Deacetylation of gaseous amidoalkylating reagents (N‐acyliminium ions) via iminium/ketene complexes: An unprecedented isotope effect
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AbstractThe prototype amidoalkylating reagent, the N‐acetylmethyliminium ion CH3C(O)N(H)CH2+ (a), was generated in the gas phase and its unimolecular chemistry was studied. Metastable ions a dissociate by deacetylation to CH2NH2+ with an exceedingly small translational energy release (0.03 kJ mol−1). A remarkable ‘all‐or‐nothing’ deuterium isotope effect is associated with this reaction. Hence although the deuterium‐labelled isotopomers CH3C(O)N(D)CH2+ (a‐d4) and CD3C(O)N(D)CH2+ (a‐d4) cleanly form CH2NHD+ + CH2CO and CH2ND2+ + CD2C‐O, respectively, the isotopomer CD3C(O)‐N(H)CH2+ (a‐d3) does not produce CH2NHD+ + CD2CO, but rather CH2ND2+ + CHDCO. By integration of results obtained from mass spectrometry‐based experiments (metustable ion (MI) and collisional activation (CA) Spectrometry and D‐labelling) and from ab initio molecular orbital calculations executed at the MP3/6–31G*//4–31G level of theory, a mechanism for deacetylation could be derived. The reaction commences with elongation of the C(O)N(H) bond in a and this is accompanied by a proton transfer to produce the hydrogen‐bridged complex OCCH2 …︁ H …︁ N(H)CH2+ (b) which then dissociates endothermically. The equilibration a ⇌ b leads to complete exchange of the NH and CH3 hydrogen atoms in a. The isotope effect observed for a‐d3 is interpreted in terms of differences in the zero‐point vibratioual energies (ZPVEs) of the sets of products CH2NHD+ + CD2CO versus CH2ND2+ + CHDCO. The NH bond in CH2NH2+ is shorter, stronger and has a higher harmonic frequency than the CH bond in ketene and thus the decrease in ZPVE is larger for CH2ND2+ + CHDCO than for CH2NHD+ + CD2CO (by 1.7 kJ mol−1). It is argued that such isotope effects can be expected for dissociations of ion‐molecule complexes and this provides a powerful indication for their intermediacy. The isomeric C3H6NO+ ions H2CCO …︁H…︁ N(H)CH2+ (c), magnified image CH2C(OH)N(H)CH2+ (e), H2NCH2CH2CO+ (f), CH3C(OH)NCH2+ (g) and protonated azetidin‐2‐one (h) were also briefly examined and characterized but with the possible exception of c and f they do not participate in the gas‐phase ion chemist of a.
