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The reactions of photochemically generated Cl(2PJ) atoms with a number of fluorohydrocarbons have been investigated in the temperature range 8–95 °C by the competitive photochlorination technique using CH4 as a primary standard. Relative and absolute rate parameters are reported for CH3F, CH2F2, CH3CH2F, CH2FCH2F, CH3CHF2, CH3CF3, CH2FCHF2, CHF2CHF2, and several auxiliary reactions including CH3Cl, C2H6, and C3H8. The internal competition for hydrogen abstraction in asymmetric fluorethanes is examined in detail. The reactivity trends are discussed and it is found that the activation energies in the fluoromethane series correlate with the known C–H bond dissociation energies. The hydrogen reactivity in the fluoroethane series for which a sufficient data base of DH°(C–H) values is not available is best rationalized in terms of inductive effects and resonance interactions.

Parts of the potential energy surface for the reactions CH3 + HX -*• CH4 + X (X = Cl, Br) were evaluated at the Gaussian-l (Gl) level of theory. Both reactions proceed via a loosely bonded complex (la, lb) which is formed without activation energy.For each reaction, the complex is hydrogen bridged, has C3l) symmetry, and is lower in energy at Gl level than reactants by 0.67 and 0.28 kcal/mol for CH3 + HC1 and CH3 + HBr systems, respectively. The transition state structures (2a, 2b) for decomposition of the complexes to products were located. The transition state (2a) for the CH3 + HC1 reaction has energy 2.53 kcal/mol relative to the reactants, while for the CH3 + HBr system the transition state (2b) is calculated to lie only 0.67 kcal/mol above reactants at Gl level. Heats of reaction for the two reactions compared to experimental values at the same temperature reaffirm error limits for Gl theory, 1.3 kcal/mol (X = Cl) and 2.2 kcal/mol (X = Br).

Ab initio molecular orbital calculations were performed using the GAUSSIAN 92 system of programs a t the RHF/6-3 l G * level of theory for 1,1,2-trifluoroethane (CHzFCHF2) and 1,1,2,2-tetrafluoroethane (CHF2-CHF2). Equilibrium geometries, barriers for internal rotation, and harmonic vibrational frequencies were thereby determined. Electron correlation energy corrections were performed by single-point calculations a t both second-and fourth-order Mdler-Plesset perturbation theory. Two conformational minima in the potential energy surface were located for each molecule. The anti structure found in the CH2FCHF2 molecule was determined to be 1.66 kcal/mol more stable than the gauche structure a t the MP4/6-31 lG**//6-31G* level after inclusion of zero-point vibrational energy correction. Similar calculations at the same level of theory determined that the trans structure for the CHFzCHF2 molecule was 1.73 kcal/mol more stable than the gauche conformer. Two rotational transition structures were located for each molecule. The barriers for rotation about the C-C bond a t the MP4/6-311G** level were determined to be 2.80 and 5.61 kcal/mol for CHzFCHF2 and 3.90 and 7.1 1 kcal/mol for CHF2CHF2. Computed thermodynamic properties including heat capacity, entropy, enthalpy, and free energy functions are reported as a function of temperature. A theoretical heat of formation was evaluated for both molecules from applicable homodesmic reactions (AH0fp8(CH2FCHF2) = -1 56.8 f 1.3 kcal/mol and AHof,298(CHF2CHF2) = -209.1 f 1.7 kcal/mol). These data were used to evaluate u o f , T , AGof,T, and K f , T for both molecules over the temperature range of 0-1500 K.