The reactions of CH3CHO + and of CH3COH + with water yield the same products, at almost the same rate. It is shown, by using a characteristic reaction of the carbene structure, that a molecule of water converts CH3COH + into its more stable isomer CH3CHO +, which is a new example of catalyzed 1,2-H transfer. The dominant product is the proton-bound dimer of water which, in fact, comes from the [H2OH+…CH3] and [H2OH+…CO] primary products whose observed abundances are poor. In a related system, ionized formamide/water, a water molecule catalyzes the 1,3-transfer leading from the solvated carbene to the [H2O…H+…H2N–C=O] stable intermediate, which eliminates CO without back energy. In contrast, such a process does not take place in the studied system since the cleavage of the so formed [H2OH+…CH3CO] transient intermediate involves a high back energy; this is explained by the charge repartition within this intermediate. In fact, a different pathway takes place. The solvated acetaldehyde ion isomerizes into a terbody intermediate in which protonated water is bonded to a CO molecule on the one hand and to a methyl radical on the other hand. Simple cleavages of this complex yield the observed products.
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