Ab initio Potential-Energy Surfaces and Electron-Spin-Exchange Cross Sections for H-O2 Interactions

Accurate quartet- and doublet-state potential-energy surfaces for the interaction of a hydrogen atom and an oxygen molecule in their ground states have been determined from an ab initio calculation using large-basis sets and the internally contracted multireference configuration interaction method. These potential surfaces have been used to calculate the H-O2 electron-spin-exchange cross section; the square root of the cross section (in a(sub 0)), not taking into account inelastic effects, can be obtained approximately from the expressions 2.390E(sup -1/6) and 5.266-0.708 log10(E) at low and high collision energies E (in E(sub h)), respectively. These functional forms, as well as the oscillatory structure of the cross section found at high energies, are expected from the nature of the interaction energy. The mean cross section (the cross section averaged over a Maxwellian velocity distribution) agrees reasonably well with the results of measurements

APPROXIMATE G1 ORBITALS

$^{1}$ W. A. Goddard, 111, Phys. Rev. 174, 659 (1968) $^{2}$ V. Kaldor, J. Chem. Phys. 48 , 835 (1968) $^{3}$ P. O. Lowdin and H. shull, Phys. Rev. 101, 1730 (1956)Author Institution:A generalized valence bond approach, the G1 method developed by $Goddard^{1}$, has several attractive features. For example, (1) it allows a single particle interpretation, (2) it yields the proper molecular dissociation, (3) it accounts for substantial part of the correlation energy, and (4) it produces an improved spin distribution near the nuclei. The G1 formulation, however, becomes unwieldy for systems with more than six electrons. To treat large systems, we have investigated an approximation to the G1 function. We expand the G1 symmetry operator and retain terms only to first order in electron exchange similar to kaldor’$s^{2}$ treatment of the Hartree-Fock method. The natural orbital $representation^{3}$ for each pair of G1 orbitals improves the convergence of this expansion. Our approximation is in the spirit similar to the Dirac-Van Vleck approach, but more general because our orbitals are nonorthogonal and are obtained variationally. We compare our results with those from exact G1 calculations for small atoms. Since the orbitals are nodeless, the G1 description should lead to unique well founded effective core potentials for application to large molecular structure calculations

THE 1Σ+^{1}\Sigma^{+} STATES OF HEAVY ALKALI HYDRIDES

$^{1}$ L. R. Kahn and W. A. Goddard, J. Chem. Phys., 56, 2685 (1972). $^{2}$ J. R. Stallcop, submitted for publication. $^{3}$ A. C. Tam and W. Happer, J. Chem. Phys. 64, 2456 (1978)Author Institution:The valence structure of the lowest $^{1}\Sigma^{+}$ states of KH, RbH, and CsH have been computed by a configuration Interaction method using effective core potentials. The core potentials of Kahn and $co-workers^{1}$ were modified to have the proper long-range $behavior^{2}$ and Include the effect of core polarization. The potential curves for the $X^{1}\Sigma^{+}$ and $A^{1}\Sigma^{+}$ stales agree with measured values generally to within 0.1 eV. Also, the $A^{1}\Sigma^{+}-X^{1}\Sigma^{+}$ transition moments of CsH compare favorably with the results of laser-excited fluorescence $experiments.^{3}

Potential Energy Curves and Transport Properties for the Interaction of He with Other Ground-state Atoms

The interactions of a He atom with a heavier atom are examined for 26 different elements, which are consecutive members selected from three rows (Li - Ne, Na - Ar, and K,Ca, Ga - Kr) and column 12 (Zn,Cd) of the periodic table. Interaction energies are determined wing high-quality ab initio calculations for the states of the molecule that would be formed from each pair of atoms in their ground states. Potential energies are tabulated for a broad range of Interatomic separation distances. The results show, for example, that the energy of an alkali interaction at small separations is nearly the same as that of a rare-gas interaction with the same electron configuration for the dosed shells. Furthermore, the repulsive-range parameter for this region is very short compared to its length for the repulsion dominated by the alkali-valence electron at large separations (beyond about 3-4 a(sub 0)). The potential energies in the region of the van der Waals minimum agree well with the most accurate results available. The ab initio energies are applied to calculate scattering cross sections and obtain the collision integrals that are needed to determine transport properties to second order. The theoretical values of Li-He total scattering cross sections and the rare-gas atom-He transport properties agree well (to within about 1%) with the corresponding measured data. Effective potential energies are constructed from the ab initio energies; the results have been shown to reproduce known transport data and can be readily applied to predict unknown transport properties for like-atom interactions

Reactive Resonances in N+N2 Exchange Reaction

Rich reactive resonances are found in a 3D quantum dynamics study of the N + N2 exchange reaction using a recently developed ab initio potential energy surface. This surface is characterized by a feature in the interaction region called Lake Eyring , that is, two symmetric transition states with a shallow minimum between them. An L2 analysis of the quasibound states associated with the shallow minimum confirms that the quasibound states associated with oscillations in all three degrees of freedom in Lake Eyring are responsible for the reactive resonances in the state-to-state reaction probabilities. The quasibound states, mostly the bending motions, give rise to strong reasonance peaks, whereas other motions contribute to the bumps and shoulders in the resonance structure. The initial state reaction probability further proves that the bending motions are the dominating factors of the reaction probability and have longer life times than the stretching motions. This is the first observation of reactive resonances from a "Lake Eyring" feature in a potential energy surface

Chemistry Modeling for Aerothermodynamics and TPS

Recent advances in supercomputers and highly scalable quantum chemistry software render computational chemistry methods a viable means of providing chemistry data for aerothermal analysis at a specific level of confidence. Four examples of first principles quantum chemistry calculations will be presented. The study of the highly nonequilibrium rotational distribution of nitrogen molecule from the exchange reaction N + N2 illustrates how chemical reactions can influence the rotational distribution. The reaction C2H + H2 is one example of a radical reaction that occurs during hypersonic entry into a methane containing atmosphere. A study of the etching of Si surface illustrates our approach to surface reactions. A recently developed web accessible database and software tool (DDD) that provides the radiation profile of diatomic molecules is also described

Quantum Scattering Study of Ro-Vibrational Excitations in N+N(sub 2) Collisions under Re-entry Conditions

A three-dimensional time-dependent quantum dynamics approach using a recently developed ab initio potential energy surface is applied to study ro-vibrational excitation in N+N2 exchange scattering for collision energies in the range 2.1- 3.2 eV. State-to-state integral exchange cross sections are examined to determine the distribution of excited rotational states of N(sub 2). The results demonstrate that highly-excited rotational states are produced by exchange scattering and furthermore, that the maximum value of (Delta)j increases rapidly with increasing collision energies. Integral exchange cross sections and exchange rate constants for excitation to the lower (upsilon = 0-3) vibrational energy levels are presented as a function of the collision energy. Excited-vibrational-state distributions for temperatures at 2,000 K and 10,000 K are included

Quantal Study of the Exchange Reaction for N + N2 using an ab initio Potential Energy Surface

The N + N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground A" state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N2(J=O). The J - K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasi-classical calculation using a LEPS PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate 3D quantal dynamics study for N + N2 reaction system and the ab initio PES reported here is the first such surface for N3