A Theoretical Study on the Relevance of Protonated and Ionized Species of Methanimine and Methanol in Astrochemistry

摘要

Under the low T conditions of the interstellar medium or planetary atmospheres like that of Titan, which presents a surface temperature of 94 K and a temperature of ca. 180 K in the upper layers of the atmosphere, reactions involving neutral species, which usually show relatively high or even huge energy barriers, are not efficient processes unless a significant external energy source is provided and transient species, like atomic or molecular radicals, are involved. A completely different picture holds when ionized or protonated species are involved, since most reactions in this case are barrierless. In order to show this point we will present two case studies: the dimerization of methanimine and the production of dimethyl ether from methanol. Methanimine is a molecule of interest in astrobiology, as it is considered an abiotic precursor of the simplest amino acid glycine. Methanimine has been observed in the interstellar medium and in the upper atmosphere of Titan. In particular, it has been speculated that its polymerization can contribute to the formation of the haze aerosols that surround the massive moon of Saturn. To assess its potential role in the formation of Titan's aerosol, we have performed a theoretical investigation of a possible dimerization process. The aim of this study is to understand whether dimerization of methanimine and, eventually, its polymerization, is possible under the conditions of the atmosphere of Titan. The second case study presented is the reaction of methanol with protonated methanol which has been considered for a long time to be an important step towards the formation of gaseous interstellar dimethyl ether. Results of high-level electronic structure calculations are reported.