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

• Main Authors: Olivia G. Haney (14351739), Brent R. Westbrook (9989978), Taylor J. Santaloci (9616982), Ryan C. Fortenberry (1597267)
• Format: Other Non-Article Part of Journal/Newspaper
• Language: unknown
• Published: 2023
• Subjects: Biophysics; Biochemistry; Neuroscience; Pharmacology; Ecology; Immunology; Environmental Sciences not elsewhere classified; Chemical Sciences not elsewhere classified; Physical Sciences not elsewhere classified; warmer temperature experiments; traditional optimized minima; internal heavy atoms; carbonic acid monomers; carbonic acid ices; 30 ° away; exhibit excitation energies; excitation energies decrease; shift observed spectra; 2 </ sup; excitation energy; observed ice; oscillator strengths; often expected; experimental treatment; energy distortions; energy cost; dimers shows; data imply; colder temperatures; 80 k; 200 nm; 20 k; 139 nm; 11 kcal; Carbonic acid
• Online Access: https://doi.org/10.1021/acs.jpca.2c07589.s001

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 τ(OCOO) dihedral is actually a relatively floppy motion unlike what is often expected for sp 2 carbons and can be distorted by 30° away from equilibrium for an energy cost of only 11 kcal/mol. As this dihedral decreases beyond 30°, the excitation energies decrease further. The oscillator strengths do, as well, but only to a point. Hence, the lower-energy distortions of τ(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.