摘要:

We discuss a cyclic chain ofNidentical sites, each with two possible quantum states, whose Hamiltonian may be written in terms of the spin operators forNspinsS= 1/2, and transformed into the Fermi site excitation representation. We show that in the Fermion wave representation the requirement that the states transform according to irreducible representations of the translational symmetry group demands that the quasiparticle momenta are drawn from the representations of the double group. They must be all integral or all half-integral when the number of quasiparticles is odd or even respectively. The requirement that a quantum-mechanical operator has a definite symmetry type leads to very complicated conditions unless the operator conserves the odd or even particle number. If it does the symmetry operator can be defined in terms of Fermi wave operators and projection operators. The time reversal transformation is developed in detail and applied to problems of degeneracy in McConnell's model of singlet excitons and in the anisotropic transverse Heisenberg antiferromagnet. In the Bogoliubov-Valatin transformation time reversal shifts the quasiparticle momenta by ± π, converting integral into half-integral momenta whenNis odd. The energies of the excited states are doubly-periodic functions of the momentum.

ISSN:0026-8976    

DOI:10.1080/00268976700101541

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Wave functions are calculated for the ground state and low-lying excited states of a linear antiferromagnetic chain ofNspinsS= 1/2 with alternate strong and weak exchange integrals, using the Fermi operator method (Nis even). There are two degenerate ground states with alternating spin distributions, formed by taking a Hartree-Fock state vector composed of ‘alternant spin waves’ with different amplitudes on the two sub-lattices. The normal Hartree-Fock state is unstable and collapses into one or other alternating state. One of these is equivalent to the pseudo-spin ground state of Soos as well as the wave functions used by Ruijgrok and Rodriguez, and Oguchi. The other is derived by the time-reversal transformation or by a translation of the regular chain, and is asymptotically orthogonal to the first whenNis large. New states are constructed which have the full translational symmetry of the Hamiltonian, and there is a doubling of the low-lying spectrum of states in which the excitation density is of order 1/N, as compared with Soos' theory. This affects the magnetic susceptibility since the higher states are not doubled and do not remain orthogonal.

ISSN:0026-8976    

DOI:10.1080/00268976700101551

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

We have described the shift of the first intense absorption band in theu.v.region of some complexes of 2,2′-dipyridyl with transition metal ions in terms of the perturbational molecular orbital theory. The observed trends seem to agree with the results of this treatment.

ISSN:0026-8976    

DOI:10.1080/00268976700101561

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Solvent effects on proton chemical shifts in a number of substituted pyridines have been investigated. The systems chosen were asymmetric 3 and 4 spin systems, which were analysed by the ‘Effective Larmor Frequency Method’ of Pople and Schaefer. The chemical shifts were then investigated in a number of solvents ranging from the non-polarn-hexane to the highly polar water. The shifts could be qualitatively interpreted on the basis of Buckingham's theory. The rather interesting but anomalous shifts obtained on dilution with water have been explained on the basis of the highly ordered water structure around the solute molecule.

ISSN:0026-8976    

DOI:10.1080/00268976700101571

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Spin density distributions in a number of sigma electron radicals have been computed, using the unrestricted Hartree-Fock (UHF) [1] method with the CNDO/2 [2] approximations for the integrals involved. Coupling constants were calculated using simple relationships, and the results in general show good agreement with experiment. Similar calculations on fluorine containing radicals were performed, but core electrons need to be included explicitly to obtain meaningful results.

ISSN:0026-8976    

DOI:10.1080/00268976700101581

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Many different spin-Hamiltonians may have the same set of matrix elements and there is, therefore, some arbitrariness in the choice of spin-Hamiltonian to represent a particular situation. This problem is discussed, with especial reference to the transformations induced by a reassignment of the correspondence between the true states and the fictitious spin states.

ISSN:0026-8976    

DOI:10.1080/00268976700101591

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

ISSN:0026-8976    

DOI:10.1080/00268976700101601

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Calculations have been made on fluorobenzene, chlorobenzene, aniline and phenol by a Pariser-Parr-Pople method which includes both σ and π-inductive effects. The results of the calculations are compared with1H and13C chemical shifts at the positionparato the substituent, and with the transition energies and oscillator strengths of the four singlet-singlet transitions of lowest energy.

ISSN:0026-8976    

DOI:10.1080/00268976700101611

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

The electronic structure of AlH4-has been studied by using a determinantal wave function constructed from a minimal basis set of one-centre atomic orbitals of analytical form. Besides the determination of the molecular energy and bond length, values were also found for the ‘breathing’ force constant, the coherent x-ray scattering factor and the purely diamagnetic contribution to the molar magnetic susceptibility. The results of the present treatment for AlH4-were compared with the recent calculations of Albasiny and Cooper who used a one-centre Hartree-Fock approach within a spherical and a non-spherical approximation. In the light of the relative labour involved in each treatment, the comparison of physical properties proved to be quite favourable.

ISSN:0026-8976    

DOI:10.1080/00268976700101621

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

14N chemical shift values have been measured in benzthiazole and some derivatives using the double irradiation method. It is shown that14N chemical shifts can be used to decide the structures of two-substituted benzthiazoles, where tautomerism involving the thiazole and thiazoline forms is possible.

ISSN:0026-8976    

DOI:10.1080/00268976700101631

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor