Experimental and theoretical study on the infrared spectroscopy of astrophysically relevant PAH derivatives 2- and 9-vinylanthracene

TL;DR

This study combines laboratory measurements and density functional theory simulations to analyze the infrared spectra of vinyl-substituted PAHs, assessing their potential contribution to astrophysical IR emission features.

Contribution

It identifies the most suitable computational methods for modeling IR spectra of vinyl-PAHs and evaluates their relevance to astrophysical IR emission.

Findings

Optimal basis set for modeling is 6-31G, or 6-31G(d) with steric interactions.

Laboratory and theoretical spectra were compared to validate computational approaches.

Vinyl-substituted PAHs do not match the mid-infrared aromatic emission bands.

Abstract

We propose to evaluate the contribution of polycyclic aromatic hydrocarbon molecules that carry side groups to the mid-infrared emission spectra. Within this framework, the IR absorption spectra of 2- and 9-vinylanthracene were measured in Ar matrices at 12 K and in CsI and polyethylene pellets at room temperature. The laboratory spectra were analyzed with the support of simulations based on the density functional theory. For each PAH molecule, eight IR spectra were computed by combining the B3LYP functional with as many different basis sets, namely 4-31G, 4-31G(d), 6-31G, 6-311G, 6-31G(d), 6-31G(d,p), 6-31+G(d,p), and 6-31++G(d,p). The comparison of the theoretical spectra with the laboratory data allowed us to determine the most suitable combinations for modeling the IR spectra of neutral PAH molecules that carry a vinyl side group. It was concluded from the examples of 2- and 9-VA that the optimum basis set is 6-31G unless a steric interaction has to be taken into account, in which case the optimum basis set is 6-31G(d). Thus, in the presence of such an interaction, the use of d-type polarization functions is recommended. We discuss the possibility for neutral vinyl-substituted PAHs to contribute to the mid-infrared emission spectra and find that their specific features do not match with the mid-infrared aromatic emission bands.