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

Abstract

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 sp2 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.

MQM 2022: The 10th Triennial Conference on Molecular Quantum Mechanics

“…Their analysis reveals features of potential relevance to the ultimate astrochemical observation of these species. 30 Next, Workman, Firth, and Gichuhi at Tennessee Tech University have simulated the negative-ion photoelectron spectra of three cyano-substituted benzenes. 31 Their results may lead to identification of these species � which are comparable to recently detected cyanonaphthalene and cyanoindene � in the interstellar medium.…”

“…In the sole paper of the MQM VSI appearing in section A2, “Aerosols; Environmental and Atmospheric Chemistry; Astrochemistry,” Haney et al in the Fortenberry group at Ole Miss explore the dependence of the lowest excitation energies of carbonic acid clusters on their dihedral O–C–O–O angles. Their analysis reveals features of potential relevance to the ultimate astrochemical observation of these species . Next, Workman, Firth, and Gichuhi at Tennessee Tech University have simulated the negative-ion photoelectron spectra of three cyano-substituted benzenes .…”

Toward the IR Detection of Carbonic Acid: Absorption and Emission Spectra

“…The present work will compute exceptionally high-accuracy quantum chemical characterization of both conformers of carbonic acid in order to provide the means for the first astronomical observation of cis-cis-H 2 CO 3 as well as IR observation of the known cis-trans. Locating astronomical environments where both conformers of carbonic acid reside as determined from remote observations requiring comparison to accurate standards will also assist in analysis of how UV photons affect its potential role in electronic spectral properties in ices and disks (Ioppolo et al 2021;Wallace & Fortenberry 2021Haney et al 2023).…”