Spectroscopic Constants and Vibrational Frequencies for l-C3H+ and Isotopologues from Highly-Accurate Quartic Force Fields: The Detection of l-C3H+ in the Horsehead Nebula PDR Questioned

TL;DR

This study uses high-accuracy quantum chemical calculations to verify the spectroscopic constants of l-C3H+ and its isotopologues, questioning the previous identification of this molecule in the Horsehead nebula based on spectral data.

Contribution

The paper provides highly accurate computational spectroscopic data for l-C3H+ and isotopologues, challenging prior astronomical identifications based on spectral line analysis.

Findings

Computed rotational constants show discrepancies with observed data.

Differences in D and H constants cast doubt on the molecular assignment.

The previous identification of l-C3H+ in the Horsehead nebula is questionable.

Abstract

Very recently, molecular rotational transitions observed in the photon-dominated region of the Horsehead nebula have been attributed to l-C3H+. In an effort to corroborate this finding, we employed state-of-the art and proven high-accuracy quantum chemical techniques to compute spectroscopic constants for this cation and its isotopologues. Even though the B rotational constant from the fit of the observed spectrum and our computations agree to within 20 MHz, a typical level of accuracy, the D rotational constant differs by more than 40%, while the H rotational constant differs by three orders of magnitude. With the likely errors in the rotational transition energies resulting from this difference in D on the order of 1 MHz for the lowest observed transition (J = 4 <- 3) and growing as J increases, the assignment of the observed rotational lines from the Horsehead nebula to l-C3H+ is questionable.