Showing papers by "Ernest R. Davidson published in 1982"

Potential surface for the methylenecyclopropane rearrangement

Abstract: MCSCF calculations with an STO-3G basis set have been carried out to obtain the optimal geometries and associated energies for a grid of points on the lowest singlet potential surface connecting methylenecyclopropane (MCP) and trimethylenemethane (TMM). Two methylene rotation angles were used to generate a two-dimensional projection of the 24-dimensional hypersurface, and a double Fourier fit to the computed energies gave a closed form expression for the energy on the two-dimensional surface. Three minima, connected by four transition states, were located on the surface. The effect of basis set expansion and inclusion of CI on the relative energies was determined. In accord with Hammond's postulate, the energies of transition states leading to orthogonal and planar TMM were found to be in the same order as the energies of these two intermediates. The conrotary transition state leading to planar TMM was found to be slightly lower in energy than the disrotary one. From a Fourier fit to the triplet energies, calculated at the optimal singlet geometries on the twedimensional surface, the line along which the singlet and triplet surfaces intersect has been found. The lowest point along this line was computed to be higher in energy than orthogonal TMM. The implications of this finding are discussed. The thermal rearrangements of methylenecyclopropane (MCP) derivatives have been the subject of numerous experimental investigations.] These studies suggest that the major pathway involves the orthogonal geometry (1) in which one methylene group H. 'C / H . The preference for 1 over 2 in the lowest singlet state contrasts with the geometry of the lowest triplet state. Both the null value of the zero field splitting parameter E" and the hyperfine coupling patternI2 suggest a triplet geometry of at least C, symmetry. Ab initio calculations predict the triplet geometry to be planar &,.I3 The difference between the preferred geometry of the singlet and triplet has been rationalized in terms of electron repulsion effects.14 A large number of calculations have been carried out to determine the relative energies of the planar and orthogonal singlet states, and all predict that the orthogonal singlet is 1 0 w e r . ' ~ ~ ~ H \ / H H \ / l H \ , / F. *, / I I1 H\C/:+C/H H\, /C\ , /H

ESR investigation of matrix isolated B 16 O and B 17 O radicals: Comparison of nuclear hyperfine structure with ab initio calculations

Abstract: An extensive ESR investigation of the 2Σ B16O and B17O radicals in rare gas matrices has been conducted. Several different trapping sites in neon matrices have been found and investigated as a function of temperature over the range 3–10 K both during and after deposition. Motional averaging of the nuclear hyperfine tensor seems to be occurring for two neon sites and the argon matrix. The large isotropic boron hyperfine coupling of these sites and the 17O hfs show excellent agreement with ab initio theoretical results. BO radicals trapped in a third neon site do not appear to be undergoing motional behavior. Isotropic and dipolar hyperfine parameters for this site show reasonable agreement with theoretical results. BO radicals in one neon site which is extremely sensitive to temperature effects exhibit nearly perfect ’’single‐crystal‐like’’ preferential orientation which has not been previously observed in rare gas matrix ESR studies. The results of this BO investigation help to correct erroneous conclusio...

A theoretical determination of the electron affinity of methylene

Abstract: Large basis set configuration interaction calculations yield an electron affinity of 0.42 eV for CH2(3B1). Application of an empirical correction, based on the known deficiencies of the basis set and CI method in atomic calculations, suggests that this estimate should be increased to approximately 0.63 eV. This provides futher evidence in favor of a reinterpretation of the photoelectron spectrum work of Zittel et al., which indicated an electron affinity of only 0.210 eV while giving a 3B1–1A1 energy gap of 19.6 kcal/mol.

Porphyrins 42. Ground and excited state calculations on the isomers of free base porphine and sirohydrochlorin

Abstract: Symmetry instabilities were encountered during MINDO/3 geometry optimizations of the sirohydrochlorin and porphine isomers leading to bond alternating optimal structures. Transition energies and oscillator strengths were calculated with INDO/S/CI. Our calculations predict the ground state cis and trans isomers of sirohydrochlorin to be close in energy and confirm the experimental assignment of the absorptions bands, with the cis tautomer having a red shifted spectrum.

Integral dependent spin couplings in CI calculations

Abstract: Integral dependent spin‐couplings are constructed which yield one spin coupling per product of orbitals and which reproduce the energy through third order. These couplings yield about 98.5% of the correlation energy obtained by using all spin couplings. Extension of this method to multireference zeroth order states requires little extra effort.

Oxygen K hole photoionization cross section of CO2

Abstract: The oxygen K hole photoionization cross section of CO2 has been computed in the discrete basis improved virtual orbital (IVO) and CI virtual orbital (CIVO) approximations. Reasonable agreement with the experimental result is obtained. Effects of basis set, hole localization, and imaging procedure are examined, and shake‐up effects are discussed.

Many‐body perturbation theory and phosphorescence: Application to CH2

Abstract: A diagrammatic formalism is derived which facilitates the calculation of phosphorescent lifetimes. This formalism uses double perturbation theory where the spin–own–orbit interaction and the two electron fluctution potential are the perturbations. The transition amplitude between the ground state and the phosphorescent state is calculated to first order in both perturbations. The phosphorescent lifetime of the 1A1 state of methylene is evaluated, by way of illustration, and is predicted to be in the range of 40–100 s. The transition amplitude, in this case, is due primarily to the direct spin–orbit mixing of the 1A1 state with the 3B1 ground state.

Porphyrins. 42. ground- and excited-state calculations on the isomers of free-base porphine and sirohydrochlorin

Abstract: Symmetry instabilities were encountered during MINDO/3 geometry optimizations of the sirohydrochlorin and porphine isomers leading to bond alternating optimal structures. Transition energies and oscillator strengths were calculated with INDO/S/CI. Our calculations predict the ground state cis and trans isomers of sirohydrochlorin to be close in energy and confirm the experimental assignment of the absorptions bands, with the cis tautomer having a red shifted spectrum.