UV–Vis Spectra of Carbonic Acid: Rationalizing Experimental Redshifts between Monomer and Bulk based on (H₂CO₃)n Calculations

The UV–Vis spectra of H 2 CO 3 are investigated in a combined experimental and theoretical approach. A sample of solid H 2 CO 3 , prepared by electron irradiation of water–carbon dioxide ice, shows characteristics of both amorphous and crystalline H 2 CO 3 in the infrared spectrum. To rationalize the experimentally observed redshift in the UV–Vis spectra between monomer and bulk H 2 CO 3 , a systematic computational study is devised using time‐dependent density functional theory. H 2 CO 3 is investigated from the monomer to (H 2 CO 3 ) n clusters, with n up to 66; in addition regular oligomer arrangements derived from previously proposed ambient‐pressure H 2 CO 3 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 H 2 CO 3 monomer. It is highlighted how these shifts emerge due to 1) increasing cluster size, 2) nonplanar arrangements, and 3) noncovalent interactions between H 2 CO 3 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 H 2 CO 3 oligomers.