摘要:

A critical discussion of the theory of spin interactions in molecules is presented. We emphasize that the conventional spin hamiltonian can only be used in first-order perturbation theory, otherwise a singular hamiltonian theory is obtained. The (singular) perturbation of atomic states by the contact operator and an inverse square potential is discussed in detail. The introduction of a spatially extended charge and magnetization for nuclei is suggested as a way of developing a mathematically satisfactory theory of electron-nuclear spin coupling effects; an analogous phenomenology for electrons alone however is not possible.

ISSN:0026-8976    

DOI:10.1080/00268977500102231

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

Les temps de relaxation (RMN) du deuterium de pyridines deuteriées spécifiquement ont été mesurés pour des solutions pyridine-eau pour des fractions molaires de pyridine comprises entre 0,1 et 1 à une température de 25°C. Ces valeurs combinées à celle du temps de relaxation de l'azote calculée à partir des largeurs de raies14N permettent d'étudier les modifications des réorientations de la pyridine en fonction de la composition du mélange dans le cadre d'un modèle de diffusion rotationnelle.

ISSN:0026-8976    

DOI:10.1080/00268977500102241

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

A method has been developed and applied to sodium methylphosphonate for determining the values ofJHXfrom the temperature dependence of the anisotropic motion of the solute. The method reduces the uncertainty ofDHXdue to solvent effects onJHX. As in methyl fluoride discrepancies are observed between structural parameters calculated from different sets of observed dipolar couplings. Unlike the case of methyl fluoride, correction of the dipolar couplings for vibrations improves consistency of these values. Large correction terms found in the present molecule result from contributions of low frequency vibrations whose excited states are appreciably populated at room temperature. The structural parameters and anisotropic motion determined in a lyotropic mesophase are discussed.

ISSN:0026-8976    

DOI:10.1080/00268977500102251

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

It is shown that the E.S.R. spectra of long-chain nitroxide biradicals represent the superposition of the spectrum of the non-reacting radical moieties in the elongated conformation and of that of the cage where the radical moieties are close together and can interact with each other. Inside the cage a fast intramolecular motion is observed. Some thermodynamic parameters of the cage as well as thermodynamic parameters of the motion inside the cage are calculated from the experimental E.S.R. spectra of four biradicals.

ISSN:0026-8976    

DOI:10.1080/00268977500102261

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

Ab initiovalence bond calculations have been performed for the ground and excited states of H2S, H2S+, H2S-, SH, SH+, SH-. The dependence of calculated properties such as energy, ionization potential, dissociation energy, and dipole moment on the starting basis set, on the amount of configuration interaction and on the number of electrons explicítly included in the calculation is studied. It is found that freezing the electron core is a satisfactory approximation leading to a drastic reduction of computation time while preserving the quality of the results as compared with experiment and MO calculations. In addition, the geometry of the ground state of H2S was optimized.

ISSN:0026-8976    

DOI:10.1080/00268977500102271

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

In the context of an SCFπ-electron theory an exact expression is derived which yieldsxπ⊥, the totalπ-electron contribution to the magnetic susceptibility perpendicular to the plane of a polycyclic conjugated hydrocarbon. Once given the first-order correction to the bond-order matrix of the molecule in question (which, in the present calculation, is obtained via a coupled Hartree-Fock procedure), this expression makes no further appeal to the London integral approximation. The terms which occur in the expression forxπ⊥due to the ‘integrated current densities’ correspond to those which would have arisen if the London approximation had been invoked, the remaining terms being, in general, non-zero if this approximation is not made. The relative ‘integrated current densities’ associated with the various constituent rings in 12 molecules agree well with the corresponding ‘ring current’ intensities estimated via the London-Pople-McWeeny method.

ISSN:0026-8976    

DOI:10.1080/00268977500102281

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

A new computational method for a previously suggested Multi-Configuration SCF molecular wave function is developed and applied to some simple molecules. The method involves the use of a single Fock-like hamiltonian matrix for all the MCSCF orbitals and does not involve orthogonality constraints or coupling operators. Application to lithium hydride shows that this method is capable of recovering a large part of the correlation energy and compares very favourably with conventional lengthy CI methods.

ISSN:0026-8976    

DOI:10.1080/00268977500102291

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

A simplified Time-Dependent Hartree-Fock scheme is discussed, involving the application of the method solely to the valence shell, and numerical results are presented for a number of atomic systems. An extensive study is made of the magnesium isoelectronic series and excellent agreement is observed between the TDHF oscillator strengths and the most accurate theoretical and experimental values. Results are also presented for the argon sequence.

ISSN:0026-8976    

DOI:10.1080/00268977500102301

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

Transition probabilities for thermal H-H* collisions are calculated in the close coupling, distorted wave, and Landau-Zener approximations using a variety of electronic basis sets. The 3s/dσgand 4s/dσgpairs of states of H2have very close avoided crossings almost at the potential minima. It is found that all the non-adiabatic basis sets give equally good decoupling, whereas the adiabatic states are strongly coupled at all energies. The Landau-Zener approximation is in all cases too crude.

ISSN:0026-8976    

DOI:10.1080/00268977500102311

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor

摘要:

The electronic structure of the diborane molecule is examined within a simple group function model. The group functions are taken to be a product of singly-occupied non-orthogonal orbitals. This model yields a molecular wavefunction which provides a simple interpretation while incorporating a substantial amount of chemically significant electron correlation. The resulting description makes possible a direct quantitative verification of the qualitative valence-shell electron-pair repulsion theory of directed valence.

ISSN:0026-8976    

DOI:10.1080/00268977500102321

出版商:Taylor & Francis Group    

年代:1975

数据来源: Taylor