Experimental and computational evidence for C=O π-bonding in [CH₂OH]⁺ and related oxonium ions

The question of whether [CH₂OH]⁺ should be described as the hydroxymethyl cation, ⁺CH₂OH, or protonated formaldehyde, CH₂=OH⁺, is reconsidered in the light of experimental information and new computational evidence. Previous arguments that the charge distribution in [CH₂OH]⁺ may be probed by considering the incremental stabilisation of [CH₂OH]⁺ induced by homologation on carbon (to give [CH₃CHOH]⁺) or oxygen (to produce [CH₂OCH₃]⁺) are critically examined. Cation stabilisation energies are shown to be better indicators of the nature of these oxonium ions. Further insight into the structure of larger C_nH_2n+1O⁺ oxonium ions is obtained by considering the site of protonation of enol ethers and related species. Computational information, including AIM (Atoms and Molecules) and NBA (Natural Bond Analysis) charges on the carbon and oxygen atoms in [CH₂OH]⁺ and related species, is considered critically. Particular attention is focused on the calculated bond lengths and barriers to rotation about the C-O bond(s) in [CH₂OH]⁺, [CH₃CHOH]⁺, [(CH₃)₂COH]⁺, CH₃OH and [CH₂OCH₃]⁺ and the C-N bond in [CH₂NH₂]⁺. Trends in these data are consistent with appreciable π-bonding only in the C-O connections which correspond to the C=O bond in the parent aldehyde or ketone from which the oxonium ion may be considered to be derived by protonation or alkyl cationation.