摘要:

The nuclear spin functions for the A3(X3)3system have been developed and used to analyse the NMR spectra of (CH3)3C6H3and (CF3)3C6H3. The π-electron coupled hyperfine splitting constants, obtained from the Ramsey theory with simple Hückel M.O.'s for benzene and without the assumption of an average energy of excitation for the virtual processes which give rise to the coupling, are in agreement with the hyperfine constants observed between the methyl groups and the ring protons in the above compounds.

ISSN:0026-8976    

DOI:10.1080/00268976200100011

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

Recent theoretical work suggests that there is an alternation in carbon-carbon bond lengths throughout a sufficiently long polyene chain. Such alternation implies the possibility of defects in which even-numbered bonds along the chain change over from short to long. In this paper, the electronic properties of such defects are examined by Hückel molecular orbital theory, with the parameters used by Longuet-Higgins and Salem [2]. It is shown that there is a localized non-bonding molecular orbital associated with each defect (if they are well separated). In the neutral molecule, there will be an odd electron in each such molecular orbital, which should be detectable by electron spin resonance. An approximate estimate of the energy required to form a defect suggests that there should be about one per seventy carbon atoms.

ISSN:0026-8976    

DOI:10.1080/00268976200100021

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

The e.s.r. spectra of the negative ions of a variety of nitrogen heterocyclic molecules have been measured and the proton and nitrogen hyperfine coupling constants determined. The proton splittings are interpreted using Hückel-type molecular orbital calculations with an adjustable Coulomb parameter for the nitrogen atom. The results indicate that the nitrogen splitting is proportional to the unpaired electron density on the nitrogen atom, the proportionality constantQNhaving the value 25 ± 2 gauss. No significant dependence of the nitrogen splitting on the spin density on adjacent carbon atoms is observed.

ISSN:0026-8976    

DOI:10.1080/00268976200100031

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

We have found strong evidence that the main interaction between methyl groups and the ring system in the positive ions of aromatic hydrocarbons involves hyperconjugation rather than an inductive effect. The methyl proton hyperfine splittings have been compared in the e.s.r. spectra of both positive and negative ions of 9-methylanthracene and 9,10-dimethylanthracene. The splittings are about twice as large in the positive ions, which is expected if hyperconjugation takes place, but cannot be explained by an inductive model. The ring proton splittings have also been measured, and show small variations which can be explained satisfactorily by either theory.

ISSN:0026-8976    

DOI:10.1080/00268976200100041

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

The e.s.r. spectra of the anions of mono- and para-di-alkyl benzenes are described. The molecular orbitals which are degenerate in the case of the benzene negative ion are split apart by the introduction of the alkyl groups but the splitting is relatively small. It is suggested that the deviation of the proton hyperfine splitting constants from the values predicted by simple Hückel theory is due either to vibration-electronic interaction or to a mixing of electronic states through thermal agitation.

ISSN:0026-8976    

DOI:10.1080/00268976200100051

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

In the lowest triplet states of naphthalene, pyrene and chrysene certain ring carbon atoms should carry a negative spin density because the exchange field of the excited electron and ‘hole’ polarizes the spins of the remaining pairedπelectrons. Extended Hartree-Fock and configurational mixing theories give almost equivalent descriptions, and both predict that the negative spin densities in the triplet state are nearly the same as in the positive or negative ion radicals of the hydrocarbon. Similar exchange effects produce antiparallel correlation between the spins of atoms with opposite spin densities. These will reduce the zero-field splittingD, but probably not be strong enough to change its sign.

ISSN:0026-8976    

DOI:10.1080/00268976200100061

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

The method of Pariser and Parr, and Pople, is used to examine various effects of the interaction between two ethylene molecules. Changes in absorption frequencies and oscillator strengths, van der Waals energies due to the dispersion force, and time constants for resonance transfer of excitation energy are calculated for several relative positions of the two molecules. The asymptotic expressions ordinarily used in the theory of such effects are compared with calculated values for several intermolecular distances. Although the absorption frequency shift can have either sign, depending on the angular displacement of one molecule with respect to the axis of the other, the intensity shift is always of the same sign (hypochromic). These effects fall off as the inverse third and sixth powers, respectively, of the intermolecular distance, and should be of greatest importance for near-neighbour molecules, for which the asymptotic formulas cannot be expected to be quantitatively valid.

ISSN:0026-8976    

DOI:10.1080/00268976200100071

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

The Jahn-Teller effect in aromatic ions (and radicals) is studied, using a model in which the total energy is regarded as due to that of the original neutral molecule (or charged radical) for which no such effect arises, together with the energy of the odd electron in the degenerate orbital. It is shown that a general formula exists for the potential energy, analogous to, but more general than, that of Moffitt and Liehr. Three configurations of minimum energy, and three saddle-points, exist, and formulae are derived which make possible the calculation of the corresponding energies and bondlength changes. Numerical applications to benzene and triphenylene show excellent agreement with direct full-scale Roothaan-type calculations by Colpa.

ISSN:0026-8976    

DOI:10.1080/00268976200100081

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

It is shown that steric repulsions stabilize one of the three rotational isomers of 2 : 3-dibromobutyric acid at the expense of the others. The high resolution proton magnetic resonance spectra may be analysed as ABX3systems, but preferential solvent shifts cause the observed patterns to change drastically with solvent. The spin coupling between the methyl group and the proton in the gauche configuration, an interaction over four chemical bonds, is found to be -0·35 c/s.

ISSN:0026-8976    

DOI:10.1080/00268976200100091

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor

摘要:

A steepest descent procedure which can be used in an iterative calculation of the unrestricted bond order matrices for any system is described. The method is used to calculate unrestricted bond order matrices for the ions and lowestπtriplet states of naphthalene, anthracene, phenanthrene and azulene. Spin densities and net charges on each carbon atom in these systems together with triplet excitation energies and ionization potentials are found and compared with experiment. Unrestricted wave functions are not in general eigenfunctions of the total spin operator but this situation can be improved by using projection operators and annihilators. Spin properties of the wave function both before and after a single annihilation are discussed together with the effect of annihilation on spin densities and energy values.

ISSN:0026-8976    

DOI:10.1080/00268976200100101

出版商:Taylor & Francis Group    

年代:1962

数据来源: Taylor