, 203. (23) Johnston, H. S.;Heicklen, J. J . Chem. Phys. 1966, 66, 532.
6341
factor, the calculated Arrhenius parameters A and E,, and the theoretical log k . The major question raised is that a t 295 K (the temperature at which the total rate constant for the 0 FCO reaction was measured) what is the contribution to the total measured rate constant by the 0 + FCO CO + FO reaction. The calculated rate for this reaction at 295 K is found to be negligible. In fact, it appears that the onset of significant contributions by this reaction does not arise until 1000 K. As aforementioned, the calculated rate constant could be as much as a factor of 4 to the experimental rate with this procedure. However, if we assume that the calculated rate constant is in error by a factor of 100, the resulting rate at 295 K is still negligibly small, and therefore has no contribution to the total rate constant reported by Ryan and Plumb for the loss of FCO radical via reaction with O(3P) atoms. Consequently, the reported rate constant of (9.3 f 2.1) X lo-" cm-3 molecule-' s-I due to reaction 8.
-
+
Summary Reaction pathways were calculated for the O(3P)and FC0(2A') system. A transition-state structure was found for the abstraction of fluorine from FCO by O(3P), while no transition state was located for the addition of O(3P) to FCO to form an FC(0)O radicals which is suggested to exist only transiently as it proceeds to dissociate to form F(*P) and C02('Z8+).The rate constant for the abstraction reaction at 295 K was determined to be negligible. This establishes that the previously measured rate coefficient of (9.3 f 2.1) X lo-" cm3 molecule-' s-' for the reaction of O(jP) with FC0(2A') radical is due solely to the following reaction: O(jP) + FCO('A') [FC(O)O]* -.+ F(2P) + C02('Z8+)
-
Supplementary Material Available: Table 1 listing total energies of species involved in the O('P) + FC0(2A') systems (1 page). Ordering information is given on any current masthead page.
Collisional Deactivation of Highly Vibrationally Excited Benzene Pumped at 248 nm Murthy L. Yerram; Jerrell D. Brenner, Keith D. King,*.' and John R. Barker*,$ The Department of Atmospheric, Oceanic, and Space Sciences, Space Physics Research Laboratory, The University of Michigan. Ann Arbor, Michigan 48109-2143 (Received: December 5, 1989; In Final Form: April 3, 1990) Highly vibrationallyexcited gas-phase benzene was prepared with a pulsed excimer laser operating at 248 nm, and the subsequent collisional deactivation was monitored with time-resolved infrared fluorescence (IRF) from the C-H stretch modes near 3050 cm-I. Very low pressure photolysis mass spectrometric experiments were carried out to determine whether reaction of the benzene occurs at this laser wavelength and can complicate the energy-transfer investigation. The quantum yield for benzene loss was determined to be 3 f I%, consistent with previous experiments (Nakashima, N.; Yoshihara, K. J . Chem. Phys. 1982, 77,6040) that found photoproducts as the result of multiphoton excitation. The only detectable gas-phase product was H,,which had a quantum yield of 0.8 f 0.3%. Energy-transfer data were obtained for 19 collider gases, including unexcited , bulk average energy, was examined benzene. The inversion technique for converting the observed IRF decay to ( ( E ( t ) ) ) the the bulk average energy transferred in detail, and a careful propagation of errors analysis performed. For most colliders, ((AE)), per collision, exhibited an approximately linear dependence on vibrational energy for energies
