Vibrational spectra of formic acid and its dimer: II. Towards identifying structural motifs in atmospheric organic clustering.

This work constitutes part II of a series on vibrational spectroscopy of formic acid, focusing on the molecular interactions of its carboxyl group during dimerization. Such interactions drive the earliest steps of clustering in organic-mediated atmospheric new particle formation (NPF), and are difficult to access with the mass spectrometric methods typically used to study NPF. While theoretical thermochemistry provides binding free energies for various cluster conformers, these conformers have not been experimentally probed to date, and it is unclear which conformers ultimately participate in atmospheric organic clustering. Vibrational spectroscopy using matrix-isolation provides a route to experimentally investigate the conformers of small clusters. To identify simple conformers in the clustering of the carboxyl group, we use the model system of the formic acid monomer and dimer in cryogenic matrices in combination with ab initio calculations. We find that: (1) efficient harmonic frequency calculations, even at a low level of density functional theory, reproduce both the direction and magnitude of the experimentally observed dimer-formation frequency shifts across different conformers. (2) Matrix effects effectively cancel out when investigating such shifts. (3) The most reliable region to study these shifts is the νCO and νC-O regions, supported secondarily by the broader and less intense νOH and δoopCOH regions. (4) Under cryogenic matrix-isolation conditions, thermodynamically less favorable conformers appear with relatively high abundance, suggesting that the matrix serves as a kinetic trap. Altogether, we can capture structural information about short-lived conformers and thus demonstrate their experimental existence. This supports the common practice of including less favorable conformers in ab initio computational nucleation models, as they are considered intermediates in atmospheric organic clustering. Our insights from this model system of formic acid pave the way for future studies of organic clustering analyzed in cryogenic matrices to investigate the earliest steps of sub-nanometer NPF.