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coefficients of HCI. This is attributedg9 to deficiencies in the short-range repulsion potential. We are not aware of comparisons between HNC equations and transient kinetic data. We hope that our results will motivate a more comprehensive effort of comparing the statistical mechanics theories with experiment.At large salt concentrations, the salt effect is not merely "physical" screening of the Coulombic interaction. When the interionic separation is small, "chemical" bond strengths (of the RO-H bond in HPTS, for example) are modified. In this limit the kinetic parameters Kd and K, are no longer independent of salt concentrations, as assumed above. In fact, it has been showngsd that Kd decreases with increasing salt concentration, in proportion to the solvent activity to some power. It would be interesting to extend the present measurements to higher concentrations. Another aspect is the (weak) dependence of the diffusion coefficient on salt concentration. Such effects would modify our model in the direction of an even weaker dependence of QY on salt concentration.We have already reportedZo a preliminary investigation of salt effects in time-resolved measurements. A more extended com-parison between theory and transient measurements is deferred to subsequent publications. Finally, extension of the results obtained in this paper to spatially limited reactions, such as those in inverse micelles,34 could provide an important tool for understanding similar processes in biological systems.42 Note Added in Proof. A recently published investigation (Suwaiyah, A,; AI-Adel, F.; Hamdan, A,; Klein, U. K. A. J. Phys. Chem. 1990, 94, 7423) finds that the absolute QY of HPTS in water is about 0.75. This contrasts with earlier report^'^,^^ of an absolute QY of about unity. If substantiated, this would slightly affect the quantitative assessment of the relative QY data. (42) Gutman, M.; Nachliel, E. A kinetic study of the reaction of hydrogen atoms with methane was performed using the Brookhaven flash photolysis-shock tube (FP-ST) facility. Experiments were conducted in the reflected shock regime over the temperature range 897-1729 K under pseudo-first-order conditions ( [CH4]/[H] 1 200). Hydrogen atoms were monitored by using atomic resonance absorption and rate constants were derived directly from decays of absorbance signals. Separate experiments, in which the initial H-atom and radical concentrations were varied, demonstrated the absence of kinetic complications due to secondary reactions. This observation was further verified by the results of a computer modeling study. The rate constant values obtained in this research may be fitted with equal uncertainty to either a two-parameter expression, k,(T) = (1.78 f 0.12) X exp[-(6440 f 80) KIT)], or a three-parameter expression, kl(T) = 1.6 X 10-'9T2.57fo.76 exp[-(3340 f 920) K/U. Unitsare cm3 molecule-' s-l with uncertainties quoted at the one standard deviation level. The preexponential term for the three-parameter expression and its uncertainty are given in logarithmic form:...

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Rate coefficients for the reaction CH3 + O2 = CH3O + O were measured behind reflected shock waves in a series of lean CH4−O2−Ar mixtures using hydroxyl and methyl radical diagnostics. The rate coefficients are well represented by an Arrhenius expression given as k = (1.6 ) × 1013 exp(−15813 ± 587 K/T) cm3 mol-1 s-1. This expression, which is valid in the temperature range 1575−1822 K, supports the downward trend in the rate coefficients that has been reported in recent determinations. All measurements to date, including the present study, have been to some extent affected by secondary reactions. The complications due to secondary reactions, choice of thermochemical data, and shock−boundary layer interactions that affect the determination of the rate coefficients are examined.