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. Calculations of the UV-vis absorbance spectra are then carried out for different clusters such as C 6H 5COOH 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. The solutions of different pH provide insights into the contributions from both the non-dissociated acid molecule and the deprotonated anionic species. 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 absorption spectra of molecular organic chromophores in aqueous media are of considerable importance in environmental chemistry.
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