ANALYSIS OF COMPLEXATION REACTIONS THROUGH MULTISPECTRUM FITTING
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Date
2002
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Ohio State University
Abstract
Chemical reactions having equilibrium constants in the range 0.01 - 100 generally have significant amounts of both reactants and products present at equilibrium. If the reaction components have spectral signatures in the UV-visible region, such processes can be analyzed with remarkable precision through nonlinear least-squares fitting of multiple absorption spectra recorded as a function of temperature and composition. As one example, the classic complexation of $I_{2}$ with mesitylene in solvents like heptane and $CCl_{4}$ yields a 1:1 complex with a well-defined charge-transfer band in the near-UV and also a weaker ``molecular'' band at longer wavelengths. The latter has long been attributed to absorption within the $I_{2}$ of the complex, but because of extensive overlap with the absorption by free $I_{2}$ in the solution, has never been well defined. The multispectrum fitting approach precisely characterizes this band. Even routine spectrophotometric data can yield 1% precision in the equilibrium constants K. In the case of the gas-phase reaction yielding BrCl from the parent halogens, this precision translates into a $\sim 1-cm^{-1}$ uncertainty in the molecular dissociation energy, which is a factor of ${\sim} 5$ better than the current best spectroscopic estimate. For reference, values of $K^{\circ}$ for this reaction appearing in the recent literature range from 6 to 10.
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Author Institution: Department of Chemistry, Vanderbilt University