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 [CH 3 OH 2 ] + . Stable isomers and transition structures have been characterized using gradient techniques. Protonated methanol is found to be the only stable isomer in the [CH 5 O] + potential surface. There is no evidence for a tightly‐bound complex, [HOCH 2 ] + …H 2 , analogous to the preferred structure [CH 3 ] + …H 2 of [CH 5 ] + . 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 [CH 2 OH] + 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 ).