UV-Vis Spectra of Carbonic Acid: Rationalizing Experimental Redshifts between Monomer and Bulk based on (HCO) Calculations.

The UV-Vis spectra of HCO are investigated in a combined experimental and theoretical approach. A sample of solid HCO, prepared by electron irradiation of water-carbon dioxide ice, shows characteristics of both amorphous and crystalline HCO in the infrared spectrum. To rationalize the experimentally observed redshift in the UV-Vis spectra between monomer and bulk HCO, a systematic computational study is devised using time-dependent density functional theory. HCO is investigated from the monomer to (HCO) clusters, with n up to 66; in addition regular oligomer arrangements derived from previously proposed ambient-pressure HCO crystal structures are also examined. The calculations explain the UV-Vis absorption of solid carbonic acid, which is redshifted by ≈2 eV and ≈5 eV compared to the experimentally observed adiabatic ionization energy of the HCO monomer. It is highlighted how these shifts emerge due to 1) increasing cluster size, 2) nonplanar arrangements, and 3) noncovalent interactions between HCO chains and sheets. The study aims to establish spectrum-to-structure relationships and serves as computational reference data for astrochemical applications in the absence of experimental laboratory data of HCO oligomers.