Reactions between neutral molecules and cation-radicals in the gas-phase: Can protonation occur without proton transfer?

Abstract

Gas-phase collisions of Gly and Ala with free electrons were employed to form positive ions at well determined energies. At elevated molecular vapor pressure, protonated [Gly+H]+ and [Ala+H]+ ions were also observed. The appearance energies of [Gly+H]+ and [Ala+H]+ were both determined as 0.30±0.14eV higher than the ionization energies of their M+ congeners. Quantum chemical calculations predict proton transfer reactions rather than hydrogen atom abstraction reactions as the likely [M+H]+ forming method in our experiment. Microcanonical rate coefficient calculations using B3LYP/6-311++G(2d,p) frequencies and rotational constants, support abstraction of a Calpha proton for glycine, whereas the C-terminal proton abstraction mechanism was found as being more favorable for alanine. Additionally, substantial immonium ion formation was observed at experimental energies identical to either the protonated product or cation radical for glycine and alanine, respectively. Theoretical barriers to M+ cation radical fragmentation to HRCNH2+ indicate that they are product-limited and that the glycine barriers are generally larger than those for alanine.