A three dimensional potential energy surface for the F+H2→HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self-consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple-zeta plus polarization basis set employed in previous nine-electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H2 fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h functions on fluorine, a linear barrier height of 1.84 kcal/mol and an exothermicity of 31.77 kcal/mol (exp. 31.73 kcal/mol) was obtained. The true saddle point has a bent structure and the barrier height is predicted to be (1.45±0.25) kcal/mol. About 700 points on the three-dimensional potential energy surface have been computed using a slightly smaller basis set, which yield F–HH barrier heights of 1.92 kcal/mol (linear), 1.54 kcal/mol (bent), and an exothermicity of 31.3 kcal/mol. The barrier height for the H+FH→HF+H exchange reaction is predicted to be 41.2 kcal/mol. In the entrance channel cuts through the three potentials correlating with F(2P3/2,1/2)+H2(1Σ+g) have been computed, and the effect of spin–orbit coupling is investigated. It is found that the spin–orbit coupling increases the barrier height relative to the asymptotic F(2P3/2)+H2(1Σ+g) ground state by about 0.35 kcal/mol, leading to final estimates for the effective collinear and bent barriers of (2.18±0.25) kcal/mol and (1.80±0.25) kcal/mol, respectively. An accurate global analytical fit of the potential (without the effect of spin–orbit coupling) has been obtained using the method of Aguado and Paniagua. Our new ab initio potential is compared to various potentials used so far in dynamics calculations for the F+H2 reaction.

Quantum mechanical integral and differential cross sections have been calculated for the title reaction at the three collision energies studied in the 1985 molecular beam experiment of Lee and co-workers, using the new ab initio potential energy surface of Stark and Werner (preceding paper). Although the overall agreement between the calculated and experimental center-of-mass frame angular distributions is satisfactory, there are still some noticeable differences. In particular, the forward scattering of HF(v′=3) is more pronounced in the present calculations than it is in the experiment and the calculations also predict some forward scattering of HF(v′=2). A comparison with the quasiclassical trajectory results of Aoiz and co-workers on the same potential energy surface shows that the forward scattering is largely a quantum mechanical effect in both cases, being dominated by high orbital angular momenta in the tunneling region where the combined centrifugal and potential energy barrier prevents classical trajectories from reacting. The possible role of a reactive scattering resonance in contributing to the quantum mechanical forward scattering is also discussed in some detail.

The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.

A theoretical study of a mechanism for ethylene trimerization, catalyzed by Cr-pyrrolyl complexes, is proposed and investigated with density functional theory methods. The selective formation of 1-hexene is generally accepted to follow a metallacycle mechanism. A detailed spin state analysis for active species in the mechanism shows that the triplet spin state represents the ground spin state for all stationary points. Complete Gibbs free energy (298.15 K) surfaces are mapped for both η5- and σ-bonding modes of pyrrole, as well as a stripped-down Cl anion model and a full ClAlMe3 anion model. From the calculated results it is shown that the proposed metallacycle mechanism is energetically favorable, with metallacycle growth identified as the rate-determining step. In addition, it is demonstrated that different bonding modes of pyrrole are preferred at different stages in the proposed mechanism, effectively suggesting that ring slippage of the pyrrole occurs on the minimum energy path on the potential energy surface. From the calculated results important insight is gained into the hemilabile nature of the pyrrole ring in the mechanism, which in turn sheds light on the general requirements for an effective ligand in Cr-catalyzed ethylene trimerization.

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.