Conformers of Vanillic Acid and Its Monohydrate: A Rotational Spectroscopic and Theoretical Study.

Organic acids are released during wildfire combustion and can influence aerosol formation and growth. Conformational flexibility is thought to be advantageous in stabilizing the precritical nucleus in the process of aerosol particle formation and allowing for further complexation with other atmospheric constituents. We describe here a study of the conformational flexibility of vanillic acid and its monohydrate using electronic structure calculations and Fourier transform microwave spectroscopy. Computationally, 12 and 28 conformers were found for the monomer and monohydrate, respectively. The two lowest energy conformers of both the vanillic acid monomer and the vanillic acid-water complex could be experimentally identified. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TZVP levels of theory is less than 1%. No tunneling splittings were observed, which suggests a relatively high barrier to methyl internal rotation, in agreement with other, previously studied vanillin derivatives. Furthermore, no -type transitions could be observed for the vanillic acid monomer, in agreement with the computed zero -dipole moment component of the two lowest energy structures. For the monohydrate the absence of -type transitions is rationalized by averaging over a large amplitude motion involving the free H atom of the water unit. From the theoretical structures, it is apparent that intramolecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further characterize the intramolecular interactions in the monomer and intra- and intermolecular interactions in the monohydrate, quantum theory of atoms-in-molecules (QTAIM), noncovalent interactions (NCI), and intrinsic bond strength index (IBSI) analyses were performed. The atmospheric abundance of the vanillic acid monohydrate relative to the monomer was evaluated to assess its atmospheric significance.