The Role of State-of-the-Art Quantum-Chemical Calculations in Astrochemistry: Formation Route and Spectroscopy of Ethanimine as a Paradigmatic Case.

Carmen Baiano,Jacopo Lupi,Vincenzo Barone,Nicola Tasinato,Cristina Puzzarini

doi:10.3390/molecules25122873

Carmen Baiano, Jacopo Lupi + Show 3 more

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https://doi.org/10.3390/molecules25122873

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Abstract

The gas-phase formation and spectroscopic characteristics of ethanimine have been re-investigated as a paradigmatic case illustrating the accuracy of state-of-the-art quantum-chemical (QC) methodologies in the field of astrochemistry. According to our computations, the reaction between the amidogen, NH, and ethyl, C2H5, radicals is very fast, close to the gas-kinetics limit. Although the main reaction channel under conditions typical of the interstellar medium leads to methanimine and the methyl radical, the predicted amount of the two E,Z stereoisomers of ethanimine is around 10%. State-of-the-art QC and kinetic models lead to a [E−CH3CHNH]/[Z−CH3CHNH] ratio of ca. 1.4, slightly higher than the previous computations, but still far from the value determined from astronomical observations (ca. 3). An accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the E and Z isomers of ethanimine combined with millimeter-wave measurements up to 300 GHz, allows for predicting the rotational spectrum of both isomers up to 500 GHz, thus opening the way toward new astronomical observations.

Highlights

Until the latter part of the 20th century, it was believed that the vastness of interstellar space consisted mainly of hydrogen atoms
The thermochemical aspects of the gas-phase reaction between the ethyl and imidogen radicals are first discussed, and the attention is focused on the results of RRKM kinetic simulations
Since the channel leading to CH3 + CH2 NH requires the smallest number of steps and the lowest barrier (TS2; breaking of the C – C bond), thermodynamic considerations should favor this process

Summary

Introduction

Until the latter part of the 20th century, it was believed that the vastness of interstellar space consisted mainly of hydrogen atoms. Apart from molecular hydrogen and a few other simple diatomic species, the harsh conditions of the interstellar medium (ISM) were thought to be incompatible with polyatomic molecules exhibiting even a small degree of complexity This idea began to be questioned roughly fifty years ago with the discovery of the first ‘complex’ molecules (formaldehyde in 1969 [1], methanol in 1970 [2], and formic acid in 1971 [3]). Molecules detected in space range from simple hydrides (such as H2 , H2 O, and NH3 ), to hydrocarbon chain species (such as the cyanopolyynes, HC2n+1 N), to simple organics (such as alcohols and aldehydes), to highly reactive species such as ions and radicals (e.g., H2 COH+ , HC3 NH+ , and CH3 O), to polycyclic aromatic hydrocarbon molecules (PAHs) [5] The latter are the most abundant polyatomic species in space, no individual PAH has yet been identified. The contribution of hydrogen and helium amounts to about 98%; that of heavier elements

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