Karimova Natalia V, Luo Man, Grassian Vicki H, Gerber R Benny
Department of Chemistry, University of California, Irvine, CA 92697, USA.
Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093, USA.
Phys Chem Chem Phys. 2020 Mar 4;22(9):5046-5056. doi: 10.1039/c9cp06728k.
The absorption spectra of molecular organic chromophores in aqueous media are of considerable importance in environmental chemistry. In this work, the UV-vis spectra of benzoic acid (BA), the simplest aromatic carboxylic acid, in aqueous solutions at varying pH and in the presence of salts are measured experimentally. The solutions of different pH provide insights into the contributions from both the non-dissociated acid molecule and the deprotonated anionic species. The microscopic interpretation of these spectra is then provided by quantum chemical calculations for small cluster models of benzoic species (benzoic acid and benzoate anion) with water molecules. Calculations of the UV-vis absorbance spectra are then carried out for different clusters such as C6H5COOH·(H2O)n and C6H5COO-·(H2O)n, where n = 0-8. The following main conclusions from these calculations and the comparison to experimental results can be made: (i) the small water cluster yields good quantitative agreement with observed solution experiments; (ii) the main peak position is found to be very similar at different levels of theory and is in excellent agreement with the experimental value, however, a weaker feature about 1 eV to lower energy (red shift) of the main peak is correctly reproduced only by using high level of theory, such as Algebraic Diagrammatic Construction (ADC); (iii) dissociation of the BA into ions is found to occur with a minimum of water molecules of n = 8; (iv) the deprotonation of BA has an influence on the computed spectrum and the energetics of the lowest energy electronic transitions; (v) the effect of the water on the spectra is much larger for the deprotonated species than for the non-dissociated acid. It was found that to reproduce experimental spectrum at pH 8.0, additional continuum representation for the extended solvent environment must be included in combination with explicit solvent molecules (n ≥ 3); (vi) salts (NaCl and CaCl2) have minimal effect on the absorption spectrum and; (vii) experimental results showed that B-band of neutral BA is not sensitive to the solvent effects whereas the effect of the water on the C-band is significant. The water effects blue-shift this band up to ∼0.2 eV. Overall, the results demonstrate the ability to further our understanding of the microscopic interpretation of the electronic structure and absorption spectra of BA in aqueous media through calculations restricted to small cluster models.
分子有机发色团在水介质中的吸收光谱在环境化学中具有相当重要的意义。在本工作中，对最简单的芳香族羧酸苯甲酸（BA）在不同pH值的水溶液以及存在盐的情况下的紫外可见光谱进行了实验测量。不同pH值的溶液有助于深入了解未离解的酸分子和去质子化的阴离子物种的贡献。然后通过对苯甲酸物种（苯甲酸和苯甲酸盐阴离子）与水分子的小簇模型进行量子化学计算，对这些光谱进行微观解释。接着对不同的簇，如C6H5COOH·(H2O)n和C6H5COO-·(H2O)n（其中n = 0 - 8）进行紫外可见吸收光谱计算。从这些计算以及与实验结果的比较中可以得出以下主要结论：（i）小水簇与观察到的溶液实验结果在定量上有很好的一致性；（ii）在不同理论水平下，主峰位置非常相似，并且与实验值高度吻合，然而，只有使用高级理论，如代数图示构建（ADC），才能正确再现主峰低约1 eV能量处（红移）的较弱特征；（iii）发现BA离解为离子时，最少需要n = 8个水分子；（iv）BA的去质子化对计算光谱和最低能量电子跃迁的能量学有影响；（v）对于去质子化物种，水对光谱的影响比对未离解酸的影响大得多。发现要在pH 8.0时再现实验光谱，必须将扩展溶剂环境的额外连续介质表示与明确的溶剂分子（n ≥ 3）结合起来；（vi）盐（NaCl和CaCl2）对吸收光谱的影响最小；（vii）实验结果表明，中性BA的B带对溶剂效应不敏感，而水对C带的影响显著。水的效应使该带蓝移高达约0.2 eV。总体而言，结果表明通过局限于小簇模型的计算，有能力进一步加深我们对水介质中BA电子结构和吸收光谱微观解释的理解。
