Sequences of basis sets that systematically converge towards the complete basis set (CBS) limit have been developed for the first-row transition metal elements Sc-Zn. Two families of basis sets, nonrelativistic and Douglas-Kroll-Hess (-DK) relativistic, are presented that range in quality from triple-zeta to quintuple-zeta. Separate sets are developed for the description of valence (3d4s) electron correlation (cc-pVnZ and cc-pVnZ-DK; n = T,Q, 5) and valence plus outer-core (3s3p3d4s) correlation (cc-pwCVnZ and cc-pwCVnZ-DK; n = T,Q, 5), as well as these sets augmented by additional diffuse functions for the description of negative ions and weak interactions (aug-cc-pVnZ and aug-cc-pVnZ-DK). Extensive benchmark calculations at the coupled cluster level of theory are presented for atomic excitation energies, ionization potentials, and electron affinities, as well as molecular calculations on selected hydrides (TiH, MnH, CuH) and other diatomics (TiF, Cu2). In addition to observing systematic convergence towards the CBS limits, both 3s3p electron correlation and scalar relativity are calculated to strongly impact many of the atomic and molecular properties investigated for these first-row transition metal species.

Recently developed correlation consistent basis sets for the first row transition metal elements Sc-Zn have been utilized to determine complete basis set (CBS) scalar relativistic electron affinities, ionization potentials, and 4s(2)3d(n-2)-4s(1)d(n-1) electronic excitation energies with single reference coupled cluster methods [CCSD(T), CCSDT, and CCSDTQ] and multireference configuration interaction with three reference spaces: 3d4s, 3d4s4p, and 3d4s4p3d'. The theoretical values calculated with the highest order coupled cluster techniques at the CBS limit, including extrapolations to full configuration interaction, are well within 1 kcal/mol of the corresponding experimental data. For the early transition metal elements (Sc-Mn) the internally contracted multireference averaged coupled pair functional method yielded excellent agreement with experiment; however, the atomic properties for the late transition metals (Mn-Zn) proved to be much more difficult to describe with this level of theory, even with the largest reference function of the present work.

A global potential energy surface (PES) for the (1)A' ground state of HgBr(2) has been constructed in order to determine the rate constants for atmospherically important reactions involving mercury and bromine. The total energy of HgBr(2) was calculated by the multireference configuration interaction level of theory with series of correlation consistent basis sets up to quadruple-zeta quality with subsequent extrapolation to the complete basis set limit. An additive correction for spin-orbit coupling was also included. The global PES was represented piecewise by interpolating three separate parts of the surface with the reproducing kernel Hilbert space method and connecting them smoothly by switch functions. Quasiclassical trajectory calculations carried out on the surface yielded 298 K thermal rate constants of 3.89 x 10(-11) cm(3)/(mol.s) for the abstraction reaction HgBr + Br --> Hg + Br(2), 2.98 x 10(-11) cm(3)/(mol.s) for the recombination reaction Br + HgBr --> HgBr(2), and 3.97 x 10(-11) cm(3)/(mol.s) for the exchange reaction Br + HgBr --> BrHg + Br. The insertion reaction Hg + Br(2) --> HgBr(2) was found to have a high barrier of 27.2 kcal/mol and a very small rate constant of just 2.74 x 10(-31) cm(3)/(mol.s) determined by the microcanonical variational transition state theory method. The implications of the obtained results to the description of the mechanism of recently observed polar tropospheric mercury depletion events are briefly discussed.

Accurate reaction enthalpies have been calculated using the CCSD(T) method and sequences of correlation consistent basis sets for the reactions of Hg with a series of small halogen-containing molecules (Cl 2 , Br 2 , BrCl, ClO, and BrO). Explicit extrapolations to the complete basis set limit are used together with accurate treatments of core-valence correlation, scalar relativity, and spin-orbit effects in order to predict both reaction enthalpies of the title reactions and heats of formation of the intermediates (HgCl 2 , HgBr 2 , HgBrCl, HgClO, and HgBrO) to an estimated accuracy of approximately 1 kcal/mol. All of the intermediates are predicted to be strongly bound and the structures and vibrational frequencies of HgClO and HgBrO are reported for the first time. The present results are expected to be useful in modeling the gas-phase oxidation of mercury in the troposphere by halogen-containing species.

Accurate 0 K enthalpies have been calculated for reactions of mercury with a series of small iodine-containing molecules (I2, IBr, ICl, and IO). The calculations have been carried out with the coupled cluster singles and doubles method with a perturbative correction for connected triple excitations [CCSD(T)] using sequences of correlation consistent basis sets and accurate relativistic pseudopotentials. Corrections have been included to account for core-valence correlation, spin-orbit coupling, scalar relativity, and the Lamb shift. In a few cases coupled cluster calculations with iterative triple (CCSDT) and quadruple (CCSDTQ) excitations have been carried out to estimate the effects of higher order electron correlation. The pseudopotential calculations have also been compared to all electron calculations using second- and third-order Douglas-Kroll-Hess Hamiltonians. In addition to the reaction enthalpies, heats of formation, bond lengths, and harmonic vibrational frequencies have been calculated for the stable triatomic products HgI2, HgIBr, HgICl, and HgIO. Accurate dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies have also been calculated for each of the diatomic molecules involved in this study (HgI, HgBr, HgCl, HgO, I2, IBr, ICl, and IO). The reported enthalpies are expected to have accuracies of 1 kcal/mol or better.