Rotational spectrum of 4-methylcyanoallene (CH₃CH=C=CH-CN), a chiral molecule of potential astrochemical interest

Abstract

Context. A successful identification of an interstellar compound requires that its spectrum has first been assigned in the laboratory. New and sensitive radiotelescopes, such as ALMA, will make it possible to detect interstellar molecules in much smaller concentrations than before. Cyanoallene (CH 2 =C=CH-CN) has recently been observed in the dense molecular cloud TMC-1 by means of its rotational spectrum. Its methyl congener, 4-methylcyanoallene (CH 3 CH=C=CH-CN), may also be present in the interstellar medium (ISM). This chiral compound exists in two forms, which are mirror images. Chirality is an essential feature of life. So far, no chiral compounds have been detected in the ISM. Aims. The synthesis and assignment of the rotational spectrum of CH 3 CH=C=CH-CN, will facilitate the potential detection of this compound in the ISM. Methods. The spectrum of 4-methylcyanoallene has been recorded between 13 and 116 GHz using the microwave spectrometer at the University of Oslo. The spectroscopic study has been augmented with high-level quantum chemical calculations at the B3LYP/ccpVTZ and CCSD/cc-pVTZ levels of theory. Results. The rotational spectra of CH 3 CH=C=CH-CN in the ground vibrational state and in the first vibrationally excited state are reported for the first time and accurate spectroscopic constants have been obtained from a large number of transitions.

Methyl Acetate and Its Singly Deuterated Isotopomers in the Interstellar Medium

“…Accuracy of computed rotational frequencies depends on the choices of the models. Recently, Carles et al (2013Carles et al ( , 2014 explained experimental rotational frequencies of 4-methylcyanoallene and ethyl cyanoacetylene with high-level quantum chemical calculations at the B3LYP/cc-pVTZ level of theory by using Gaussian 09W program. Their results are in excellent agreement with the experiment.…”

“…In fact, there are no options in Gaussian 09W program to compute rotational and distortional constants at the CCSD level of theory. This prompted us to use B3LYP functional along with the Peterson and Dunnings correlation consistent (cc-pVTZ) basis set (Peterson & Dunning 2002;Carles et al 2013) which are proven to yield satisfactory results (Carles et al 2013;Carles et al 2014;Das et al 2015) and also takes less CPU time than other highly correlated methods. Many molecules have been detected in interstellar space with internal rotations (Nguyen et al 2014).…”

Potential formation of three pyrimidine bases in interstellar regions

“…Accuracy of the computed spectroscopic constant depends on the choices of the models. For example, Carles et al (2013Carles et al ( , 2014 program for the computation. In Table 3, we compare our results with the existing results.…”

“…Accuracy of the computed spectroscopic constant depends on the choices of the models. For example, Carles et al (2013Carles et al ( , 2014 -23an equilibrium geometry, while the measured ones are subject to some vibrational effects also (Csazar 1989). Experimentally extracted values might contain some uncertainties.…”