New ab initio calculations of the potential energy surface of the recombination reaction H ם O 2 ם( M) → HO 2 ם( M), together with statistical unimolecular rate theory employing extensive classical trajectory calculations on this potential, allowed the characterization of thermally averaged and energy E-and angular momentum J-specific rate constants of the H ם O 2 ⇔ HO 2 recombination/dissociation system. On the basis of these results, falloff curves of the reaction over wide temperature and pressure ranges were constructed that are consistent with the limited available experimental data and that allow for extrapolations into experimentally unexplored ranges of conditions. The calculations go beyond earlier work in a variety of ways: (1) a potential with improved precision in the kinetically relevant range of interfragment distances was used, (2) the dynamics was shown to be neither adiabatic nor sudden such that neither simplified SACM-nor RRKM-type approaches apply, (3) the falloff curves were dominated by rotational and weak collision broadening, illustrating the necessity of using E-and J-specific rate constants k(E,J) in the calculations, (4) centrifugal barriers E 0 (J) were revised by considering the full dimensionality of the problem, and (5) vibrational anharmonicities were approximated in an improved way by including stretchbend coupling contributions.
The paper provides analytical representations of the pressure and temperature dependence of the rate coefficients over wide ranges of conditions. In the simplest form, limiting low-pressure rate constants k rec,0 / [N 2 ] ס (5.4 ע 0.5) ן 10 23מ (T/300 K) 4.1מ cm 6 molecule 2מ s 1מ and k rec,0 /[Ar] ס (2.2 ע 0.3) ן 10 23מ (T/ 300 K) 2.1מ cm 6 molecule 1מ s 1מ as well as limiting high-pressure rate constants k rec,ϱ ס (9.5 ע 1.0) ן 10 11מ (T/300 K) 44.0ם cm 3 molecule 1מ s 1מ are recommended over the range 300-2000 K. Falloff curves are constructed with F cent Ϸ 0.5 ע 0.1 independent of the temperature over the range 300-2000 K.