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.
Thermal rate coefficients are calculated for the reaction, (1), H+O2→OH+O and its reverse, (−1), O+OH→O2+H, using the quasiclassical trajectory method and the most recently reported double many-body expansion (DMBE IV) potential energy surface for the ground electronic state of the hydroperoxyl radical. The full range of temperatures for which experimental data is available in the literature has been covered, namely, 1000≤T≤3000 K for reaction (1) and 150≤T≤3000 K for reaction (−1). The equilibrium constant has also been evaluated. In addition, calculations of the isotopic effect on the thermal rate coefficient for the deuterated reactions D+O2→OD+O and O+OD→O2+D, and equilibrium constant, are reported. The theoretical results are shown to be in good agreement with the most recent and accurate experimental measurements.
A new potential energy surface for the O((3)P) + H(2) system in the lowest (3)A(") state is built using ab initio data calculated by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)] and the double many-body expansion formalism. It incorporates a semiempirical model of long-range interactions, which should play an important role at low collision energies. Preliminary quasiclassical trajectory results at 12.6 kcal/mol collision energy, show that the deeper van der Waals region described in this new surface translates into a four times higher cross section than that of Rogers' (3)A(") surface. To confirm this hypothesis, a second surface was calibrated. The two surfaces are fitted with rmsd<0.5 kcal/mol and differ mainly on the depth of the van der Waals region. That difference in the van der Waals region corresponds to a 22% lower cross section of the less deep surface, which is still three times higher than the equivalent results from Rogers' (3)A(") surface. This study reflects the importance of a correct description of van der Waals forces on potential energy surfaces.
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.
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