Thermodynamic and kinetic aspects of glycine and its radical cation under interstellar medium conditions

Abstract

Glycine is the smallest amino acid, frequently found in meteorites samples and interstellar ices. Studies about its formation and survival on the hostile interstellar medium (ISM) have been reported. In this work, the hypothesis that high-energy collision phenomena promote desorption of a fraction of the icy glycine to the gas phase, possibly assisted by molecular ionization, is assumed. In order to simulate the chemical behaviour of glycine and its radical cation under interstellar environments, a decomposition kinetics scheme is proposed. Calculations have been performed at the CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) level. Rate coefficients have been calculated at different temperatures (50–300 K), adopting the canonical variational transition state theory. In order to mitigate the ISM non-thermalized conditions, microcanonical variational rate coefficients have also been predicted. Eight stationary points, characterized as minimum energy points, have been located for neutral glycine, while four have been found for the glycine radical cation. The lowest energy conformer of neutral form accounts for 75 per cent of the population, whereas the radical cation global minimum accounts for approximately 100 per cent, at 300 K. The most favourable decomposition channel for neutral glycine is the deamination, with a barrier height of 44.76 kcal mol−1. The most favourable reaction channel for glycine radical cation is the dissociation forming |${\rm H}$|⁠, |${\rm CO_{2}}$|⁠, and |$\rm{CH_{2}NH_{2}^{+}}$| with the dissociation limit of 18.03 kcal mol−1, with respect to the lowest energy conformer.