Competition in the formation of ion-neutral complexes. Expulsion of methane from the molecular ion of acetamide

Abstract

An ion–neutral complex is a non-covalently bonded aggregate of an ion with one or more neutral molecules in which at least one of the partners rotates freely (or nearly so) in all directions. A density-of-states model is described, which calculates the proportion of ion–neutral complex formation that ought to accompany simple bond cleavages of molecular ions. Application of this model to the published mass spectrum of acetamide predicts the occurrence of ions that have not hitherto been reported. Relative intensities on the order of 0.1 (where the abundance of the most intense fragment ion = 1) ere predicted for [M – HO]+ and [M – CH4]+˙ ions, which have the same nominal masses as the prominent [M – NH3]+˙ and [M – NH2]+ fragments. High-resolution mass spectrometric experiments confirm the presence of the predicted fragment ions. The [M – HO]+ and [M – CH4]+˙ fragments were observed with relative abundances of 0.02 and 0.04, respectively. Differences between theory and experiment may be ascribed to effects of competing distonic ion pathways.