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Interaction with polar neutral molecules can cause a-distonic radical cations and conventional molecular ions t o interconvert in the gas phase, in spite of substantial energy barriers for the unassisted isomerization; the interconversion is particularly facile when the proton affinity of the neutral molecule is close t o that of the deprotonated radical cation.

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The reaction of ionized formamide H(2)NCHO(*)(+) with water leads to an exclusive loss of CO from the complex. This contrasts with the unimolecular reaction of low-energy ionized formamide, which loses exclusively one hydrogen atom. The unimolecular loss of CO is not observed because it involves several H-transfers corresponding to high-energy barriers. Experimental and theoretical studies of the role of solvation by water on the fragmentation of ionized formamide leads to three different results: (i) In contrast with different systems previously studied, in which solvation plays only a role on one or two steps of a reaction, a molecule of water is efficient in the catalysis of the decarbonylation process because water catalyzes all the steps of the reaction of ionized formamide, including the final dissociation of the amide bond. (ii) The catalyzed isomerization of carbonylic radical cations into their carbene counterparts is shown to be an important step in the process. To study this step, a precise probe, characterizing the carbene structure by ion-molecule reaction, is for the first time described. (iii) Finally, decarbonylation of ionized formamide yields the [NH(3), H(2)O](*)(+) ion, which has not been generated and experimentally studied previously. By this method, the [NH(3), H(2)O](*)(+) ion is generated in abundance and with a low internal energy content, allowing one either to prepare, by ligand exchange, a series of other solvated radical cations or to generate covalent structures such as distonic ions. First results on related systems indicate that the conclusions obtained for ionized formamide are widespread.

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By using a new “titration” technique, the activation barrier for the strictly unimolecular 1,3‐hydrogen migration 1.+→2.+ has been determined (0.74±0.06 eV; see reaction profile). With this technique the internal energy of 1.+ is controlled by variable photoionization of acetamide and the chemistry of the resulting enol ion 2.+ is then probed by structure‐specific ion–molecule reactions.

Spontaneous and catalyzed isomerizations of the acetamide radical cationThe use of mass spectrometry for the study of peptides (sequencing, cationization, H-D exchanges, etc.) is a rapidly growing field. 1 Therefore, the chemistry of ionized amides, studied by mass spectrometry, is of considerable interest. This letter reports some new findings concerning the unimolecular and bimolecular chemistry of ionized acetamide, mainly focused on spontaneous and catalyzed 1,3-H migrations, resulting in tautomerization of the acetamide radical cation.Among the four possible tautomers 1-4 of ionized acetamide (Scheme 1), only 1 and 2 have been experimentally described, 2 although no definitive proof was given of their structures. Structure 1 was assigned on the basis of direct electron ionization of acetamide, and structure 2 was logically given to the m/z 59 ion from ionized hexanamide, as the most stable fragment resulting from the McLafferty rearrangement.G2(MP2) level).

The mechanism of the oxidation of alcohols by silver carbonate on Celite was thoroughly investigated to ascertain the nature of the transition state and the possible intervention of reaction intermediates. Kinetic, stereochemical, and isotopic labeling techniques were used to differentiate among the various theoretically plausible mechanistic alternatives. The effects of surface adsorption and solvent composition on the outcome of the reaction were also studied. The data were consistent with a concerted process for which a model is proposed.