Benjamin C. Shepler

Roaming Dynamics in CH3CHO Photodissociation Revealed on a Global Potential Energy Surface

We present a quasiclassical trajectory study of the photodissociation of CH3CHO to molecular and radical products, CH4 + CO and CH3 + HCO, respectively, using global ab initio-based potentials energy surfaces. The molecular products have a well-defined potential barrier transition state (TS) but the dynamics exhibit strong deviations from the TS pathway to these products. The radical products are formed via a variational TS. Calculations are reported at total energies corresponding to photolysis wavelengths of 308, 282, 264, 248 and 233 nm. The results at 308 nm focus on a comparison with experiment [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] and the elucidation of the nature and extent of non-TS reaction dynamics to form the molecular products, CH4 + CO. At the other wavelengths the focus is the branching ratio of these products and the radical products, CH3 + HCO.

The group 12 metal chalcogenides: an accurate multireference configuration interaction and coupled cluster study

The near-equilibrium potential energy functions of the lowest two electronic states (1Σ+, 3Π) of the group 12 chalcogenides, i.e. {Zn, Cd, Hg} + {O, S, Se, Te, Po}, have been calculated from large-scale multireference configuration interaction (MRCI) and coupled cluster calculations. Use of a sequence of correlation consistent basis sets with accurate relativistic pseudopotentials allowed for the extrapolation of the potential functions to the complete basis set (CBS) limit. Inclusion of spin–orbit coupling yields an avoided crossing between the Ω = 0+ components of the 1Σ+ and 3Π states, which in some cases strongly affects the ground state spectroscopic constants. In almost all cases the calculated ground electronic state corresponds to , except for CdPo, HgSe, HgTe, and HgPo, where the 3Π2 state is calculated to be lowest. Spectroscopic constants and dissociation energies are determined for all states both before and after the inclusion of spin–orbit coupling. Accurate equilibrium dipole moments (without spin–orbit effects) are also reported for the 1Σ+ states. The present work confirms that the dissociation energies of all the group 12 chalcogenides except ZnO should be revisited by experiment.

Hg+Br-->HgBr recombination and collision-induced dissociation dynamics.

A global potential energy surface has been constructed for the system HgBr+Ar-->Hg+Br+Ar to determine temperature dependent rate constants for the collision-induced dissociation (CID) and recombination of Hg and Br atoms. The surface was decomposed using a many-body expansion. Accurate two-body potentials for HgBr, HgAr, and ArBr were calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)], as well as the multireference averaged coupled pair functional method. Correlation consistent basis sets were used to extrapolate to the complete basis set limit and corrections were included to account for scalar and spin-orbit relativistic effects, core-valence correlation, and the Lamb shift. The three-body potential was computed with the CCSD(T) method and triple-zeta quality basis sets. Quasiclassical trajectories using the final analytical potential surface were directly carried out on the CID of HgBr by Ar for a large sampling of initial rotational, vibrational, and collision energies. The recombination rate of Hg and Br atoms is a likely first step in mercury depletion events that have been observed in the Arctic troposphere during polar sunrise. The effective second order rate constant for this process was determined in this work from the calculated CID rate as a function of temperature using the principle of detailed balance, which resulted in k(T) = 1.2 x 10(-12) cm(3) molecule(-1) s(-1) at 260 K and 1 bar pressure.

Determination of the rate constant for sulfur recombination by quasiclassical trajectory calculations

The sulfur recombination reaction has been thought of as one of the most important chemical reactions in the volcanic activities of the planet. It is also important in determining the propagation of elemental sulfur in the atmosphere. There have been two experimental attempts to determine the reaction rate of the S+S-->S(2) recombination, however their results differ by four orders of magnitude. In this work, we determine the rate constant of S+S-->S(2) from quasiclassical trajectory calculations. The third order rate constant at 298.15 K predicted by the present calculations is 4.19 x 10(-33) cm(6) molecules(-2) s(-1), which is in excellent agreement with the determination of Fair and Thrush [Trans. Faraday Soc. 65, 1208 (1969)]. The temperature dependent rate constant is determined to be 3.94 x 10(-33) exp[205.56(1T-1298.15)], which was determined from the temperature range of 100-500 K.