Thermochemistry of aqueous hydroxyl radical from advances in photoacoustic calorimetry and ab initio continuum solvation theory

Photoacoustic signals from dilute (∼30 mM) solutions of H2O2 were measured over the temperature range from 10 to 45 °C to obtain the reaction enthalpy and volume change for H2O2(aq) → 2 OH(aq) from which we ultimately determined ΔfG°, ΔfH° and partial molal volume, v°, of OH (aq). We find ΔrH = 46.8 ± 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and ΔVr = 6.5 ± 0.4 mL/mol. The v° for OH in water is 14.4 ± 0.4 mL/ml: smaller than the v° of water. Using ab intio continuum theory, the hydration free energy is calculated to be ?3.9 ± 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by rolling" a three-dimensional electron density isocontour of water on the ab initio water?OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): ΔfH° = ?0.2 ± 1.4 and ΔfG° = 5.8 ± 0.4 kcal/mol that are in agreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory toward ab initio-defined solute cavities. Copyright © 2004 American Chemical Society."