Electron transfer to protonated beta-alanine N-methylamide in the gas phase: an experimental and computational study of dissociation energetics and mechanisms

Ammonium radicals derived from protonated beta-alanine N-methyl amide (BANMA) were generated by femtosecond collisional electron transfer to gas-phase cations prepared by chemical ionization and electrospray. Regardless of the mode of precursor ion preparation, the radicals underwent complete dissociation on the time scale of 5.15 micros. Deuterium isotope labeling and product analysis pointed out several competitive and convergent dissociation pathways that were not completely resolved by experiment. Ab initio calculations, which were extrapolated up to the CCSD(T)/6-311++G(3df,2p) level of theory, provided the proton affinity and gas-phase basicity of BANMA as PA = 971 kJ mol-1 and GB = 932 kJ mol-1 to form the most stable ion structure 1c+ in which the protonated ammonium group was internally solvated by hydrogen bonding to the amide carbonyl. Ion 1c+ was calculated to have an adiabatic recombination energy of 3.33 eV to form ammonium radical 1c*. The potential energy surface for competitive and consecutive isomerizations and dissociations of 1c* was investigated at correlated levels of theory and used for Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. RRKM unimolecular rate constants suggested that dissociations starting from the ground electronic state of radical 1c* were dominated by loss of an ammonium hydrogen atom. In contrast, dissociations starting from the B excited state were predicted to proceed by reversible isomerization to an aminoketyl radical (1f*). The latter can in part dissociate by N-Calpha bond cleavage leading to the loss of the amide methyl group. This indicates that apparently competitive dissociations observed for larger amide and peptide radicals, such as backbone cleavages and losses of side-chain groups, may originate from different electronic states and proceed on different potential energy surfaces.