Dissociation of proton-bound complexes and proton affinity of benzamides

Abstract

Proton-bound heterodimers of substituted benzamides 1–15 and N,N-dimethyl benzamides 16–30, respectively, with a series of reference bases were generated under chemical ionization conditions. Their dissociation into the protonated amide AH + and protonated reference base BH + was studied by metastable ion techniques and by collision-induced dissociation (CID) to examine substituent effects on the proton affinity (PA) of the benzamides and to elucidate some aspects of the dissociation dynamics of proton-bound clusters. The PAs of the substituted benzamides were determined by bracketing the amide by a pair of reference bases to give rise to more and less abundant signals of the protonated base in the mass-analyzed ion kinetic energy (MIKE) spectra of the proton-bound heterodimers. The substituent effects observed agree with O-protonation in both the primary and the tertiary benzamides. However, the susceptibility of the benzamide to polar substituent effects is remarkably small, which indicates a “resonance saturation” of the amide group. The relative abundances of AH + and BH + in the MIKE and collisional activation (CA) mass spectra depend strongly on the pressure of the collision gas during CID, and in certain cases a reversal of the relative abundances with increasing pressure that favors the formation of BH + from a less basic reference base is observed. Although this effect underlines the limited possibilities of the “kinetic method” for PA determination by CID of proton-bound heterodimers, it uncovers important kinetic effects during the dissociation of proton-bound heterodimers and of proton transfer reactions in the gas phase. In the case of the protonated amide clusters, the observed intensity effects in the CA mass spectra are explained by a double-well potential energy surface caused by solvation of the protonated base by the polar amide in the protonated heterodimer.