HIGH-PRESSURE DISCHARGE FLOW KINETICS AND FRONTIER ORBITAL MECHANISTIC ANALYSIS FOR OH + CH2CCL2, CIS-CHCLCHCL, TRANS-CHCLCHCL, CFCLCF2, AND CF2CCL2-] PRODUCTS

The kinetics and mechanism of the gas-phase reactions of five halogenated ethenes with the hydroxyl radical are studied by high-pressure discharge flow and by frontier orbital analysis. In contrast to the vast majority of OH/small hydrocarbon reaction series, the rate constants for the OH/halogenated ethene reactions do not correlate with the electron-donating ability (ionization potential) of the halogenated ethene, and the reactions are substantially slower than the majority of OH/alkene reactions. It is suggested that the suppressed reactivity with OH is partly due to a substantial nonbonding contribution to the frontier orbital interaction between the doubly occupied, pi-bonding HOMO of the haloethene and the singly-occupied pi-orbital of OH. In addition, the energy required to distort the ethene plane of symmetry in the transition state is likely to be raised by halogen substitution. SCF ab initio calculations of the halogenated ethene wave function and of the potential energy surface for OH + C2H4 --> CH2CH2OH are used to aid in the frontier orbital mechanistic analysis, to differentiate between the OH/haloethene and OH/alkene systems, and to interpret the strong negative temperature dependences observed in some of these reactions. The high-pressure-limit rate constants for the five OH/halogenated ethene addition reactions, measured from 297 to 368 K, are as follows: for CH2CCl2, (1.6 +/- 0.6) x 10(-12)e(560 +/- 110)/T, (1.02 +/- 0.10) x 10(-11) at 297 K; for CF2CCl2, (1.5 +/- 0.5) x 10(-12)e(480 +/- 100)/T, (7.44 +/- 0.40) x 10(-12) at 297 K; for CFClCF2, (1.0 +/- 0.5) x 10(-12)e(590 +/- 150)/T, (7.57 +/- 0.40) x 10(-12) at 297 K; for cis-CHClCHCl, (2.0 +/- 0.5) x 10(-12)e(70 +/- 120)/T, (2.40 +/- 0.20) x 10(-12) at 297 K; for trans-CHClCHCl, (2.1 +/- 0.8) x 10(-12)e(20 +/- 120)/T, (2.10 +/- 0.20) x 10(-12) at 297 K, where the units are cm3 molecule-1 s-1 and the uncertainties are at the 95% confidence level.