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| - Spectroscopic and Thermodynamic Study of Charge Transfer Interaction of DoxycyclineHydrochloride with Riboflavin in Aqueous Ethanol Media of Varying Compositions
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| - Doxycycline hydrochloride has been shown to form charge transfer (CT) complexes with a number of electronacceptors in 50% (v/v) aqueous ethanol medium. CT absorption bands of complexes with four acceptors ofknown electron affinity have been determined, and they correlate well with the electron affinities of theacceptors. From this correlation, the electron affinity of riboflavin has been estimated to be 1.02 eV. The CTabsorption bands have also been utilized to determine the vertical ionization potential of doxycyclinehydrochloride (7.05 eV). The enthalpy and entropy of formation of the complex between riboflavin anddoxycycline hydrochloride have been determined by estimating the formation constant (K) spectrophotometrically at five different temperatures in pure water. The pronounced effect of the dielectric constant of themedium on the magnitude of K has been observed by determining K in aqueous ethanol mixtures of varyingcompositions. This has been rationalized in terms of ionic dissociation of the hydrochloride DH+Cl-, hydrolysisof the cation DH+, and complexation of the free base D with riboflavin.
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