Rate constants have been measured by the flowing afterglow technique at 300 °K for the quenching of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) by a large number of small molecules. For the same reagent, the magnitudes of the cross-sections usually increase in the series Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2). The Ar(3P2) and Ar(3P0) data are compared to results in the literature for these states and to data for Ar(3P1) and Ar(1P1). The set of thermal quenching cross sections are used to test the correlations between the magnitudes of the cross sections and properties of the reagents as predicted by the orbiting, absorbing-sphere, golden rule, and curve-crossing mechanisms for quenching. The best correlation is between the cross sections and the C6 coefficient. The analysis supports the proposition that the orbiting-controlled, curve-crossing model is the general mechanism governing the magnitude of the thermal cross sections for quenching of the metastable states. This model explains the very large quenching cross sections of F2 and OF2 (relative to other molecules composed of first row elements) because covalent–ionic curve crossing occurs outside the conventional orbiting radius. The validity of the simple van der Waals dispersion forces as being the dominant entrance channel interaction between the excited state rare gas atoms and the reagents is discussed.

The infrared chemiluminescence from the HF elimination reactions of CF3H, CF3CH3, CZHSF, CzFSH, n-C3F7H, and i-C,F7H has been used to assign the vibrational and rotational distributions of HF. The chemically activated fluoroalkane molecules were formed by H atom recombination with the appropriate fluoroalkyl radicals, which were generated by reactions of H atoms with the fluoroalkyl iodide precursor molecules. The HF vibrational distributions decline monotonically with increasing energy. The mean HF vibrational energy is larger than the statistical expectation, and 2 5 3 5 % of the potential energy of the exit channel is specifically released as HF vibrational energy. The HF(u) rotational excitation is modest, and (EK(HF)) seems to be equal to or less than the statistical expectation. The HF(o,J) distributions are used to discuss the dynamics of these HF elimination reactions. The energy disposal pattern from the HF elimination reaction from CF3H is compared to the vibrational energy distributions of HC1 from the CF,HCI, CFH,CI, and CFHCI, molecules that were generated by secondary reactions in the F + CFHzCI, CH3CI, and CH2C12 systems. In general, three-centered reactions of halomethanes release a larger fraction of the potential energy as (Ev(HX)) than do four-centered reactions of haloethanes.

The recombination of CH(2)Cl and CH(2)F radicals generates vibrationally excited CH(2)ClCH(2)Cl, CH(2)FCH(2)F, and CH(2)ClCH(2)F molecules with about 90 kcal mol(-1) of energy in a room temperature bath gas. New experimental data for CH(2)ClCH(2)F have been obtained that are combined with previously published studies for C(2)H(4)Cl(2) and C(2)H(4)F(2) to define reliable rate constants of 3.0 x 10(8) (C(2)H(4)F(2)), 2.4 x 10(8) (C(2)H(4)Cl(2)), and 1.9 x 10(8) (CH(2)ClCH(2)F) s(-1) for HCl and HF elimination. The product branching ratio for CH(2)ClCH(2)F is approximately 1. These experimental rate constants are compared to calculated statistical rate constants (RRKM) to assign threshold energies for HF and HCl elimination. The calculated rate constants are based on transition-state models obtained from calculations of electronic structures; the energy levels of the asymmetric, hindered, internal rotation were directly included in the state counting to obtain a more realistic measure for the density of internal states for the molecules. The assigned threshold energies for C(2)H(4)F(2) and C(2)H(4)Cl(2) are both 63 +/- 2 kcal mol(-1). The threshold energies for CH(2)ClCH(2)F are 65 +/- 2 (HCl) and 63 +/- 2 (HF) kcal mol(-1). These threshold energies are 5-7 kcal mol(-1) higher than the corresponding values for C(2)H(5)Cl or C(2)H(5)F, and beta-substitution of F or Cl atoms raises threshold energies for HF or HCl elimination reactions. The treatment presented here for obtaining the densities of states and the entropy of activation from models with asymmetric internal rotations with high barriers can be used to judge the validity of using a symmetric internal-rotor approximation for other cases. Finally, threshold energies for the 1,2-fluorochloroethanes are compared to those of the 1,1-fluorochloroethanes to illustrate substituent effects on the relative energies of the isomeric transition states.

No abstract