The bond dissociation energies and the enthalpies of formation of halogenated molecules were theoretically calculated, and the results were compared with the corresponding experimental values in order to examine the reliability of a large number of levels of theory in thermochemical calculations. Density functional theory using a multitude of exchange and correlation functionals, Møller-Plesset perturbation theory, and QCISD-(T) and CCSD(T) methods were employed, with all-electron and effective-core potential basis sets of varying complexity. A small set of 19 molecules was selected, consisting of X 2 , HX, and CH 3 X (X ) F, Cl, Br, and I), the mixed-halogen molecules ClF, BrF, BrCl, IF, and ICl, and H 2 and CH 4 . The calculated bond dissociation energies were corrected for basis set superposition errors and the first-order spin-orbit coupling in the 2 P state of halogen atoms. In addition, the enthalpies of formation of all molecules in the set as well as those of methyl CH 3 and halomethyl radicals CH 2 X were also calculated by using the corresponding atomization reactions, corrected for the spin-orbit coupling in the 3 P state of carbon atom and the 2 P state of halogen atoms. Levels of theory employing the B3P86 functional with moderately large basis sets, augmented with diffusion and polarization functions, were found to be sufficiently reliable in the calculation of bond dissociation energies of closed-shell halogenated molecules. In particular, the B3P86/6-311++G(2df,p) level of theory was found to be the most accurate, with an RMS deviation of 6 kJ mol -1 for 23 bond dissociation energies, with a negligible dependence of the accuracy on the level of theory chosen for the geometry optimization. In addition, the B3P86 functional in combination with small basis sets was found to be superior to B3LYP and MP2 in the calculation of molecular structures. Regarding the calculated enthalpies of formation, G2 theory was the most accurate, with an RMS deviation of 9 kJ mol -1 , followed by several combinations of the B3PW91 and B3LYP functionals with mostly large basis sets. However, the B3P86 functional tends to overbind openshell species, resulting in an underestimation of the enthalpies of formation for polyatomic molecules. Extension of the bond dissociation energy calculations at levels of theory employing the B3P86 functional to a larger set of 60 bonds in 41 halogen-containing molecules revealed systematic errors dependent on the molecular size. Therefore, the calculated bond dissociation energies at the B3P86/6-311++G(2df,p) level of theory were empirically improved by increasing the absolute energies of the radicals by the quantity 9 × 10 -5 ‚N e Hartrees (N e ) total number of electrons of the radical), with a subsequent lowering of the RMS deviation in the larger set to 8.0 kJ mol -1 .

The absolute rate constants for the reactions of chlorine atoms with a series of fluorinated ethanols CF 3 CH 2 -OH (1), CHF 2 CH 2 OH (2), and CH 2 FCH 2 OH (3) were measured in the gas phase over the temperature range 273-363 K, by employing the discharge-flow mass spectrometric technique with a Knudsen type reactor. The absolute rate constants are given by the following expressions (in cm 3 molecule -1 s -1 , 2σ uncertainties): k 1 ) (0.85 ( 0.19) × 10 -11 exp((-792 ( 74)/T), k 2 ) (2.61 ( 0.49) × 10 -11 exp((-662 ( 60)/T), and k 3 ) (7.57 ( 0.98) × 10 -11 exp((-408 ( 40)/T). The title reactions take place primarily via the abstraction of methylene hydrogen, yielding the corresponding fluoroethanol radicals, which subsequently may undergo oxidation reactions, resulting in fluoroacetaldehydes. Quantum mechanical calculations of the C-H bond energies in the title alcohols at the B3P86/6-311++G(2df,p) level of theory show that the methylene hydrogen bonds are the weakest. The atmospheric degradation mechanism of the title fluorinated ethanols is also presented.

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