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

The question of whether [CHOH] should be described as the hydroxymethyl cation, CHOH, or protonated formaldehyde, CH=OH, is reconsidered in the light of experimental information and new computational evidence. Previous arguments that the charge distribution in [CHOH] may be probed by considering the incremental stabilisation of [CHOH] induced by homologation on carbon (to give [CHCHOH]) or oxygen (to produce [CHOCH]) 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 CHO 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 [CHOH] 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 [CHOH], [CHCHOH], [(CH)COH], CHOH and [CHOCH] and the C-N bond in [CHNH]. 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.