Peter K. Pearson

Potential energy surface for the model unimolecular reaction HNC → HCN

Ab initio electronic structure theory has been used to determine the more important features of the potential energy surface for the simple isomerization reaction HNC → HCN. Extended basis sets were used in conjunction with both self−consistent−field (SCF) and configuration interaction (CI) wavefunctions. For nonlinear or Cs geometrical arrangements of the three atoms, the CI included 11735 configurations, i.e., all single and double excitations. This large scale CI reproduces the HCN ground state geometry quite accurately and has been used to tentatively identify HNC in the interstellar medium. The SCF calculations predict HNC to lie 9.5 kcal/mole above HCN, while CI yields 14.6 kcal/mole. Similarly, barrier heights of 40.2 and 34.9 kcal/mole are predicted by SCF and CI. Thus, the SCF approximation is qualitatively reasonable for HNC → HCN. If HNC is designated by a reaction angle of 180° and HCN by 0°, then the saddle point or transition state is predicted to lie at 73.7°, significantly closer to HCN. A...

Potential Energy Surface Including Electron Correlation for F + H2 → FH + H: Refined Linear Surface

A priori quantum mechanical calculations have been carried out at about 150 linear geometries for the fluorine plus hydrogen molecule system. An extended basis set of Gaussian functions was used, and electron correlation was treated explicitly by configuration interaction. Comparison with the experimental activation energy and exothermicity suggests that the theoretical potential surface is quite realistic.

Potential Energy Surface Including Electron Correlation for the Chemical F + H2 → FH + H I. Preliminary Surface

Rigorous quantum mechanical calculations have been carried out for about 150 linear and 200 non‐linear geometries for the FH2 system. The contracted Gaussian basis set used consisted of four s and two p functions on fluorine and two s functions on hydrogen. The barrier height and exothermicity are poorly predicted by single configuration self‐consistent‐field calculations. However, the 214‐configuration correlated results are in qualitative agreement with experiment (low barrier height and substantial exothermicity). The reaction coordinate is discussed, and pictures of the potential surface are presented. A second series of calculations is being carried out with a larger basis set. These latter calculations yield nearly quantitative agreement with experiment for both the barrier height and exothermicity.

Theoretical Potential Energy Curves for OH, HF+, HF, HF−, NeH+, and NeH

Ab initio calculations have been carried out on the ground states of OH, HF+, HF, HF−, NeH+, and NeH. Extended basis sets were used and electron correlation was included by way of first‐order wavefunctions. Dissociation energies and other spectroscopic constants are in good agreement with available experimental data except for the bond distance of HF+. Electron detachment in collisions between H and F− is discussed on the basis of the calculated potential curves. Potential curves were also obtained ab initio for the three lowest excited states of NeH. These curves are qualitatively similar to those reported earlier by Slocomb, Miller, and Schaefer for HeH. The C 3Σ+ state of NeH is predicted to have a potential maximum of 0.87 eV at internuclear separation ∼ 4 bohr.

On the H+F2→HF+F reaction. An ab initio potential energy surface

Rigorous quantum mechanical calculations have been carried out to predict the H+F2→HF+F potential energy surface. A double zeta basis set was used, and open‐shell self‐consistent‐field (SCF) calculations were carried out. In addition, electron correlation was explicitly treated using first‐order wavefunctions, made up of 555 2A′ configurations. Orbitals were optimized by the interative natural orbital method. From the SCF calculations the barrier height and exothermicity are predicted to be 12.2 and 132.4 kcal/mole, respectively. The configuration interaction (CI) values are 1.0 and 88.3 kcal, in much better agreement with the experimental values, 1.2 and 102.5 kcal. The saddle point is predicted from the CI calculations to occur for a linear geometry, R(H–F)=2.05 A, R(F–F)=1.57 A. This corresponds to an H–F separation more than twice as great as in the HF molecule but an F–F separation is only slightly (0.03 A) longer than in the isolated F2 molecule. A substantial number of calculations were carried out...

Role of Electron Correlation in a Priori Predictions of the Electronic Ground State of BeO

Ab initio wavefunctions including electron correlation have been calculated for the 3II state of BeO. A (4s2p1d) basis set of Slater functions was centered on each atom. The iterative natural orbital method was used to optimize the set of molecular orbitals employed in each 591 configuration first‐order wave‐function. The 3II energy calculated here is 0.73 eV above the 1Σ+ energy obtained in a comparable calculation. Since near Hartree‐Fock calculations result in a 3II energy below the 1Σ+ energy, it seems clear that electron correlation plays a crucial role in the ordering of these states. Predicted spectroscopic constants for the 3II state are: Re=1.463 A, ωe= 1270 cm−1, and Be= 1.365 cm−1. Natural orbital occupation numbers and coefficients of important configurations in the CI wavefunctions are presented to describe the electronic structure of 3II BeO. First‐order calculations (519 configurations) were also carried out for the lowest 3Σ− state of BeO. These calculations confirm our previous SCF predic...

Repulsive 3∑− and low-lying (⩾ 1.9 eV) 3∑+ states of BeO

Abstract Ab initio calculations have been carried out on the lowest 3 ∑ − and 3 ∑ + states of beryllium oxide. A “double zeta plus polarization” set of Slater functions was used. The self-consistent-field wavefunction for the 3 ∑ − state dissociates properly to ground state Be and O atoms and is repulsive. Electron correlation was explicitly considered for the 3 ∑ + state using “first-order” wavefunctions, which have yielded reliable dissociation energies for other diatomic molecules. The 3 ∑ + state, which has not been observed experimentally, is predicted to lie 1.91 eV above the lowest 1 ∑ + state. Predicted spectroscopic constants for the 3 ∑ + state are r e = 1.384 A, ω e = 1234 cm −1 , and B e = 1.527 cm −1 .

Potential curves and inelastic cross sections for low energy collisions of O+ and He

Potential curves corresponding to all the valence states of HeO+ have been calculated with a minimum basis set and full configuration interaction. The principal inelastic process in low energy collisions of ground state He and O+ is seen to be the 4S→ 2D excitation O+, the transition arising from a spin‐orbit interaction at a crossing of the lowest 4Σ and 2Π states of HeO+. Much more accurate calculations were thus carried out for these two states, as well as a semiclassical calculation of the cross section for He+O+(4S)→ He+O+(2D). The cross section has no activation energy other than its energetic threshold (3.3 eV) and rises to a maximum of ∼8.6× 10−3 A2 at ∼ 6 eV. There is a residual oscillatory structure in the energy dependence of the cross section, and it is shown how experimental observation of this could be used to obtain precise information concerning the relevant potential curves.