Accurate Spectroscopic Characterization of Oxirane: A Valuable Route to Its Identification in Titan's Atmosphere and the Assignment of Unidentified Infrared Bands

Abstract

In an effort to provide an accurate spectroscopic characterization of oxirane, state-of-the-art computational methods and approaches have been employed to determine highly accurate fundamental vibrational frequencies and rotational parameters. Available experimental data were used to assess the reliability of our computations, and an accuracy on average of 10 cm for fundamental transitions as well as overtones and combination bands has been pointed out. Moving to rotational spectroscopy, relative discrepancies of 0.1%, 2%-3%, and 3%-4% were observed for rotational, quartic, and sextic centrifugal-distortion constants, respectively. We are therefore confident that the highly accurate spectroscopic data provided herein can be useful for identification of oxirane in Titan's atmosphere and the assignment of unidentified infrared bands. Since oxirane was already observed in the interstellar medium and some astronomical objects are characterized by very high D/H ratios, we also considered the accurate determination of the spectroscopic parameters for the mono-deuterated species, oxirane-1. For the latter, an empirical scaling procedure allowed us to improve our computed data and to provide predictions for rotational transitions with a relative accuracy of about 0.02% (i.e., an uncertainty of about 40 MHz for a transition lying at 200 GHz).

Computational challenges in Astrochemistry

“…While there is no theoretical justification for the inclusion of diffuse function effects (Δ r (aug)) once the extrapolation to the CBS limit is performed, the latter correction is often introduced to recover on an empirical basis the limitations affecting extrapolation procedures carried out with small‐ to medium‐sized basis sets. Based on the comparison with best‐estimated equilibrium structures obtained using the ‘gradient’ scheme as well as with experiment, the overall conclusion is that the so‐called ‘cheap’ geometry scheme is robust, reliable and accurate, even when dealing with rather flexible molecules like glycine and pyruvic acid . For bond distances, for instance, it is noted that the maximum absolute deviation with respect to CCSD(T)/CBS + CV is smaller than 0.001 Å.…”

Astronomical complex organic molecules: Quantum chemistry meets rotational spectroscopy

“…Recently, oxirane, and protonated oxirane have been suggested as potential prebiotic species present in Titan's atmosphere . While the rotational and infrared spectra of the former have been experimentally well characterized, for the latter a spectroscopic characterization was completely missing.…”

“…In Ref. a thorough investigation has been carried out by means of the state‐of‐the‐art computational approach described in the methodology section, with the spectroscopic parameters of oxirane computed at the same level of theory and the corresponding experimental data used to assess the accuracy of the predicted rotational spectrum. While we refer interested readers to Refs.…”

“…While we refer interested readers to Refs. , Figure shows a small portion of the computed and experimental rotational spectrum of oxirane together with the computed one for protonated oxirane. From the comparison of experiment and theory for oxirane the accuracy of the predictions for the protonated species can be derived.…”

“… A small portion (84–86 GHz) of the computed and experimental rotational spectrum of oxirane together with the computed one for protonated oxirane . For details, see text…”