(Sub-)millimeter-wave spectroscopy of gauche-propanal

TL;DR

This study provides a detailed millimeter-wave spectral analysis of gauche-propanal, revealing its vibrational states, tunneling interactions, and accurately determining its fundamental vibrational frequency.

Contribution

It offers the first comprehensive analysis of both ground and excited vibrational states of gauche-propanal, including tunneling interactions and resonance effects, with high experimental accuracy.

Findings

Identified and assigned weak $c$-type transitions using DM-DR measurements.

Determined the fundamental vibrational frequency $ u_{24}$ as 68.75037(30) cm$^{-1}$.

Developed a detailed spectral model accounting for tunneling, Coriolis, and Fermi interactions.

Abstract

A detailed analysis of (sub-)millimeter-wave spectra of the vibrational ground state ($\upsilon=0$) combined with the energetically lowest excited vibrational state ($\upsilon_{24}=1$; aldehyde torsion) of gauche-propanal (g-C$_2$H$_5$CHO) up to 500 GHz is presented. Both vibrational states, $\upsilon=0$ and $\upsilon_{24}=1$, are treated with tunneling rotation interactions between their two respective tunneling states, which originate from two stable degenerate gauche-conformers; left- and right-handed configurations separated by a small potential barrier. Thanks to double-modulation double-resonance (DM-DR) measurements, important but weak $c$-type transitions connecting the tunneling states could be unambiguously assigned. In addition, Coriolis interaction as well as Fermi resonance between the two vibrational states needed to be taken into account to derive fits with experimental accuracy using Pickett's SPFIT program in a reduced axis system (RAS). Based on the rotational analysis, the fundamental vibrational frequency $\nu_{24}$ of gauche-propanal is redetermined to 68.75037(30) cm$^{-1}$.