UV-Vis Spectra of Carbonic Acid: Rationalizing Experimental Redshifts between Monomer and Bulk based on (H2CO3)n
Dennis F Dinu1, Bastian Klein1, Chaojiang Zhang2
1Institute of Materials Chemistry, TU Wien, Getreidemarkt 9/165, Vienna, 1060, Austria.
None:
The UV-Vis spectra of H2CO3 are investigated in a combined experimental and theoretical approach. A sample of solid H2CO3, prepared by electron irradiation of water-carbon dioxide ice, shows characteristics of both amorphous and crystalline H2CO3 in the infrared spectrum. To rationalize the experimentally observed redshift in the UV-Vis spectra between monomer and bulk H2CO3, a systematic computational study is devised using time-dependent density functional theory. H2CO3 is investigated from the monomer to (H2CO3)n clusters, with n up to 66; in addition regular oligomer arrangements derived from previously proposed ambient-pressure H2CO3 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 H2CO3 monomer. It is highlighted how these shifts emerge due to 1) increasing cluster size, 2) nonplanar arrangements, and 3) noncovalent interactions between H2CO3 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 H2CO3 oligomers.
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