Enthalpies of formation and bond dissociation energies at 298.15 K for molecules containing all four halogen atoms were calculated by CCSD(T) using double- and triple-ζ correlation-consistent basis sets, extrapolated to the complete basis set limit by the infinite basis (IB) method. The small molecules X_2, HX, CH_2X, CH_3X (X = H, F, Cl, Br, I), CIF, BrF, BrCl, IF, ICl, CX (X = H, F, Cl, Br), and CHX (X = H, F, Cl) constituted the benchmark set. The cc-pV(n+d) Z (n = D,T) basis sets were used for chlorine, and two different sequences of conventional basis sets for iodine. The sequence consisting of the smaller SV4P and 6-311G (3df) basis sets for iodine-denoted by (cc)-led to a slightly better performance. The IB extrapolation parameters were obtained by the minimization of the deviation between the zero-point exclusive atomization energies calculated by IB and those calculated by a combined Gaussian/exponential function using a sequence of three cc-pV-(n+d)Z basis sets, with n = D, T, Q. All geometry optimizations and vibrational frequency calculations were performed at the MP2/6-311G(d) level of theory. A slight improvement of the calculated bond lengths, vibrational frequencies, and enthalpies of formation for diatomic molecules was achieved by a geometry optimization at levels of theory employing CCSD(T) and complete basis set limits. The calculated thermochemical properties were corrected for spin-orbit effects, and were further improved by the inclusion of core/valence correlation calculated at the CCSD(T)/(cc)-pV(D+d)Z level of theory and scalar relativistic corrections calculated at the MCPF-MVD/(cc)-pV(T+d)Z level. The application of the IB method in a larger set of molecules, including halomethanes CH_((4-k))X_k (X = F, Cl; k = 3,4), CH_2XY (X, Y = F, Cl, Br, I), and haloethanes CH_3CH_2X (X = H, F, Cl, Br, I), revealed a systematic failure in molecules containing more than one chlorine atoms, attributed to the inadequacy of the two-point (D,T) extrapolation of correlation energy. The agreement with experimental data was improved by lowering the infinitely extrapolated total energies by the amount Q(N_(eff))~γ, N_(eff) being the sum of the effective number of electrons for all constituent atoms, defined as the number of valence electrons for H,C,F,Br,I, and the total number of electrons for Cl. The parameters Q and γ were appropriately adjusted by the minimization of the root-mean-square (RMS) deviation from the experimental enthalpies of formation. Thus, by using the parameters α = 5.02, β_(CCSD(T)) = 2.41, Q = 9.37 * 10~(-6), and γ = 1.80, RMS deviations of 5.7 and 6.3 kJ mol~(-1) were obtained for 57 enthalpies of formation and 76 bond dissociation energies, respectively.
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