Deacetylation of gaseous amidoalkylating reagents (N‐acyliminium ions) via iminium/ketene complexes: An unprecedented isotope effect

Abstract The prototype amidoalkylating reagent, the N ‐acetylmethyliminium ion CH 3 C(O)N(H)CH 2 + (a), was generated in the gas phase and its unimolecular chemistry was studied. Metastable ions a dissociate by deacetylation to CH 2 NH 2 + 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 CH 3 C(O)N(D)CH 2 + ( a ‐ d 4 ) and CD 3 C(O)N(D)CH 2 + ( a ‐ d 4 ) cleanly form CH 2 NHD + + CH 2 CO and CH 2 ND 2 + + CD 2 C‐O, respectively, the isotopomer CD 3 C(O)‐N(H)CH 2 + ( a ‐ d 3 ) does not produce CH 2 NHD + + CD 2 CO, but rather CH 2 ND 2 + + CHDCO. 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 OCCH 2 …︁ H …︁ N(H)CH 2 + ( b ) which then dissociates endothermically. The equilibration a ⇌ b leads to complete exchange of the NH and CH 3 hydrogen atoms in a . The isotope effect observed for a ‐ d 3 is interpreted in terms of differences in the zero‐point vibratioual energies (ZPVEs) of the sets of products CH 2 NHD + + CD 2 CO versus CH 2 ND 2 + + CHDCO. The NH bond in CH 2 NH 2 + 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 CH 2 ND 2 + + CHDCO than for CH 2 NHD + + CD 2 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 C 3 H 6 NO + ions H 2 CCO …︁H…︁ N(H)CH 2 + ( c ), magnified image CH 2 C(OH)N(H)CH 2 + ( e ), H 2 NCH 2 CH 2 CO + ( f ), CH 3 C(OH)NCH 2 + ( 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 .