Theoretical Characterization of Carbonic Acid Clusters in the UV.

Two theoretical structural motifs are proposed to match two experimental solid carbonic acid UV spectra from previous literature ( Astron. Astrophys. 2021, 646, A172): a linear ribbon structure as a single octamer and nonplanar orientations of carbonic acid clusters. The latter have some contribution from approximated amorphous solid carbonic acid in the form of 40 different clusters of 8 carbonic acid molecules ensemble-averaged together, but unoptimized pairs of optimized dimers oriented perpendicular to one another give the strongest intensities of lower energy UV transitions. The linear ribbon structure's predicted spectrum computed with CAM-B3LYP/6-311G(d,p) agrees well with Experimental Solid B─the ß-carbonic acid experimental data in the UV region. Meanwhile, the 40 amorphous clusters are built with a randomization script, and the electronically excited states are calculated with both CAM-B3LYP/6-311G(d,p) and ωB97XD/6-311G(d,p). The resulting theoretical spectrum is constructed by employing a Boltzmann distribution of the intensities and artificially broadening the simulated spectra. The nonplanar dimer pairs are computed with CAM-B3LYP and B3LYP with the 6-311G(d,p) basis set. The results of the amorphous simulation weakly correspond with the Experimental Solid A spectrum, but the fully nonplanar motif matches the experiment much more convincingly. As a result, the previous work appears to have observed the traditional crystalline phase of solid carbonic acid in Experimental Solid B, whereas the nonplanar orientations of the carbonic acids in the clusters appear to correlate with Experimental Solid A. This spectral classification will aid in future laboratory work exploring the role that carbonic acid can play in low temperature, low pressure desorbed environments with potential application to astrochemistry.

Linear and Helical Carbonic Acid Clusters.

Crystallization of carbonic acid likely begins with a linear or ribbon-esque oligomerization, but a helical spiral is shown here to be a new, competing motif for this process. The present combined density functional theory and coupled-cluster theory work examines both the ribbon and the new helical spiral motifs in terms of relative energies, sequential binding energies, and electronic spectra which could potentially aid in distinguishing between the two forms. The helix diverges in energy from the ribbon by roughly 0.2 eV (∼4 kcal/mol) per dimer addition, but the largest intensity absorption features at 9.16 eV (135 nm) and 7.11 eV (175 nm), respective of the ribbon and spiral, will allow these to be separately observed and classified via electronic spectroscopy to determine more conclusively which motif holds in the earliest formation stages of solid carbonic acid.