Received November 26, 2003

Abstract:

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 _fG, _fH and partial molal volume, v, of OH (aq). We find _rH = 46.8 ± 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and V_r = 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): _fH = -0.2 ± 1.4 and _fG = 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.

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