The structure and dissociation pathways of protonated methanol: An ab initio molecular orbital study

Ab initio molecular orbital calculations with moderately large polarization basis sets and including valence-electron correlation have been used to examine the structure and dissociation mechanisms of protonated methanol [CH₃OH₂]⁺. Stable isomers and transition structures have been characterized using gradient techniques. Protonated methanol is found to be the only stable isomer in the [CH₅O]⁺ potential surface. There is no evidence for a tightly-bound complex, [HOCH₂]⁺…?H₂, analogous to the preferred structure [CH₃]⁺…?H₂ of [CH₅]⁺. Protonated methanol is found to possess a pyramidal arrangement of bonds at the oxygen atom with a barrier to inversion of 8kJ mol^?1. The lowest energy fragmentation pathways are dissociation into methyl cation and water (predicted to require 284 kJ mol^?1 with zero reverse activation energy) and loss of molecular hydrogen (endothermic by 138 kJ mol^?1 but with a reverse activation barrier of 149 kJ mol^?1). The results offer a possible explanation as to why production of [CH₂OH]⁺ from the reaction of methyl cation with water is not observed. Other dissociation processes examined include loss of a hydrogen atom to yield the methylenoxonium radical cation or methanol radical cation (requiring 441 and 490 kJ mol^?1, respectively) and loss of a proton to yield neutral methanol (requiring 784 kJ mol^?1).