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.

Benzothiazole (BTH) and its derivatives are amongst a group of emerging contaminants that are widely distributed in the environment due to their extensive use in many different consumer products. In air, reaction with the hydroxyl radical (OH) is expected to be a major loss process for BTH in the gas phase, but the kinetics and mechanisms are unknown. Here, we report a combination of experiments and theory to determine both the rate constant and products of the reaction of OH with the smallest member of the series, benzothiazole, in the gas phase. The mechanism first involves an attack by OH on BTH to produce several OHBTH intermediates. This is followed by O₂ reactions with OHBTH, leading to several stable products successfully predicted by theory. Relative rate studies at 1 atm in air and 298 K using benzene as a reference gave a rate constant for the BTH + OH reaction of 2.1 ± 0.1 × 10⁻¹² (1σ) cm³ per molecule per s, which translates to a lifetime in air of 5.5 days at 1 × 10⁶ OH cm⁻³. Four hydroxybenzothiazole products reflecting attack on different carbon atoms of the benzene ring were measured (n-OHBTH, where n = 4, 5, 6, 7), with the relative product yields well predicted by the calculated formation energies of the pre-reaction OH⋯BTH complex. Attack of OH on the –CH of the thiazole ring leads to the formation of 2-OHBTH, representing a smaller fraction of the overall reaction, and is shown to proceed through a more complex mechanism than attack on the benzene ring. A theoretical approach to predicting chromatographic retention times of the products based on solvation free energies (ΔG_solv) was successful for most of the products. These studies illustrate how the powerful combination of experiment and theory can be used to predict products of atmospheric oxidation of emerging contaminants and ultimately used to assess their impacts on the environment.