Conformer-Specific Photoelectron Spectroscopy of Carbonic Acid: H₂CO₃

Carbonic acid (H₂CO₃) is a fundamental species in biological, ecological, and astronomical systems. However, its spectroscopic characterization is incomplete because of its reactive nature. The photoionization (PI) and the photoion mass-selected threshold photoelectron (ms-TPE) spectra of H₂CO₃ were obtained by utilizing vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence spectroscopy. Two carbonic acid conformers, namely, cis–cis and cis–trans, were identified. Experimental adiabatic ionization energies (AIEs) of cis–cis and cis–trans H₂CO₃ were determined to be 11.27 ± 0.02 and 11.18 ± 0.03 eV, and the cation enthalpies of formation could be derived as Δ_fH°_0K = 485 ± 2 and 482 ± 3 kJ mol^–1, respectively. The cis–cis conformer shows intense peaks in the ms-TPES that are assigned to the C=O/C–OH stretching mode, while the cis–trans conformer exhibits a long progression to which two C=O/C–OH stretching modes contribute. The TPE spectra allow for the sensitive and conformer-selective detection of carbonic acid in terrestrial experiments to better understand astrochemical reactions.