Journal article
Spectroscopic Evidence of Keto−Enol Tautomerism in Deliquesced Malonic Acid Particles
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, Vol.115(17), pp.4373-4380
05/05/2011
DOI: 10.1021/jp112360x
PMID: 21480651
Abstract
Scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and optical microscopy coupled with Fourier transform infrared spectroscopy (micro-FTIR) have been applied to observe hygroscopic growth and chemical changes in malonic acid particles deposited on substrates. The extent of the hygroscopic growth of particles has been quantified in terms of the corresponding water-to-solute ratios (WSR) based on STXM/NEXAFS and micro-FTIR data sets. WSR values derived separately from two applied methods displayed a remarkable agreement with previous data reported in the literature. Comparison of NEXAFS and FTIR spectra acquired at different relative humidity (RH) shows efficient keto−enol tautomerization of malonic acid, with the enol form dominating at higher RH. The keto−enol equilibrium constants were calculated using relevant peak intensities in the carbon and oxygen K-edge NEXAFS spectra as a function of RH. We report strong dependence of the equilibrium constant on RH, with measured values of 0.18 ± 0.03, 1.11 ± 0.14, and 2.33 ± 0.37 corresponding to 2, 50, and 90% RH, respectively. Enols are important intermediates in aldol condensation reactions pertaining to formation and atmospheric aging of secondary organic aerosol (SOA). The present knowledge assumes that constituents of atmospheric deliquesced particles undergo aqueous chemistry with kinetic and equilibrium constants analogous to reactions in bulk solutions, which would estimate absolute dominance of the keto form of carboxylic acids. For instance, the keto−enol equilibrium constant of malonic acid in diluted aqueous solution is <10−4. Our results suggest that in deliquesced micrometer-size particles, carboxylic acids may exist in predominantly enol forms that need to be explicitly considered in atmospheric aerosol chemistry.
Details
- Title: Subtitle
- Spectroscopic Evidence of Keto−Enol Tautomerism in Deliquesced Malonic Acid Particles
- Creators
- Suman GhoraiAlexander LaskinAlexei V Tivanski
- Resource Type
- Journal article
- Publication Details
- The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, Vol.115(17), pp.4373-4380
- Publisher
- American Chemical Society
- DOI
- 10.1021/jp112360x
- PMID
- 21480651
- ISSN
- 1089-5639
- eISSN
- 1520-5215
- Language
- English
- Date published
- 05/05/2011
- Academic Unit
- Chemistry
- Record Identifier
- 9983985932502771
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