IR Spectra of Protonated Carbonic Acid and Its Isomeric H3O+[sdot]CO2 ComplexThis study was supported by the Deutsche Forschungsgemeinschaft (DO[emsp14]729/2), the Fonds der Chemischen Industrie, and the Bavaria-California Technology Center (BaCaTeC)

Carbonic acid, H2CO3, and its isomeric weakly bound H2O·CO2 complex are short-lived intermediates in the hydrolysis of CO2, a process of fundamental importance in geochemistry and biology. Whereas H2O·CO2 has been characterized by spectroscopy, 7] there is only indirect experimental evidence for the existence of H2CO3 in the gas phase. Theoretical calculations predict that isolated H2CO3 should be stable but that it decomposes into H2O and CO2 in the presence of water. There have been several unsuccessful attempts to generate and characterize gas-phase H2CO3 by heating solid H2CO3 [3] or NH4HCO3. [12] Herein we describe the properties of gas-phase protonated H2CO3 and report the first spectroscopic identification of the fundamental C(OH)3 + carbocation and its H3O ·CO2 isomer by argon-tagging photodissociation spectroscopy. Quantum chemical calculations demonstrate that there are several isomers on the [C,H3,O3] + potential-energy surface (1–4, Figure 1). The most stable structure corresponds to the hydrogen-bonded H3O ·CO2 complex (4, Erel= 0). The attraction is dominated by charge-induced dipole and charge–quadrupole interaction leading to nearly linear hydrogen bonding. 16] The predicted dissociation energy of 62 kJmol 1 is in good agreement with the binding enthalpy of 64 kJmol 1 measured by high-pressure mass spectrometry. Protonation of H2CO3 at the carbonyl group results in two planar isomers of the trihydroxycarbenium ion, antiand synC(OH)3 . The anti structure (1, C3h, Erel= 18 kJmol ) is separated by a barrier of 50 kJmol 1 from the less stable syn structure (2, Cs, Erel= 47 kJmol ). Conversion between the C(OH)3 + isomers and 4 is suppressed by a high isomerization barrier (214 kJmol ). In the NMR, IR, Raman, and X-ray crystallography spectra of C(OH)3 + in superacid solutions or corresponding salts only the anti isomer was observed. Finally, a fourth nonplanar [C,H3,O3] + isomer can be generated by protonation of H2CO3 at a hydroxyl group (3, C1, Erel= 118 kJmol ), leading to a hitherto undetected complex of H2O and OCOH +[14] with a binding energy of 105 kJmol . Electron ionization (EI) of diethyl carbonate is known to selectively produce C(OH)3 + ions through consecutive elimination of a vinyl radical and ethene. 14] They could be distinguished from H3O ·CO2 complexes made by EI of a H2O/CO2 mixture by their different fragmentation patterns in metastable decay (MD) and high-energy (8 keV) collisioninduced dissociation (CID). This work takes advantage of low-energy (8 eV) CID spectra for the assignments of isomers. Specifically, the major collision fragments of C(OH)3 + and H3O ·CO2 correspond to OCOH + and H3O , respectively (Figure 2). Experimental conditions can be optimized to favor exclusive production of either C(OH)3 + or H3O ·CO2 isomers. Although mass spectrometry experiments can distinguish between different isomers, they do not provide direct structural information. Therefore, the highly sensitive Figure 1. Structures, relative energies (left, in kJmol ), and intermolecular binding energies (right, in kJmol ) of various isomers of [C,H3,O3] + and [C,H3,O3] ·Ar calculated at the B3LYP/6-311G** level. The positive charge has been omitted in each structure for clarity.