Conformer-Specific Photoelectron Spectroscopy of Carbonic Acid: H 2 CO 3 .

Carbonic acid (H 2 CO 3 ) 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 H 2 CO 3 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 H 2 CO 3 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 f H ° 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.