Ab Initio Treatment of Bond-Breaking Reactions: Accurate Course of HO3 Dissociation and Revisit to Isomerization.

An efficient scheme is devised for accurate studies of bond-breaking/forming reactions and illustrated for HO3. It is suggested and numerically demonstrated that an accurate dissociation path for the title system can be obtained by defining the central OO bond as the reaction coordinate. The approach consists of optimizing the dissociation path at the full-valence-complete-active space level of theory followed by single-point multireference configuration interaction calculations along it. Using large diffusely augmented basis sets of the correlation consistent type, accurate dissociation curves are then obtained for both the cis- and trans-HO3 isomers by extrapolating the calculated raw energies to the complete basis set limit. The profiles show a weak van der Waals type minimum and a small barrier, both lying below the dissociation asymptote. It is shown that this barrier arises at the break-off of the central OO chemical bond and onset of the OH···O2 hydrogen bond. The calculated dissociation energies (De) are 4.5 ± 0.1 and 4.7 ± 0.1 kcal mol(-1) for the cis- and trans-HO3 isomers, respectively, with a very conservative estimate of the dissociation energy (D0) for trans-HO3 being 2.7 ± 0.2 kcal mol(-1) and a more focused one being 2.8 ± 0.1 kcal mol(-1). This result improves upon our previous estimate of this quantity while overlapping in the lower range of 2.9 ± 0.1 kcal mol(-1), the commonly accepted value from the low-temperature CRESU experiments. Since the cis-HO3 isomer is predicted to be 0.15 kcal mol(-1) less stable than trans-HO3, this may partly explain the failure to obtain a clear characterization of the former. The isomerization (torsional) potential is also revisited and a comparison presented with a curve inferred from spectroscopic measurements. Good agreement is observed, with the accuracy of the new calculated data commending its use for the reanalysis of the available vibrational-rotational spectroscopic data.