DEACETYLATION OF GASEOUS AMIDOALKYLATING REAGENTS (NORMAL-ACYLIMINIUM IONS) VIA IMINIUM KETENE COMPLEXES - AN UNPRECEDENTED ISOTOPE EFFECT

The 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-d1) and CD3C(= O)N(D) = CH2+ (a-d4) cleanly form CH2NHD+ + CH2 = C = O and CH2ND2+ + CD2 = C = O, respectively, the isotopomer CD3C(= O)-N(H) = CH2+ (a-d3) does not produce CH2NHD+ + CD2 = C = O, but rather CH2ND2+ + CHD = C = O. By integration of results obtained from mass spectrometry-based experiments (metastable 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 O = C = CH2...H...N(H)-CH2+ (b) which then dissociates endothermically. The equilibration a half arrow right over half arrow left 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 vibrational energies (ZPVEs) of the sets of products CH2 = NHD+ + CD2 = C = O versus CH2 = ND2+ + CHD = C = O. The N-H bond in CH2 = NH2+ is shorter, stronger and has a higher harmonic frequency than the C-H bond in ketene and thus the decrease in ZPVE is larger for CH2 = ND2+ + CHD = C = O than for CH2 = NHD+ + CD2 = C = O (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 H2C = C = O...H...N(H) = CH2+ (c), CH2OCN(H)CH3+, CH2 = C(OH)N(H)CH2+ (e), H2NCH2CH2C = O+ (f), CH3C(OH) = N = CH2+ (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 chemistry of a.