Observing the completion of the first solvation shell of carbon dioxide in argon from rotationally resolved spectra

TL;DR

This study provides the first experimental rotational spectra of CO2 in argon clusters, revealing structural details of the first solvation shell completion and identifying vibrational modes, thus filling a gap in experimental data for medium-sized molecular clusters.

Contribution

It offers the first experimental rotationally resolved spectra of CO2 in argon clusters, confirming shell completion and structural symmetry, which were previously only theoretically studied.

Findings

Confirmed completion of the first solvation shell in CO2-Ar clusters

Determined precise rotational parameters and vibrational frequency shifts

Identified low-frequency vibrational modes (~2 cm-1) in the clusters

Abstract

Widespread interest in weakly bound molecular clusters of medium size (5-50 molecules) is motivated by their complicated energy landscapes, which lead to hundreds or thousands of distinct isomers. But most studies are theoretical in nature, and there are no experimental results which provide definitive structural information on completion of the first solvation shell. Here we assign rotationally resolved mid-infrared spectra to argon clusters containing a single carbon dioxide molecule, CO2-Ar15 and CO2-Ar17. These mark completion of the first solvation shell for CO2 in argon. The assignments are confirmed by nuclear spin intensity alternation in the spectra, a marker of highly symmetric structures for these clusters. Precise values are determined for rotational parameters, and for shifts of the CO2 vibrational frequency induced by the argon atoms. The spectra indicate possible low frequency (~2 cm-1) vibrational modes in these clusters, posing a challenge for future cluster theory.