Millimeter and Submillimeter Wave Spectroscopy of Propanal

TL;DR

This study provides detailed millimeter and submillimeter wave spectra of propanal's conformers, improving spectroscopic parameters for astronomical detection and analysis, including internal rotation effects and large amplitude motions.

Contribution

First comprehensive spectroscopic analysis of propanal's conformers in the millimeter/submillimeter range with improved parameters for astronomical applications.

Findings

Enhanced spectral predictions for propanal's conformers.

Demonstrated application to ALMA astronomical data.

Detailed analysis of internal rotation and tunneling effects.

Abstract

The rotational spectra of the two stable conformers syn- and gauche-propanal (CH$_3$CH$_2$CHO) were studied in the millimeter and submillimeter wave regions from 75 to 500 GHz with the Cologne (Sub-)Millimeter wave Spectrometer. Furthermore, the first excited states associated with the aldehyde torsion and with the methyl torsion, respectively, of the $syn$-conformer were analyzed. The newly obtained spectroscopic parameters yield better predictions, thus fulfill sensitivity and resolution requirements in new astronomical observations in order to unambiguously assign pure rotational transitions of propanal. This is demonstrated on a radio astronomical spectrum from the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (ALMA-PILS). In particular, an accurate description of observed splittings, caused by internal rotation of the methyl group in the $syn$-conformer and by tunneling rotation interaction from two stable degenerate $gauche$-conformers, is reported. The rotational spectrum of propanal is of additional interest because of its two large amplitude motions pertaining to the methyl and the aldehyde group, respectively.