Tunneling Isomerizations on the Potential Energy Surfaces of Formaldehyde and Methanol Radical Cations

Abstract

Formaldehyde (H2CO+) and methanol (H3COH+) radical cations, well-known in mass spectrometry, potentially form from radiative ionization or ion–molecule reactions in the interstellar medium. For both ions, other tautomeric forms exist that are accessible via [1,2]hydrogen shifts involving reaction barriers in excess of 25 kcal mol–1. Here, we compute the tunneling rates of the isomerization processes connecting the hydroxymethylene radical cation (HCOH+) to its more stable formaldehyde isomer (H2CO+) and the methanol radical cation (H3COH+) to its methylene oxonium isomer (H2COH2+) using the Wentzel–Kramers–Brillouin method at the CCSD(T)/cc-pVQZ//B3LYP/cc-pVTZ level of theory. While the hydroxymethylene radical cation features a half-life of over 3500 years and thus represents a potentially observable molecule, the methanol radical cation is predicted to decay with a half-life of about 4 days and is thus not likely to be present in appreciable quantities in space. We discuss the potential relevance of the...