Coupled-Cluster Study of the Lower Energy Region of the Ground Electronic State of the HSO2 Potential Energy Surface.

This work reports CCSD(T)/aug-cc-pV(T+d)Z ab initio calculations for the lower energy region of the ground electronic state of the HSO2 system. Optimized geometries, total energies, zero-point vibrational energies, frequencies, complete basis set extrapolations, and reaction paths are reported at the same level of calculation. The connection of the two minima (synperiplanar HOSO and HSO2) with the dissociation limit H + SO2 through the van der Waals minimum H···SO2 was established. An important quantitative discrepancy with previous works is the fact that at the present level of calculation the energy difference between transition states connecting the global minimum synperiplanar HOSO to the HSO2 minimum (TS5) and to the van der Waals minimum H···SO2 (TS6) is negligible, implying that the forward barriers after the synperiplanar HOSO global minimum have practically the same height. This result suggests that these two transition states may be involved in the path of the global minimum toward the exit channel H + SO2. As a consequence, trajectories for the OH + SO collisions could evolve through the well formed by the HSO2 minimum, therefore opening two competitive channels for the OH + SO → H + SO2 reaction, a fact never reported in trajectory calculations.

Connection between the upper and lower energy regions of the potential energy surface of the ground electronic state of the HSO2 system.

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless, controversy still exists about a possible connection between the upper and lower energy regions of the potential energy surface (PES) for the ground electronic state of the system. Very recently, a path to connect these regions was proposed based on studies at the CASPT2/aug-cc-pV(T+d)Z level of calculation but the small energy difference between some of the transitions states along that path suggested the necessity of calculations at a higher level of theory. In the present work, we report a CCSD(T)/aug-cc-pV(T+d)Z study of the stationary states associated to the proposed connection path, including assessment of the most reliable complete basis set (CBS) extrapolation scheme for the system. Among the new features, the present calculations show that there are no structures corresponding to the HSO(2)(b) minimum and the TS3 saddle point obtained at the CASPT2 level and that the connection path between the upper and lower energy regions of the PES for the ground electronic state involves only one transition state and most probably more than one electronic state.