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 interaction of water vapor with ice remains incompletely understood despite its importance in environmental processes. A particular concern is the probability for water accommodation on the ice surface, for which results from earlier studies vary by more than 2 orders of magnitude. Here, we apply an environmental molecular beam method to directly determine water accommodation and desorption kinetics on ice. Short D2O gas pulses collide with H2O ice between 170 and 200 K, and a fraction of the adsorbed molecules desorbs within tens of milliseconds by first order kinetics. The bulk accommodation coefficient decreases nonlinearly with increasing temperature and reaches 0.41 ± 0.18 at 200 K. The kinetics are well described by a model wherein water molecules adsorb in a surface state from which they either desorb or become incorporated into the bulk ice structure. The weakly bound surface state affects water accommodation on the ice surface with important implications for atmospheric cloud processes.

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Molecular scattering experiments are used to investigate water interactions with methanol and n-butanol covered ice between 155 K and 200 K. The inelastically scattered and desorbed products of an incident molecular beam are measured and analyzed to illuminate molecular scale processes. The residence time and uptake coefficients of water impinging on alcohol-covered ice are calculated. The surfactant molecules are observed to affect water transport to and from the ice surface in a manner that is related to the number of carbon atoms they contain. Butanol films are observed to reduce water uptake by ice by 20%, whereas methanol monolayers pose no significant barrier to water transport. Water colliding with methanol covered ice rapidly permeates the alcohol layer, but on butanol has mean surface lifetimes of ≲0.6 ms, enabling some molecules to thermally desorb before reaching the water ice underlying the butanol. These observations are put into the context of cloud and atmospheric scale processes, where such surfactant layers may affect a range of aerosol processes, and thus have implications for cloud evolution, the global water cycle, and long term climate