摘要:

Dirac-Fock calculations near the SCF limit using the recently developed basis set expansion technique of Mark and Schwarz have been performed on the F2ground state with particular consideration of the relativistic splitting of the πgand πuorbitals. The magnetic contribution to the Breit interaction has been included by first-order perturbation theory. Fine structure splittings of the ionic states F2+(X2Πg) and F2+(A2Πu) have been calculated by first-order perturbation theory within the Breit-Pauli framework at three levels of approximation for the zero-order wavefunction. The results of the Dirac-Breit and the Breit-Pauli approaches are compared with experimental data. The calculated splittings are analysed by partitioning them into physically meaningful contributions. Aided by results of numerical Dirac-Fock calculations on atoms the general conclusion is drawn that ionic reorganization of the wavefunction increases the fine structure splitting at the Breit-Pauli level, whereas it decreases the splitting at the Dirac-Breit level. Using a model of Ishiguro and Kobori the ratios of Πgand Πuspin-orbit splittings of halogen molecular ionsX2+are discussed.

ISSN:0026-8976    

DOI:10.1080/00268978400102501

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Neutron scattering by oxygen gas at 100 bar has been measured. The observed differential cross-section at highQ-values is fitted to a form-factor dependent on the molecular geometry. At lowerQ-values there are coherent contributions from intermolecular and magnetic correlations. Molecular dynamics calculations are used to estimate the nuclear part so that the magnetic form-factor can be determined for an isolated molecule. The results for the magnetic form-factor are in satisfactory agreement with the predictions of Kleiner except at very lowQ-values.

ISSN:0026-8976    

DOI:10.1080/00268978400102511

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Low temperature ENDOR measurements on dibenzene vanadium diluted in single crystals of (2,2)-paracyclophane and ruthenocene are reported. The proton hyperfine tensors of the rigid benzene rings of the paramagnetic probe yield information on site symmetry and, indirectly, on the space group of the matrix crystals at 10 K. The principal axes systems of the proton hyperfine tensors of dibenzene vanadium in (2,2)-paracyclophane essentially retain theD6hsymmetry of the molecule. However, the isotropic parts of the coupling tensors clearly classify the protons into three sets of equivalent nuclei, whereas from the symmetry of the substitution site in (2,2)-paracyclophane only two sets were to be expected. A lowering of the space group of this matrix crystal is suggested to explain the observed ENDOR spectra. The proton hyperfine data of dibenzene vanadium are discussed in terms of a LCAO-Mo model. From the orthorhombicity of the tensors the spin density in the 2pz-orbital of the adjacent carbon is calculated and compared with data from N.M.R. contact shift studies. Temperature dependence of the ENDOR spectra relating to reorientation processes of the aromatic rings will be reported in the subsequent paper.

ISSN:0026-8976    

DOI:10.1080/00268978400102521

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The temperature dependence of the proton ENDOR spectra of dibenzene vanadium diluted into (2,2)-paracyclophane and ruthenocene single crystals is reported in the range 10–200 K. The low temperature spectra are discussed in detail in the preceding paper. The observed temperature dependence is modelled by a chemical exchange formalism for the rotational reorientation of the benzene rings with respect to the matrix crystal, leading to estimates of the activation parameters of the reorientation process. In the (2,2)-paracyclophane host crystal, both rings of dibenzene vanadium show the same reorientation dynamics, whereas in the ruthenocene host crystal, the corresponding activation parameters of the two benzene rings are considerably different. Furthermore, from the temperature dependence of the proton ENDOR spectra of dibenzene vanadium and cyclopentadienyl-cycloheptatrienyl vanadium in frozen toluene, the effect of ring size on the rotational reorientation dynamics is demonstrated.

ISSN:0026-8976    

DOI:10.1080/00268978400102531

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Classical and quantum mechanical models for the phase transitions in the polyphenyls are developed. Starting with the high symmetry phases of these crystals, the models are able to predict several key features of the polyphenyl transitions. First, the nature of the phase transitions, order-disorder in p-terphenyl and displacive in biphenyl, is confirmed. Second, the transition is predicted to occur at a substantially lower temperature for biphenyl than for p-terphenyl. Third, the models predict a decrease of the p-terphenyl transition temperature with pressure, in agreement with experiment. The trasition to an incommensurate phase for biphenyl is discussed, and the origin of the observed isotope effect is explored.

ISSN:0026-8976    

DOI:10.1080/00268978400102541

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Rotational barriers in 19 molecules possessing a single internal rotation angle around a B-N, C-C, C-N, C-O, N-N, N-O, O-O central bond and geometrical isomerism in 3 molecules possessing a N=N double bond have been studiedab initioby the improved bond-orbital method. The first approximation, where the chemical groups occurring in these molecules are described in terms of non-orthogonal SCF bond-orbitals constructed from energy-optimized bond hybrids and polarities, is improved in second order of perturbation theory by admitting single excitations from bonding to antibonding orbitals and accounting for induction including exchange (polarization and delocalization). The molecules studied possess 16 to 34 electrons and a variety of functional groups differing in their chemical structure (CH3, NH2, OH, NO, CHO, CH=CH2, NH= and some of their F-derivatives). The overall results obtained using a STO-3G basis, rigid rotation and experimental geometries, are close to experiment and to the corresponding MO-SCF calculations in the same basis, but individual energy components allow us to establish a clear correlation between barriers and chemical structure, grouping the 22 molecules into 4 classes. In the first class (CH3-Xmolecules and 1,2-difluoroethane) barriers are dominated by steric interactions (Pauli repulsions) which are sufficiently well described in first order. In the second class (N2H4, NH2OH, NH=NH and its fluoroderivatives, molecules all possessing lone pairs adjacent to the central bond) barriers are due to competition between first-order Pauli repulsion and characteristicgeminalσ-σ* delocalization occurring in second order. In the third class (1,3-butadiene, glyoxal, formamide and formic acid, molecules possessing double bonds and/or π-lone pairs at both ends of the rotation axis) barriers are dominated by large π-π*vicinaldelocalization. In the fourth class (HNO2, H2O2and its fluoroderivatives, molecules presenting both previous structural features) barriers result from competition between all preceding effects.

ISSN:0026-8976    

DOI:10.1080/00268978400102551

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

It is shown that the removal of the ‘spherical approximation’ from the Atomic Hartree-Fock equations generates a set of atomic orbitals which are not always eigenfunctions ofL2. In particular, thes-type andd-type orbitals ‘mix’. The result of this mixing is to generate a set of ‘valence’ orbitals for these atoms which solves the old problem of why, if the 3dlies below the 4s, does the 3d not alwaysfill up first.

ISSN:0026-8976    

DOI:10.1080/00268978400102561

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

It is shown that the replacement of the minimization of the Hartree-Fock energy functional by the parametrized Hartree-Fock energy function has some important consequences for the calculation of interactions involving anions.

ISSN:0026-8976    

DOI:10.1080/00268978400102571

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Results of molecular dynamics simulations are reported for CH3F, CHF3, CH3Cl, CH3CN, CO2and CS2. The intermolecular pair potentials used in these calculations were derived with a new model which was introduced recently. The calculated data on thermodynamics, static structure, self-diffusion coefficient and reorientational correlation times are compared with experimental and other molecular dynamics results.

ISSN:0026-8976    

DOI:10.1080/00268978400102581

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Via optically detected ENDOR the hyperfine tensors of the protons, and partially of the nitrogen atoms, of zinc porphine in its3Eustate have been obtained. The results show that the in-plane principal axes of the fine-structure tensor make an angle of 7·8 ± 1·1° with the N-N symmetry axes. The spin densities are evaluated from the hyperfine data in a point-dipole model; a nuclear distortion ofb1gsymmetry has to be included in order to arrive at a consistent interpretation. The spin densities are compared with calculations and confirm a (eg←a2u) orbital configuration for ZnP in its3Eustate.

ISSN:0026-8976    

DOI:10.1080/00268978400102591

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor