摘要:

A wide variety of theoretical methods have been applied to a very simple but notoriously difficult problem, the calculation of the bond distance in F2+. All theoretical methods used the same basis set, the standard Huzinaga-Dunning double-zeta plus polarization (DZ + P) set, designated F (9s 5p 1d/4s 2p 1d). All methods which enforce inversion symmetry and go beyond second-order perturbation theory are qualitatively successful, giving bond distances within 005 Å of experiment. Methods not enforcing inversion symmetry are successful to within 003 Å if based on a restricted Hartree-Fock (RHF) starting point. When the wavefunction is not constrained to have inversion symmetry, methods based on an unrestricted Hartree-Fock (UHF) starting point are less satisfactory, yielding errors in the F2+bond distance ranging from 0-092 Å (full fourth-order perturbation theory, UMP4) to 0-850 Å (single-determinant UHF).

ISSN:0144-235X    

DOI:10.1080/01442358609353387

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

In photoelectrochemical cells which convert radiant energy to electrical energy the size of the built-in potentialUbis a key quantity. 17band the flat-band potential,Efb, change with a variation in the redox potential of the electrolyte. Such changes as have been reported in the literature are reviewed. In addition for the cell n-CdTe|selenide solution the changes inUbandEfbwith redox potential are reported. As Eredoiwent more negative,Uhincreased to a maximum and then decreased. The changes are attributed to changing adsorption of selenide species.

ISSN:0144-235X    

DOI:10.1080/01442358609353388

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Non-exponential relaxation in disordered systems may be interpreted in terms of a distribution of exponential rate constants that arises naturally from a narrow, random distribution in the value of a structural parameter (distance or angle) which characterizes the inequivalent sites within the system. This analysis is applied to the decay of methyl radicals in glassy methanol and to other systems. In the former case, the underlying assumptions have been tested experimentally. Such an approach indicates in general terms which kind of disorder is responsible for the observed decay. It also offers the prospect of obtaining a detailed description of the microscopic structure of a reactive site.

ISSN:0144-235X    

DOI:10.1080/01442358609353389

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The diffraction experiment provides explicit data which relate directly to the electron density (X-ray scattering) or the magnetization density (polarized neutron scattering). A summary is given of the determination of spin densities in a number of paramagnetic molecular ions with orbitally non-degenerate ground states. The observed densities are compared with theoretical models up to the level of unrestricted Hartree-Fock. Electron correlation is amongst the most important features in the molecular spin densities; observed spin transfer from the metals to the ligands can be matched only with a theory based on a quite extensive basis set.

ISSN:0144-235X    

DOI:10.1080/01442358609353390

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Polarized neutron and X-ray diffraction experimental results on simple Cr(III), Ni(II) and Co(ll) complexes are compared with theoretical calculations. A simple ionic (crystal-field) model is a useful first approximation for the description of the spin and charge densities. For the more covalently bound cases the effects of electron-electron correlation in the metal-ligand bonding are as important as the spin and charge transferred via simple covalence. Thus simple M.O. models do not offer much improvement, nor doab initiocalculations which do not include configurational interaction (viz. restricted Hartree-Fock). Unconstrained calculations which partly incorporate electron-electron correlation are in qualitative, but not quantitative, agreement with the experiments. The polarized neutron diffraction experiments provide such sensitive tests of theory because they spatially separate the small effects in the bonding region from the metal atom 3d region, whereas these are inextricably mixed in the energetic properties measured by spectroscopic techniques.

ISSN:0144-235X    

DOI:10.1080/01442358609353391

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Circular dichroism and optical activity are well known manifestations of optical activity. In recent years several novel types of optical activity have been observed. An achiral system bathed in an intense beam of circularly polarized light shows chiral behaviour which can be detected as circular dichroism or optical rotation. In another type of induced circular dichroism, a chiral solvent is used to induce chirality. Theories of natural and induced optical activity are reviewed within a unified framework using quantum electrodynamics.

ISSN:0144-235X    

DOI:10.1080/01442358609353392

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The functional groups at which nucleophilic or electrophilic substitution or addition occur in organic molecules may be considered as charge traps in the charged-radical transient (intermediate or transition) states which play a crucial part in such reactions. The role of the sigma framework in modifying the electronic properties of the functional-group trapping states may be referred to as sigma coupling. The general expressions for sigma coupling are derived in the context of a charge-excitation model for the transient species. They provide a quantitative definition of Hoffmann's through-bond concept.

ISSN:0144-235X    

DOI:10.1080/01442358609353393

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The dipole polarizability gives the first-order effect of an electric field on an electron cloud. Its calculation depends on the perturbation of the wavefunction by the electric field. The application of the variation method to the calculation of polarizabilities is discussed. Emphasis is laid on using configuration interaction wavefunctions in the calculation of transition-metal atom polarizabilities.

ISSN:0144-235X    

DOI:10.1080/01442358609353394

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The Pfeiffer effect concerns the change in optical activity of a solution containing a chiral substance, the environment compound, when a racemic mixture of another substance, classically a coordination complex of diimine ligands, is added to it. Several mechanisms have been proposed to account for the effect and these are discussed with respect to specific interactions leading to the observed chiral discriminations. Some new related systems involving Ru(II) complexes are reported.

ISSN:0144-235X    

DOI:10.1080/01442358609353395

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Difficulties caused by hadronic size effects encountered in quantum electrodynamics (QED) calculations of Lamb shifts in the hydrogen atom (H) are missing in positronium (Ps), a purely leptonic system. Precision measurements of the hyperfine structure in Ps thus provide less ambiguous tests of QED. A review of this problem, leading up to present-generation two-photon absorption experiments, is presented. For future investigations of Ps with two-colour lasers, calculations of the 13S1-23S1and 13S1-33S1two-photon transition amplitudes and probabilities are carried out. A new feature is the occurrence of a ‘destructive interference profile’ (DIP) in then= 3 pre-resonance region.

ISSN:0144-235X    

DOI:10.1080/01442358609353396

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor