Rotational and Infrared Spectroscopy of Ethanimine: A Route toward Its Astrophysical and Planetary Detection

Abstract

This%is%the%final%accepted%manuscript%of:% % A.% Melli,% M.% Melosso,% N.% Tasinato,% et% al.% ROTATIONAL% AND% INFRARED% SPECTROSCOPY% OF% ETHANIMINE:%A%ROUTE%TOWARDS%ITS%ASTROPHYSICAL%AND%PLANETARY%DETECTION.%Astrophys.%J.% 855,%123%(2018);%DOI:%10.3847/1538U4357/aaa899% % Available%at:%https://doi.org/10.3847/1538U4357/aaa899% % ABSTRACTEthanimine, a possible precursor of amino acids, is considered an important prebiotic molecule and thus may play important roles in the formation of biological building-blocks in the interstellar medium. In addition, its identification in Titan's atmosphere would be important for understanding the abiotic synthesis of organic species. An accurate computational characterization of the molecular structure, energetics and spectroscopic properties of the E and Z isomers of ethanimine, CH 3 CHNH, has been carried out by means of a composite scheme based on coupled-cluster techniques, which also accounts for extrapolation to the complete basis-set limit and core-valence correlation correction, combined with density functional theory for the treatment of vibrational anharmonic e↵ects. By combining the computational results with new millimeter-wave measurements up to 300 GHz, the rotational spectrum of both isomers can be accurately predicted up to 500 GHz. Furthermore, our computations allowed us to revise the infrared spectrum of both E -and Z -CH 3 CHNH, thus predicting all fundamental bands with high accuracy.

A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine

“…Starting from this intermediate, therefore, formation of E-,Z-CH3CHNH + H would be by far the dominant reaction channels.Finally, according to the present CCSD(T)/aug-cc-pVTZ level) calculations, the energy gap between E-,Z-CH3CHNH is 2.9 kJ/mol, a value which compares very well with that determined byMelli et al at the CCSD(T)/CBS+CV level corrected for the anharmonic ZPE at the B2PLYP-D3BJ/maug-cc-pVTZ-dH level (2.77 kJ/mol). The E-Z-interconversion barrier is found to be 117.9 kJ/mol, again in excellent agreement with the calculations byMelli et al (2018).As we are going to see in Sec. 3.2, not only this portion of the PES is the one of interest to characterize the formation of aldimines, but is also the part experienced by most of the reactive flux.…”

“…The Z-isomer is less stable than the E-isomer by 4.60 kJ/mol according to the calculations by Quan et al (2016). This value has been refined to 2.77 kJ/mol in higher level calculations by Melli et al (2018). The isomerization barrier is instead very high (115.8 kJ/mol according to the high level calculations of Melli et al) because it is necessary to break the double bond of C=N to move from the Z-to E-isomers and vice versa.…”

Prebiotic molecules in interstellar space: Rotational spectroscopy and quantum chemistry

“…The absolute values of the dipole moment components are also reported. The red circles highlight the frequency ranges for which measurements were available prior to the investigation by Melli et al (2018).…”

“…Nuclear quadrupole coupling constants at the CCSD(T)/cc-pCVQZ level augmented by vibrational corrections at the B2PLYP-D3BJ/maug-cc-pVTZ-dH level. For details, see Melli et al (2018). b Watson A-reduction.…”

“…The presence of a finite V 3 barrier, due to the hindered internal rotation of the methyl group, leads to a splitting of the threefold degeneracy into two levels, a nondegenerate A level and a doubly-degenerate E level, which renders the rotational spectrum rather complicated. The reader is referred to Melli et al (2018) for a detailed account.…”