Solving the Puzzle of the Carbonic Acid Vibrational Spectrum - an Anharmonic Story

Against the general belief that carbonic acid is too unstable for synthesis, it was possible to synthesize the solid^[1,2] as well as gas-phase carbonic acid.^[3] It was suggested that solid carbonic acid might exist in Earth's upper troposphere and in the harsh environments of other solar bodies,^[4] where it undergoes a cycle of synthesis, decomposition, and dimerization.^[5] To provide spectroscopic data for probing the existence of extraterrestrial carbonic acid,^[2,6] matrix-isolation infrared (MI-IR) spectroscopy has shown to be essential.^[3,4,6-8] However, early assignments within the harmonic approximation using scaling factors impeded a full interpretation of the rather complex MI-IR spectrum of H₂CO₃. Recently, carbonic acid was detected in the Galactic center molecular cloud,^[9] triggering new interest in the anharmonic spectrum.^[10] In this regard, we substantially reassign our argon MI-IR spectra based on accurate anharmonic calculations. We calculate a four-mode potential energy surface (PES) at the explicitly correlated coupled-cluster theory using up to triple-zeta basis sets, i. e., CCSD(T)-F12/cc-pVTZ-F12. On this PES, we perform vibrational self-consistent field and configuration interaction (VSCF/VCI) calculations to obtain accurate vibrational transition frequencies and resonance analysis of the fundamentals, first overtones, and combination bands. In total, 12 new bands can be assigned, extending the spectral data for carbonic acid and thus simplifying detection in more complex environments. Furthermore, we clarify disputed assignments between the cc- and ct-conformer.