Vibrational dynamics of formic acid and its dimer: FTIR and Raman jet spectroscopy and theory

The vibrational spectroscopy of formic acid and its dimer (including deuterated isotopologues) is investigated using Raman and FTIR jet spectroscopy. The vibrational spectroscopic record of monomeric formic acid is reviewed, corrected, and extended. The monomer database has been digitised and can be found online (https://qmbench.net). The intramolecular vibrational fingerprint spectra of formic acid dimer (FAD) are revisited, bridging the gap between the completed intermolecular van der Waals and intramolecular carbonyl stretching region, providing a plethora of environment-free combination and overtone bands. As a side product of the dimer-focussed analysis, the number of assigned formic acid trimer fundamentals is drastically increased. The measured spectra are publicly available (https://data.goettingen-research-online.de/dataverse/formic_acid_spectra). Comparisons are made to test previously published potential energy functions and high-level nuclear vibrational structure calculations (GENIUSH-Smolyak, canonical Van Vleck perturbation theory CVPT, multiconfiguration time-dependent Hartree MCTDH, vibrational configuration interaction VCI). Second-order vibrational perturbation theory (VPT2) is found to perform very well for these systems in the investigated energy regions. The VPT2 benchmarks highlight the importance of using large basis set coupled-cluster potentials. Otherwise, the inherent VPT2 error is outweighed by the underlying electronic structure errors. VPT2 with hybrid CCSD(T)-F12-edited MP2/aVDZ quartic force fields is found to be a compelling trade-off between accuracy and cost that provides reliable and accurate predictions for fundamentals and medium vibrational excitations.