In this paper we show that the usual assumption in studies of the temperature variation of equilibrium constants for equilibria of the form A + B T± AB that a plot of In K vs. 1/T (K = equilibrium constant, T = temperature in degrees kelvin) is a straight line with slope equal to -AHVH/R (AHVH = van't Hoff or apparent enthalpy, R = gas constant) is not valid in many cases. In all the cases considered here, AHVH is temperature dependent and is significantly different from the true or calorimetrically measured enthalpy, and the respective values for ACp are also significantly different.The ready availability of sensitive isothermal titration calorimeters makes possible the accurate determination of both enthalpies and equilibrium constants on the same sample for a wide variety of processes. In view of this situation, detailed comparisons of calorimetrically determined enthalpies of reaction (AHcal) and enthalpies derived from equilibrium constants by means of the van't Hoff equation (AHVH) highlighted by the fact that ACp as evaluated from the equilibrium constants is -0.287 kcal-K-1 mol-1, which is 45% larger in magnitude than that derived from the observed enthalpies-namely, -0.198 kcal K-1 mol-'.To check on the possibility that a significant contribution to AHcal might arise from an exchange of protons between the protein and the buffer, the two reactants were mixed in the absence of buffer at pH 5.5 at -25°C. The results indicated that the reaction leads to the liberation of 0.025 ± 0.05 mol of HI per mol of protein, corresponding in acetate buffer to -0.05 kcal mol-1 due to buffer protonation and probably a similar, perhaps compensating, contribution due to deprotonation of the protein.

No abstract

C-H direction. Indeed, the electron impact studies show dissociations in both directions increasing the number of low-lying states. The decrease of 0.13 eV in the adiabatic electron affinities in going from C6F6 to C6F5H is reasonable.In summary, the potential energy curves support the BCD data obtained for the negative ions of C6F6 and indirectly support the BCD data obtained for C6F5H. Thus, it appears that the BCD results are reasonable values for the ground-state electron affinities.The values of the rate constants for thermal electron attachment determined in the BCD at 1-atm pressure agree within a factor of 2 with values reported in the literature. The only experimental observation which cannot be explained is the value of the electron affinity of C6F6 determined from the endothermic charge transfer from S'.5Acknowledgment. We thank the Robert A. Welch Foundation, Grant E-095, for financial support of this work. The helpful suggestions of the reviewers are also recognized, especially in pointing out references which had been overlooked.

Glossaryconstants in eq 1, a has units of K constants in eq 2 , c has units of K constants in eq 3, x has units of K Kirkwood correlation factor Boltzmann constant, J K-' diffusion-controlled rate constant, dm3 mol-' s-' refractive index Arrhenius A factor, dm3 mol-' s-' activation energy, kJ mol-' activation energy of viscous flow, kJ mol-' molar mass, kg mol-' Avogadro number, mol-' gas constant, J mol-' K-' temperature, K relative permittivity high-frequency relative permittivity (= 1.1 X n D2) permittivity of vacuum, C2 N-' m-2 €0 17 viscosity, N s m-2 PO dipole moment, C m P density, kg m-3 Conductivity is in units of S m-' (1 siemen = 1 mho) Reglstry No. NMP, 872-50-4. Literature Cited Wuepper, J. L.; Popov, A. I. J. Am. Chem. SOC. 1969, 91, 4352. Dyke, J. D.; Sears, P. G.; Popov, A. I. J. Phys. Chem. 1967, 7 1 , 4140. Breant, M. Bull. SOC. Chim. Fr. 1971, 725 and references therein. Sowinski, A. F.; Whitesides, G. M. J. Org. Chem. 1979, 4 4 , 2369. Guarino, G.; Ortona, 0.; Sartorio, R.; Vitagiiano, V. J. Chem. Eng. Data 1985, 3 0 , 366. Kadhum, A. A.; Langan, J. R.; Salmon, G. A,; Edwards, P. P. J . Radioanal.

No abstract