Carbonic Acid Revisited: Vibrational Spectra, Energetics and the Possibility of Detecting an Elusive Molecule

TL;DR

This study calculates vibrational spectra and energetics of carbonic acid conformers and dimers using advanced computational methods, assessing their detectability in Earth's and Mars' atmospheres.

Contribution

It provides a comprehensive computational analysis of carbonic acid's vibrational frequencies and formation energetics, improving detection prospects in natural environments.

Findings

ωB97XD functional accurately predicts vibrational frequencies.

High-level extrapolation methods match ωB97XD in reaction energetics.

Large anharmonic effects influence vibrational spectra significantly.

Abstract

We calculate harmonic frequencies of the three most abundant carbonic acid conformers. For this, different model chemistries are investigated with respect to their benefits and shortcomings. Based on these results we use perturbation theory to calculate anharmonic corrections at the {\omega}B97XD/aug-cc-pVXZ, X=D,T,Q, level of theory and compare them with recent experimental data and theoretical predictions. A discrete variable representation method is used to predict the large anharmonic contributions to the frequencies of the stretching vibrations in the hydrogen bonds in the carbonic acid dimer. Moreover, we re-investigate the energetics of the formation of the carbonic acid dimer from its constituents water and carbon dioxide using a high-level extrapolation method. We find that the {\omega}B97XD functional performs well in estimating the fundamental frequencies of the carbonic acid conformers. Concerning the reaction energetics, the accuracy of {\omega}B97XD is even comparable to the high-level extrapolation method. We discuss possibilities to detect carbonic acid in various natural environments such as Earth's and Martian atmospheres.