Haney Olivia G, Westbrook Brent R, Santaloci Taylor J, Fortenberry Ryan C
Department of Chemistry & Biochemistry, Belhaven University, Jackson, Mississippi 39212, United States.
Department of Chemistry & Biochemistry, University of Mississippi, University, Mississippi 38677-1848, United States.
J Phys Chem A. 2023 Jan 19;127(2):489-494. doi: 10.1021/acs.jpca.2c07589.
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 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.
与传统优化的极小值相比，非最小碳酸簇提供的激发能和振子强度与观测到的冰相紫外吸收更相符。对碳酸单体和二聚体进行的这种运动方程耦合簇量子化学分析表明，单体中内部重原子的二面角发生变化时，会产生接近200 nm的紫外电子激发，其振子强度会产生可观测的特征。实际上，这个τ（OCOO）二面角是一种相对松弛的运动，这与通常对sp碳的预期不同，并且它可以偏离平衡30°，而能量成本仅为11千卡/摩尔。当这个二面角减小超过30°时，激发能会进一步降低。振子强度也会降低，但只会降低到一定程度。因此，τ（OCOO）的低能量畸变足以产生具有激发能和振子强度的结构，这些结构会使观测到的碳酸冰光谱从139 nm处的最高紫外吸收特征发生红移。这些数据表明，在碳酸冰的实验处理中，较低温度（20 K）在退火后会使这些结构冻结，而较高温度（80 K）的实验则不会。
