Previous studies have shown that the solitary ketene-water ion CH2=C(=O)OH2+ (1) does not isomerize into CH2=C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2=O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2C=O+. 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, 29 kcal mol-1, is reduced to mere 0.6 kcal mol-1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis
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