Rotational spectroscopy $n$-propanol: $Aa$ and $Ag$ conformers

TL;DR

This study characterizes the rotational spectra of the $Aa$ and $Ag$ conformers of $n$-propanol, providing precise spectroscopic data to enable their astronomical detection, which was previously limited to certain conformers.

Contribution

The paper provides the first detailed spectroscopic models and unambiguous transition assignments for the $Aa$ and $Ag$ conformers of $n$-propanol, facilitating their identification in space.

Findings

Derived accurate spectroscopic models for $Aa$ and $Ag$ conformers.

Unambiguously assigned transitions for $Aa$ and $Ag$ conformers.

Confirmed the existence of two tunneling states, $Ag^+$ and $Ag^-$.

Abstract

The primary alcohol $n$-propanol (i.e., $normal$-propanol or propan-1-ol; C$_3$H$_7$OH) occurs in five different conformers: $Ga$, $Gg$, $Gg'$, $Aa$, and $Ag$. All rotational spectra of the three conformers of the $G$ family are well described, making astronomical search of their spectroscopic signatures possible, as opposed to those of the $Aa$ and $Ag$ conformers. Our goal is to facilitate the astronomical detection of $Aa$ and $Ag$ conformers of $n$-propanol by characterizing their rotational spectra. We recorded the rotational spectra of $n$-propanol in the frequency domain of 18$-$505\,GHz.Additional double-modulation double-resonance (DM-DR) measurements were performed, more specifically with the goal to unambiguously assign weak transitions of the $Aa$ conformer and to verify assignments of the $Ag$ conformer. We derived a spectroscopic quantum mechanical model with experimental accuracy (with $J_\textrm{max}=70$ and $K_{a,\textrm{max}}=6$) for $Aa$ $n$-propanol. Furthermore, we unambiguously assigned transitions (with $J_\textrm{max}=69$ and $K_{a,\textrm{max}}=9$) of $Ag$ $n$-propanol; in doing so, we prove the existence of two tunneling states, $Ag^+$ and $Ag^-$. The astronomical search of all five conformers of $n$-propanol is now possible via their rotational signatures. These are applied in a companion article on the detection of $n$-propanol toward the hot molecular core Sgr B2(N2).