摘要:

The lowering of the Curie temperature of a ferromagnetic substance by random dilution with non-magnetic elements is studied on the basis of Heisenberg and Ising models having nearest-neighbour interactions between magnetic elements only. It is proved that the limiting concentration,pc, of magnetic elements, below which there is no Curie temperature, is the same for all such models, whether Heisenberg or Ising and regardless of the spin value concerned: it depends only on the lattice structure. Numerical estimates ofpcare given for various lattices. Successive approximations are developed to the full Curie temperature curve, and it is found that whereas for the Ising modelTc(p) approximates topTc(1) nearp=1,Tc(p)<pTc(1) for the Heisenberg model.

ISSN:0026-8976    

DOI:10.1080/00268976100100011

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

All possible two-centre hybrid and exchange integrals between 2pπand 3pπatomic orbitals with equal orbital exponentαhave been calculated and tabulated forα=1·00 (0·25) 9·00.

ISSN:0026-8976    

DOI:10.1080/00268976100100021

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

The self-consistent bond orders for the ground states of a number of heterocyclic molecules are calculated by a steepest descents method. The parameters used are those of McWeeny and Peacock whose value of the electronegativity parameter is much smaller than has been customary. The dipole moments are deduced and found to be in as good agreement with experiment as those calculated with larger electronegativity parameters. The CC bond lengths are also calculated for acridine and found to be in excellent agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268976100100031

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

According tolcao-mocalculations lowerπ-electron and total electron energy values result if the molecular symmetry group for pentalene is taken to be C2hinstead of D2h. Though a considerable reduction inπ-electron energy is obtained, the gain in total electron energy is much smaller. Nevertheless it may be stated that in the ground state the twofold axes C2xand C2yare not present.

ISSN:0026-8976    

DOI:10.1080/00268976100100041

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

The variation of the position of high resolution proton magnetic resonance lines of a polar solute with dielectric constant of the solvent may be calculated on the assumption that the dipole polarizes the surrounding medium, setting up an electrostatic ‘reaction field’ which alters the distribution of the electrons responsible for magnetic shielding. The theory predicts that the shape of the solute molecule should be important in determining the magnitude of this reaction field. The magnetic shielding values measured in a range of solutions of acetonitrile, representing a rod-like molecule, and of paraldehyde, a disc-like molecule, are shown to support the ‘shape theory’ rather than the simpler treatment which assumes that all solutes are spheres. Evidence is obtained that paraldehyde is essentially only one of the many possible stereoisomers, the ‘chair’ form with all the methyl groups in equatorial positions.

ISSN:0026-8976    

DOI:10.1080/00268976100100051

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

This paper uses the theory of creation and destruction operators to give a new simpler proof of the well-known pairing relation between electronic states of alternant hydrocarbon positive and negative ions. The method is like one that Heisenberg used to explain why the atomic spectra of elements such as C (with two electrons in the 2pshell), and O (with two holes) are so similar. It depends on the analogy between electrons in an orbital ψ and holes in the paired antibonding orbital ψ′ of the π-electron shell. A modified pairing relation holds also in a magnetic field. Applications of the operators to other problems are suggested.

ISSN:0026-8976    

DOI:10.1080/00268976100100061

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

Two errors in my paper “Wave functions for the methane molecule” [1] are corrected. They concern my f-harmonic approximation to the wave-function in the equilibrium configuration, for which the final expression for the wave function, the energy lowering, and the density function were all in error.

ISSN:0026-8976    

DOI:10.1080/00268976100100071

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

The variation with interatomic separation of the nuclear magnetic screening constant for H2has been studied in a previous paper. Here an estimate is made of the difference in screening which is a consequence of the different zero-point vibrational functions in H2, HD and D2. The results are in good agreement with the observed values.

ISSN:0026-8976    

DOI:10.1080/00268976100100081

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

A critical study has been made of Hafner's new heptalene and pentalene derivatives I and II. Conventional valence-bond, molecular-orbital and free-electron approximations have been used. All agree that I should be genuinely aromatic, and II should not be. The valence-bond method suggests that II should be pseudo-aromatic, the other methods suggest that it should possess cyclo-olefinic character. All methods confirm the very long wavelength absorption of I, but the agreement in the case of II is less satisfactory. Charge distributions, bond orders and lengths are calculated by the molecular-orbital method, and compared with heptalene, pentalene, azulene and acenaphthylene. The relevance of the aromatic sextet rule is pointed out in discussing charge distribution in these odd-numbered ring compounds. Localization energies are also calculated, and are used to predict the positions of greatest reactivity for electrophilic, nucleophilic and radical reactions.

ISSN:0026-8976    

DOI:10.1080/00268976100100091

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor

摘要:

Electron spin resonance investigations on aγ-irradiated single crystal of succinic acid have confirmed and extended the findings of Heller and McConnell on the hyperfine couplings in the radical (CO2H)ĊH-CH2(CO2H). Complete coupling tensors, and thegtensor, are given in the table. These confirm that the radical has almost the same orientation as the unchanged molecules. The plane of the free radical carbon is twisted 5° from the original carbon skeleton plane and this leads to appreciable non-equivalence of the hydrogen atoms of the CH2group.

ISSN:0026-8976    

DOI:10.1080/00268976100100101

出版商:Taylor & Francis Group    

年代:1961

数据来源: Taylor