Measurements of the kinetic energy released in decompositions of proton-bound dimers of primary amines

Abstract

The kinetic energy release (KER) associated with the decomposition of protonated dimers into the protonated and neutral monomer has been measured for a homologous series of primary monoamines, primary diamines, monobasic α-amino acids and dibasic α-amino acids by linked scans (B 2/E) on a two-sector instrument and by mass-analyzed ion kinetic energy spectroscopy (MIKES) on a four-sector instrument. The T 0.5 values were between 16.1 meV and 26.1 meV for the linked scans and between 10.3 meV and 17.7 meV for the MIKE spectra. For the protonated dimers of monoamines and monobasic amino acids, the KER of the decomposition increases with the number of degrees of freedom of the ion. For the protonated dimers of diamines and dibasic amino acids, the KER of the decomposition reaction is independent of the number of degrees of freedom. This difference must reflect a difference in the potential energy surfaces across which the decomposition of the two types of protonated dimers takes place. There seems to be a tendency of KER for decomposition of the proton-bound dimers of primary amines to fall with the proton affinity (PA) of the neutral within an isomeric series. This indicates a correlation between the PA of the amine and the critical energy for decomposition of the protonated dimer. For the protonated dimers of monoamines, the results are in agreement with the assumption that the reaction occurs without a barrier for the reverse reaction.