ab initio calculations and thermochemical analysis on Cl atom abstractionsof chlorine from chlorocarbons and the reverse alkyl abstractions: Cl-2+R center dot <-> Cl center dot+RCl

Thermodynamic properties (Delta H(f(298))degrees S((298))degrees and Cp(T) from 300 to 1500 K for reactants, adducts, transition states, and products in reactions of CH3 and C2H5 with Cl-2 are calculated using CBSQ//MP2/6-311 G(d,p). Molecular structures and vibration frequencies are determined at th e MP2/6-311G(d,p), with single-point calculations For energy at QCISD(T)/6- 311 + G(d,p), MP4(SDQ)/CbsB4, and MP2/CBSB3 levels of calculation with scal ed vibration frequencies Contributions of rotational frequencies for S((298 ))degrees and Cp(T)'s are calculated based on rotational barrier heights an d moments of inertia using the method of Pitzer and Gwinn [1]. Thermodynamic parameters, Delta H(f(298))degrees, S((298))degrees and C-p(T ), are evaluated for C-1 and C-2 chlorocarbon molecules and radicals. These thermodynamic properties are used in evaluation and comparison of Cl-2 + R . --> Cl . + RCl (defined forward direction) reaction rate constants from the kinetics literature for comparison with the calculations Data from some 20 reactions in the literature show linearity on a plot of Ea(fwd) vs. Del ta H-rxn fwd, yielding a slope of (038 +/- 0.04) and intercept of (10.12 +/ - 0.81) kcal/mole. A correlation of average Arrhenius preexponential factor for Cl . + RCl --> Cl-2 + R . (reverse rxn) of (4.44 +/- 1.58) x 10(13) cm (3)/mol-sec on a perchlorine basis is obtained with Ea(Rev) = (0.64 +/- 0.0 4) x Delta H-rxn,H-Rev + (9.72 +/- 0.83) kcal/mole, where Ea(Rev) is 0.0 if Delta H-rxn.Rev is more than 15.2 kcal/mole exothermic. Kinetic evaluation s of literature data are also performed for classes of reactions. Ea(fwd) = (0.39 +/- 0.11) x Delta H-rxn.fwd + (10.49 +/- 2.21) kcal/mole an d average A(fwd) = (5.89 +/- 2.48) x 10(12) cm(3)/mole-sec for hydrocarbons : Ea(fwd) = (0.40 +/- 0.07) x Delta H-rxn.fwd + (10.32 +/- 1.31) kcal/mole and average A(fwd) = (6.89 +/- 2.15) x 10(11) cm(3)/mole-sec for C-1 chloro carbons: Ea(fwd) = (0.33 +/- 0.08) x Delta H-rxn.fwd + (9.46 +/- 1.35) kcal /mole and average A(fwd) = (4.64 +/- 2.10) x 10(11) cm(3)/mole-sec for C-2 chlorocarbons. Calculation results on the methyl and ethyl reactions with C l-2 show agreement with the experimental data after an adjustment of +2.3 k cal/mole is made in the calculated negative Ea's. (C) 2000 John Wiley & Son s, Inc.