We report a new single-valued potential energy surface for the ground state of H 0 2 from the double many-body expansion (DMBE) method. This new surface conforms with the three-body energy of recent ab initio CAS SCF/CCI calculations semiempirically corrected by the DMBE-SEC method and reproduces the most accurate estimates of the experimental dissociation energy, equilibrium geometry, and quadratic force constants for the hydroperoxyl radical. Using this new H 0 2 (DMBE IV) potential energy function, exploratory dynamics calculations of the 0 + OH -O2 + PI reaction have also been carried out by the quasiclassical trajectory method. Thermal rate coefficients are reported for T = 250, 1250, and 2250 K that are shown to be in good agreement with the best reported measurements.

nuclei interacting with Ln3+ ions (in which case y in eq 3 refers to the nucleus and r to the electron-nuclear distance). As part of an NMR study of lanthanidebound micelles, we have measured the proton Ti relaxation times for SDS micelles (0.07 M surfactant) to which a variety of Ln3+ ions (0.002 M) had been added.22 The quantity of interest, plotted as hollow squares in Figure 1, is the relaxation enhancement for the CH, group in SDS bound directly to the sulfate, defined of dipolar Ti enhancements (hollow squares) does not match the pattern of k, values (filled squares). This argues against a significant dipolar contribution to k,. ConclusionWe have measured bimolecular quenching rate constants k, for interaction of lanthanide ions with the 1,9-biradical 2. The evidence so far suggests that spin exchange is the principal quenching mechanism. The dipolar mechanism does not appear to have a major influence on the quenching. Further investigations, including the magnetic field dependence and chain length dependence of k,, and lanthanide effect on intramolecular product ratios, are in progress. A double many-body expansion potential energy surface reported previously for H02(R2A") and referred to here as DMBE I is modified to produce thermal rate coefficients for the reaction 0 + OH -O2 + H in good agreement with experiment. Acknowledgment. The authors thank theThis new potential energy surface will be referred to as DMBE 11. By the further imposition that the potential function should reproduce the experimental spectroscopic force field data for the hydroperoxyl radical, another potential energy surface has been obtained, DMBE 111. Both of these improved DMBE I1 and DMBE 111 potential energy surfaces preserve the functional form used previously for DMBE I except for the long-range 0. -.OH electrostatic interaction, which is defined in the spirit of a more satisfactory adiabatic theory.

Quasi-classical trajectory calculations of the thermal rate coefficient for the title reaction have been carried out over the temperature range 250 5 T 5 2500 K by using two recently reported DMBE potential energy surfaces for the ground state of the hydroperoxyl radical. The results are compared with each other and with experiment. The agreement is g o d. Our results support previous theoretical calculations by Miller on the Melius-Blint potential energy surface in that nonstatistical 'recrossing" effects are very important. For the DMBE I1 (DMBE 111) potential energy surface, these nonstatistical corrections are found to increase from a factor of about 1.2 (1.4) at 250 K to about 2.1 (2.5) at 2500 K. However, they are considerably smaller than the nonstatistical corrections reported by Miller (factors of about 2.2 and 3.3 at the above temperatures). Although due, of course, to topographical differences between the DMBE and Melius-Blint potential energy surfaces, such discrepancy stems also from the different definitions used for H02* complex in the simple chemical model 0 + OH e H02*-+ 0 2 + H.