Department of Chemistry & Biochemistry, University of Mississippi, University, Mississippi 38677-1848, United States.
J Phys Chem A. 2022 Jun 16;126(23):3739-3744. doi: 10.1021/acs.jpca.2c00862. Epub 2022 Jun 7.
Two theoretical structural motifs are proposed to match two experimental solid carbonic acid UV spectra from previous literature ( 2021, 646, A172): a linear ribbon structure as a single octamer and nonplanar orientations of carbonic acid clusters. The latter have some contribution from approximated amorphous solid carbonic acid in the form of 40 different clusters of 8 carbonic acid molecules ensemble-averaged together, but unoptimized pairs of optimized dimers oriented perpendicular to one another give the strongest intensities of lower energy UV transitions. The linear ribbon structure's predicted spectrum computed with CAM-B3LYP/6-311G(d,p) agrees well with Experimental Solid B─the β-carbonic acid experimental data in the UV region. Meanwhile, the 40 amorphous clusters are built with a randomization script, and the electronically excited states are calculated with both CAM-B3LYP/6-311G(d,p) and ωB97XD/6-311G(d,p). The resulting theoretical spectrum is constructed by employing a Boltzmann distribution of the intensities and artificially broadening the simulated spectra. The nonplanar dimer pairs are computed with CAM-B3LYP and B3LYP with the 6-311G(d,p) basis set. The results of the amorphous simulation weakly correspond with the Experimental Solid A spectrum, but the fully nonplanar motif matches the experiment much more convincingly. As a result, the previous work appears to have observed the traditional crystalline phase of solid carbonic acid in Experimental Solid B, whereas the nonplanar orientations of the carbonic acids in the clusters appear to correlate with Experimental Solid A. This spectral classification will aid in future laboratory work exploring the role that carbonic acid can play in low temperature, low pressure desorbed environments with potential application to astrochemistry.
提出了两种理论结构基序来匹配以前文献中（2021 年，646，A172）的两个实验固态碳酸紫外光谱：一种是作为单个八聚体的线性带状结构和碳酸簇的非平面取向。后者在某种程度上来自近似无定形固态碳酸的贡献，形式为 40 个不同的 8 个碳酸分子簇集，一起进行平均，但是未优化的对优化的彼此垂直取向的二聚体给出了较低能量 UV 跃迁的最强强度。用 CAM-B3LYP/6-311G(d,p)计算的线性带状结构的预测光谱与实验固态 B（β-碳酸的实验数据在 UV 区域）非常吻合。同时，40 个无定形簇是用随机化脚本构建的，用 CAM-B3LYP/6-311G(d,p)和 ωB97XD/6-311G(d,p)计算了电子激发态。通过使用强度的玻尔兹曼分布和人工扩展模拟光谱来构建理论光谱。用 CAM-B3LYP 和 B3LYP 计算非平面二聚体对，使用 6-311G(d,p)基组。无定形模拟的结果与实验固态 A 光谱弱对应，但完全非平面基序更令人信服地匹配实验结果。因此，之前的工作似乎在实验固态 B 中观察到了固态碳酸的传统晶相，而簇中的碳酸的非平面取向似乎与实验固态 A 相关。这种光谱分类将有助于未来的实验室工作，探索碳酸在低温、低压解吸环境中所起的作用，该环境可能在天体化学中有应用。
