We report the synthesis of glycine on interstellar ice-analog films composed of water, methylamine (MA), and carbon dioxide under irradiation of ultraviolet (UV) photons. Analysis of the UV-irradiated ice films by in situ mass spectrometric methods revealed glycine and other isomers as photochemical products. Deuterium-labeling experiments were conducted to determine the structures of the photoproducts and to examine their formation pathways. The reactions occur via photocleavages of C-H and N-H bonds in MA, followed by subsequent reactions of the nascent H atom with CO 2 , leading to the formation of HOCO and then to glycine and carbamic acid. The photochemical synthesis of glycine occurs efficiently at the ice surfaces, and the competing photosynthesis and photodestruction processes can reach a steady-state kinetic balance at an extended UV exposure, maintaining a substantial population level of glycine. The observation suggests that interstellar amino acids can be created on ice grains, and that they can also be stored in the ices by maintaining a kinetic balance under interstellar UV irradiation. As such, the transport of amino acids in interstellar space may be possible without depleting the net abundance of amino acids in the ices but rather increasing the structural diversity of the molecules.
Proton transfer from the hydronium ion to NH(3), CH3NH2, and (CH3)2NH is examined at the surface of ice films at 60 K. The reactants and products are quantitatively monitored by the techniques of Cs+ reactive-ion scattering and low-energy sputtering. The proton-transfer reactions at the ice surface proceed only to a limited extent. The proton-transfer efficiency exhibits the order NH3>(CH3)NH2=(CH3)2NH, which opposes the basicity order of the amines in the gas phase or aqueous solution. Thermochemical analysis suggests that the energetics of the proton-transfer reaction is greatly altered at the ice surface from that in liquid water due to limited hydration. Water molecules constrained at the ice surface amplify the methyl substitution effect on the hydration efficiency of the amines and reverse the order of their proton-accepting abilities.
We studied the initial-stage mechanism of the electrophilic addition reaction of ethene with HCl by examining the interactions between ethene and HCl on water-ice and frozen molecular films at temperatures of 80-140 K. Cs(+) reactive ion scattering (RIS) and low-energy sputtering (LES) techniques were used to probe the reaction intermediates that were kinetically trapped on the surface, in conjunction with temperature-programmed desorption (TPD) mass spectrometry to monitor the desorbing species. The reaction initially produced the π complex of HCl and ethene at temperatures below about 93 K and an "ethyl cationic species" at temperatures below about 100 K. The ethyl cationic species was formed via direct proton transfer from the HCl molecule to ethene with the assistance of water solvation, rather than via the interaction of hydronium ions and ethene. At high temperatures, this species dissociated into ethene and hydronium and chloride ions. The reaction did not, however, complete the final transition state on the ice surface to produce ethyl chloride. The observation gives evidence that the electrophilic addition reaction of ethene occurs through an ethyl-like intermediate with an ionic character.
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