Surprising Quenching of the Spin–Orbit Interaction Significantly Diminishes H2O···X [X = F, Cl, Br, I] Dissociation Energies

Toward Accurate Spin-Orbit Splittings from Relativistic Multireference Electronic Structure Theory

Most nonrelativistic electron correlation methods can be adapted to account for relativistic effects, as long as the relativistic molecular spinor integrals are available, from either a four-, two-, or one-component mean-field calculation. However, relativistic multireference correlation methods remain a relatively unexplored area, with mixed evidence regarding the improvements brought by perturbative treatments. We report, for the first time, the implementation of state-averaged four-component relativistic multireference perturbation theories to second and third order based on the driven similarity renormalization group (DSRG). With our methods, named 4c-SA-DSRG-MRPT2 and 3, we find that the dynamical correlation included on top of 4c-CASSCF references can significantly improve the spin-orbit splittings in p-block elements and potential energy surfaces when compared to 4c-CASSCF and 4c-CASPT2 results. We further show that 4c-DSRG-MRPT2 and 3 are applicable to these systems over a wide range of the flow parameter, with systematic improvement from second to third order in terms of both improved error statistics and reduced sensitivity with respect to the flow parameter.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl^•(H₂O)_n=1-4. Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H₂O)_n-1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical-water complexes.

Pathways for the OH + Cl2 → HOCl + Cl and HOCl + Cl → HCl + ClO Reactions

High level coupled-cluster theory, with spin-orbit coupling evaluated via the Breit-Pauli operator in the interacting-states approach, is used to investigate the OH radical reaction with Cl2 and the subsequent reaction HOCl + Cl. The entrance complex, transition state, and exit complex for both reactions have been determined using the CCSD(T) method with correlation consistent basis sets up to cc-pV6Z. Also reported are CCSDT computations. The OH + Cl2 reaction is predicted to be endothermic by 2.2 kcal/mol, compared to the best experiments, 2.0 kcal/mol. The above theoretical results include zero-point vibrational energy corrections and spin-orbit contributions. The activation energy (Ea) of the OH + Cl2 reaction predicted here, 2.3 kcal/mol, could be as much as 1 kcal/mol too high, but it falls among the four experimental Ea values, which span the range 1.1-2.5 kcal/mol. The exothermicity of the second reaction HOCl + Cl → HCl + ClO is 8.4 kcal/mol, compared to experiment 8.7 kcal/mol. The activation energy for latter reaction is unknown experimentally, but predicted here to be large, 11.5 kcal/mol. There are currently no experiments relevant to the theoretical entrance and exit complexes predicted here.

Energy disposal and thermal rate constants for the OH + HBr and OH + DBr reactions: quasiclassical trajectory calculations on an accurate potential energy surface

We report reaction cross sections, energy disposal, and rate constants for the OH + HBr → Br + H2O and OH + DBr → Br + HDO reactions from quasiclassical trajectory calculations using an ab initio potential energy surface [ de Oliveira-Filho , A. G. S. ; Ornellas , F. R. ; Bowman , J. M. J. Phys. Chem. Lett. 2014 , 5 , 706 - 712 ]. Comparison with available experiments are made and generally show good agreement.