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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 C 6 H 5 COOH·(H 2 O) n and C 6 H 5 COO − ·(H 2 O) 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 CaCl 2 ) 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.
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Ultraviolet absorption spectra of acrylic acid and its conjugate base, acrylate, in aqueous solution
Acrylic acid is an important compound widely used in industry with multiple commercial applications, and it is also a key intermediate in the marine organosulfur cycle. However, the fundamental ultraviolet (UV) absorption spectrum of acrylic acid or its conjugate base, acrylate (pKa = 4.25 at 20 oC) have not been determined in water. In this paper, we determined the absorption spectrum of acrylate in aqueous solution at pH 7.2 and 20 oC between 207 and 400 nm. The molar absorptivity decreased rapidly from 3958 M‒1 cm‒1 at 207 nm to a non-detectable value at wavelengths greater than 330 nm, with weak absorption at wavelengths greater than 290 nm (e.g., ɛ290nm 2.7 M‒1 cm‒1). No discernable absorption bands were observed in the absorption spectrum. Excellent agreement was observed when comparing absorption spectra obtained (1) with two different spectrophotometers and (2) with standards prepared from either newly purchased sodium acrylate or from the base hydrolysis of dimethylsulfoniopropionate. Wavelength-dependent molar absorptivities were constant at pH 7.2 over a range of acrylate concentrations from 25 to 135 μM. The absorption spectrum red shifted when the solution pH increased from 2.8 to 8.2, with an isosbestic point observed at 214 nm indicating two exchangeable species in solution. Our study provides the first detailed UV absorption spectra of acrylic acid and acrylate in aqueous solution, with important implications regarding the detection and study of these compounds in environmental settings and commercial applications.
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- Award ID(s):
- 1756907
- PAR ID:
- 10504164
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Photochemistry and Photobiology A: Chemistry
- Volume:
- 449
- Issue:
- C
- ISSN:
- 1010-6030
- Page Range / eLocation ID:
- 115371
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jphotochem.2023.115371
