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

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.

Gas-phase Chemistry in the Interstellar Medium: The Role of Laboratory Astrochemistry

“…As mentioned above, C-substituted imines such as cyanomethanimine, ethanimine and propargylimine exist in two forms: Z and E (Melosso et al, 2018;Baiano et al, 2020;Lupi et al, 2020a). As pointed out in several works (see, e.g., Zhang et al (2020);Shingledecker et al (2020)), known reaction mechanisms are not able to explain the isomer abundance ratio obtained from astronomical observations.…”

“…It thus became clear that chemical reactions occurring on the surface of dust grains also play a crucial role in the interstellar chemistry (Caselli et al, 2004;Cuppen et al, 2017). However, the contribution of gas-phase chemistry cannot be overlooked and, indeed, a full explanation of observed molecular abundances is often obtained by accounting for both reactivity on grains and in the gas phase (Ruaud and Gorti, 2019;Laas et al, 2011;Codella et al, 2017;Baiano et al, 2020;Shingledecker et al, 2020). Despite the awareness of a rich interstellar chemistry occurring both on grains and in the gas phase, we are very far from sufficient knowledge to fully understand it: much information is still missing and only a small fraction of the elementary reactions have actually been adequately characterized.…”

Accuracy Meets Interpretability for Computational Spectroscopy by Means of Hybrid and Double-Hybrid Functionals

“…In fact, the increasing sophistication of well-established techniques like nuclear magnetic and electron paramagnetic resonance (NMR and EPR), microwave (MW), infrared (IR), Raman, visible (Vis), ultra-violet (UV), or fluorescence and the parallel blooming of new ones, e.g., vibrational, electronic, and magnetic circular dichroism (VCD, ECD, and MCD), Raman optical activity (ROA), circularly polarized luminescence (CPL), multi-photon and time-resolved methods have a huge impact in several fields of science and technology (He et al, 2007;Barone, 2012;Berova et al, 2012;Sugiki et al, 2017;Lane, 2018). In addition to being widely used to infer information about molecular structure and dynamics in both gas and condensed phases (Sugiki et al, 2017;Puzzarini and Barone, 2018), spectroscopy allows for the unequivocal identification of chemical species in hostile environments, e.g., the interstellar space (Baiano et al, 2020), or in samples of unknown composition (He et al, 2007;Lindon et al, 2017;Lane, 2018) and plays a pivotal role in the study of photochemical mechanisms in biological systems and in the development of new technological devices, including photovoltaic cells, optoelectronic devices, eco-sustainable solutions, UV-resistant materials, dyes, and fluorescent probes (Berova et al, 2012;Drummen, 2012;Lindon et al, 2017;Lane, 2018). Unfortunately, interpretation of experimental data is often a difficult task: the observed spectroscopic behavior results from the subtle interplay of stereo-electronic, dynamic and environmental effects, whose specific roles are difficult to disentangle.…”

Photodestruction of Diatomic Molecular Ions: Laboratory and Astrophysical Application

“…The next step of development is to modify and adapt presented method to enable theoretical research of photodestruction processes which involves larger and complicated molecules for further exploration and explanation of experimentally obtained results e.g., in synchrotron facility (see research of [ 40 , 41 , 42 ]) for potential industry, biological, astrobiological and astrochemistry applications [ 43 , 44 ].…”