Infrared Characterization of Isotopic Analogues of Methanediol in Aqueous Solution

Dissolved methanediol in aqueous solution has been treated as the precursor for the formation of atmospheric formic acid in multiphase environments. In this work, methanediol, CH₂(OH)₂, and its isotopic analogues, CH₂(OD)₂, CD₂(OH)₂, and CD₂(OD)₂, in aqueous solution were prepared by dissolving paraformaldehyde and deuterium-substituted paraformaldehyde powders in H₂O and D₂O under reflux. Their infrared absorption contours of formaldehyde solutions at concentrations of <1 wt % are not dependent on the concentration, mainly referring to the characteristics of the monomeric configuration, and can be categorized into two parts. At wavenumbers >2000 cm^-1, broad bands of moderate strengths were ascribed to the stretching modes of two OH or OD groups, observed at 3000-3700 and 2050-2750 cm^-1, respectively. At wavenumbers of 950-1200 cm^-1, the isotopic analogues of methanediols composed of CH₂ moieties are featured with a singlet strong band at ca. 1030 cm^-1, mainly attributed to the O-C-O stretching modes; the isotopic methanediols containing CD₂ moieties manifested two intense bands at ca. 1100 and 980 cm^-1, majorly enveloping the CD₂ deformation and O-C-O stretching modes. The aforementioned spectral features were assigned on the basis of density functional theory, ωB97XD, with the basis set aug-cc-pVTZ and the solvent effect using the conductor-like polarizable continuum model. In addition, the predicted energetics suggested that the trans-methanediol is more stable than the cis- conformer by ca. 0.62 kcal mol^-1 and majorly contributes to the infrared features. At higher concentrations of CH₂(OH)₂, extra bands at 920 and 1104 cm^-1 appeared and were attributed to the C-O-C stretching modes of the dimeric/polymeric methanediol; that is, HO(CH₂O)_nH, n ≥ 2. These infrared characterizations of the isotopic analogues of methanediols provided suitable detection windows in the relevant atmospheric and aerosol reactions in the laboratory studies.