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The excited-state proton transfer of 3-hydroxybenzoic acid and 4-hydroxybenzoic acid was studied by time-resolved laser-induced fluorescence spectroscopy with ultra-short laser pulses. The excited-state reactions were identified in aqueous media as a function of the pH value. Apart from the well-known inversion of the ordinary dissociation properties of these compounds, new species were found which exist only in the excited-state resulting from a temporal and reversible annihilation of the aromatic bond system. These species and their reaction mechanisms were detected by their absorption and fluorescence spectra.

Solution and solid-state properties of m-hydroxybenzoic acid have been investigated. Two polymorphs were found where the monoclinic modification exhibits a higher stability than the orthorhombic form. The solubility of the monoclinic polymorph was determined between 10 and 50 degrees C in methanol, acetonitrile, acetic acid, acetone, water and ethyl acetate. The solubility of the orthorhombic polymorph was determined between 10 and 50 degrees C in acetonitrile, acetic acid, acetone and ethyl acetate. A thermodynamic analysis revealed a marked correlation between the molar solubility and the van't Hoff enthalpy of solution at constant temperature. In addition, in each solvent increased temperature resulted in increased van't Hoff enthalpy of solution. It is shown that the solubility data can be used to estimate melting properties for both polymorphs. The solubility ratio of the two forms and the DSC thermogram of the orthorhombic form strongly suggest that the system is monotropic. However, according to the polymorph rules of Burger and Ramberger, the estimated higher melting enthalpy and lower melting temperature of the orthorhombic form points towards an enantiotropic system. Hence, this system appears to be an exception to the Burger and Ramberger melting enthalpy rule, and the probable reason for this is found in the difference in the heat capacity of the two solid forms., QC 20100831