Red-Shifting the Excitation Energy of Carbonic Acid Clusters Via Nonminimum Structures

Nonminimum carbonic acid clusters provide excitation energies and oscillator strengths in line with observed ice-phase UV absorptions better than traditional optimized minima. This equation-of-motion coupled cluster quantum chemical analysis on carbonic acid monomers and dimers shows that shifts to the dihedral angle for the internal heavy atoms in the monomer produce UV electronic excitations close to 200 nm with oscillator strengths that would produce observable features. This tau(OCOO) dihedral is actually a relatively floppy motion unlike what is often expected for sp(2) carbons and can be distorted by 30 degrees away from equilibrium for an energy cost of only 11 kcal/mol. As this dihedral decreases beyond 30 degrees, the excitation energies decrease further. The oscillator strengths do, as well, but only to a point. Hence, the lower-energy distortions of tau(OCOO) are sufficient to produce structures that exhibit excitation energies and oscillator strengths that would red-shift observed spectra of carbonic acid ices away from the highest UV absorption feature at 139 nm. Such data imply that colder temperatures (20 K) in the experimental treatment of carbonic acid ices are freezing these structures out after annealing, whereas the warmer temperature experiments (80 K) are not.

Automated summary

Nonminimum carbonic acid clusters provide better excitation energies and oscillator strengths for observed ice-phase UV absorptions than traditional optimized minima. Quantum chemical analysis on carbonic acid monomers and dimers reveals that shifts to the dihedral angle of the internal heavy atoms produce UV electronic excitations close to 200 nm with observable oscillator strengths. The flexibility of the OCOO dihedral angle allows for lower-energy distortions that red-shift the observed spectra of carbonic acid ices away from the highest UV absorption feature at 139 nm.