Energetics of the protonation of CO: Implications for the observation of HOC+ in dense interstellar clouds

Abstract

In this study, we report a series of large scale calculations for several studies on the H₃CO⁺ potential energy hypersurface. This study was motivated by the recent observation of an abundance ratio of HCO⁺/HOC⁺ of at least 330 if the species HOC⁺ is indeed present in dense interstellar clouds. The calculations have focused on an accurate determination of the absolute proton affinities for the species H₂, HCHO, and most importantly of CO to form both HCO⁺ and HOC⁺. These calcualtions have been performed using large polarized basis sets and include the effects of electron correlation at the CI(SDQ) level. Furthermore, the effects of zero point energy corrections have been incorporated by use of the calculated harmonic frequencies. This study has shown that the proton affinities of the species H₂ and CO to form HOC⁺ are within 1 kcal of each other. The calculations demonstrate that there is no thermodynamic driving force to form HOC⁺ in collisions of H⁺₃ with CO and that the formation of HCO⁺ in such collision is very exoergic. We have suggested a plausible mechanism to explain the differences observed between the laboratory and the interstellar medium. Finally, we have investigated the possibility of the formation of a molecular complex between H₂ and HCO⁺/HOC⁺ as a sink of much of the HOC⁺ which may be formed.