Millimeter-wave spectroscopy of the $^{13}$CH$_3$OD isotopic species of methyl alcohol

TL;DR

This study provides detailed millimeter-wave spectral data for the $^{13}$CH$_3$OD isotopologue of methanol, aiding astronomical observations and understanding of isotopic chemistry in interstellar environments.

Contribution

The paper reports new high-resolution spectral measurements and fitted molecular constants for $^{13}$CH$_3$OD across 150-510 GHz, expanding spectral data for this isotopologue.

Findings

Line positions for multiple torsional states and J levels are reported.

Spectral lines have been accurately fitted to a torsion-rotation Hamiltonian.

The data supports future astronomical detection and analysis of $^{13}$CH$_3$OD.

Abstract

The dramatic increase in sensitivity, spectral coverage and resolution of radio astronomical facilities in recent years has opened new possibilities for observation of chemical differentiation and isotopic fractionation in protostellar sources to shed light on their spatial and temporal evolution. In warm interstellar environments, methanol is an abundant species, hence spectral data for its isotopic forms are of special interest. In the present work, the millimeter-wave spectrum of the $^{13}$CH$_3$OD isotopologue has been investigated over the region from 150$-$510 GHz to provide a set of transition frequencies for potential astronomical application. The focus is on two types of prominent $^{13}$CH$_3$OD spectral groupings, namely the $a$-type $^qR$-branch multiplets and the $b$-type $Q$-branches. Line positions are reported for the $^qR(J)$ clusters for $J = 3$ to 10 for the $v_{\rm t} = 0$ and 1 torsional states, and for a number of $v_{\rm t} = 0$ and 1 $^rQ(J)$ or $^pQ(J)$ line series up to $J = 25$. The frequencies have been fitted to a multi-parameter torsion-rotation Hamiltonian, and upper level excitation energies have been calculated from the resulting molecular constants.