Raman characterization for vibrations of aqueous methanediol and its oligomers

Methanediol (CH₂(OH)₂) and its oligomers, HO(CH₂O)_nH (with n≥2), in aqueous phase are regarded to play a role in the multiphase chemistry of the atmosphere. However, the unambiguous characterization of these species via spectroscopic analysis remains limited. In this work, a Raman spectroscopic analysis was conducted on aqueous solutions of formaldehyde where its hydrate, i.e., methanediol, and corresponding formation of its oligomers (n up to 3) were equilibrated. And then, a spectral decomposition of the Raman data was performed to extract the individual Raman spectra of the monomer, dimer and trimer. The retrieved Raman features of these species were further assigned via theoretical calculation on the basis of density functional theory calculations. The results reveal that only oligomer species, i.e., dimer and trimer, exhibit two characteristic COC stretching bands at 921cm^-1 and 1121cm^-1 and one characteristic CH₂ rocking mode near 1319cm^-1. Notably, the trimer is distinguished by an additional COC stretching band near 937cm^-1. The characteristic OCO stretching band of the monomer near 1048cm^-1 exhibits a moderate shift to 1063cm^-1 upon oligomerization into the dimer and trimer.