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 HSO₂ 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 HSO₂(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