Formaldehyde mediated proton-transport catalysis in the ketene–water radical cation CH2C(O)OH2+

Previous studies have shown that the solitary ketene–water ion CH2C(O)OH2+ (1) does not isomerize into CH2C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2CO+.A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH2C(O)O(H)–H⋯OCH2]+ in which the CH2O molecule catalyzes the transformation 1→2 prior to dissociation. The barrier for the unassisted reaction, 29kcalmol−1, is reduced to a mere 0.6kcalmol−1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis.