Rotational spectra of isotopic species of methyl cyanide, CH$_3$CN, in their ground vibrational states up to terahertz frequencies

TL;DR

This study provides high-precision rotational spectra of methyl cyanide isotopologues up to 1.63 THz, improving spectroscopic parameters crucial for astrophysical observations of star-forming regions.

Contribution

The paper reports the first comprehensive laboratory spectra of methyl cyanide isotopologues up to terahertz frequencies, with significantly improved spectroscopic parameters for astrophysical applications.

Findings

Identified transitions for multiple isotopologues up to 1.2 THz.

Achieved high-accuracy spectroscopic parameters for all studied species.

Extended the frequency range of reliable methyl cyanide spectra for astronomical use.

Abstract

Methyl cyanide is an important trace molecule in star-forming regions. It is one of the more common molecules used to derive kinetic temperatures in such sources. As preparatory work for Herschel, SOFIA, and in particular ALMA we want to improve the rest frequencies of the main as well as minor isotopologs of methyl cyanide. The laboratory rotational spectrum of methyl cyanide in natural isotopic composition has been recorded up to 1.63 THz. Transitions with good signal-to-noise ratio could be identified for CH$_3$CN, $^{13}$CH$_3$CN, CH$_3^{13}$CN, CH$_3$C$^{15}$N, CH$_2$DCN, and $^{13}$CH$_3^{13}$CN in their ground vibrational states up to about 1.2 THz. The main isotopic species could be identified even in the highest frequency spectral recordings around 1.6 THz. The highest $J'$ quantum numbers included in the fit are 64 for $^{13}$CH$_3^{13}$CN and 89 for the main isotopic species. Greatly improved spectroscopic parameters have been obtained by fitting the present data together with previously reported transition frequencies. The present data will be helpful to identify isotopologs of methyl cyanide in the higher frequency bands of instruments such as the recently launched Herschel satellite, the upcoming airplane mission SOFIA or the radio telescope array ALMA.