摘要:

The line shapes of ENDOR spin-echoes of free radicals in liquid solution are calculated by exact solution of the equation of motion of the density matrix including relaxation and coherence effects. It is found that the duration of the r.f. pulse is very important in determining the ENDOR line width. The strength of the r.f. field, the various relaxation times, and the inclusion of r.f. coherence effects also contribute to the line shape in a complicated way. Although the theory is presented for free radicals in the motionally narrowed regime, it is expected that the calculated line shapes demonstrate the gross features of relaxation and coherence effects in rigid limit ENDOR spin-echo spectra.

ISSN:0026-8976    

DOI:10.1080/00268977800100231

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

General expressions are derived for the thermodynamic derivatives of a property <Q>, whereQis a function (static or time-dependent) of the phase variables and <…> is an average over an equilibrium grand canonical ensemble. As an example of the use of these expressions we then consider the case where <Q> is a time-dependent density-density correlation function. Thermodynamic derivatives of the time-dependent Van Hove correlation functions are considered in detail, and examples of how the resulting expressions can be used to interpret neutron-scattering data are given. The expressions developed lead to more stringent ways of testing theories of fluids, and provide a method for studying triplet correlation functions which have been nearly inaccessible in the past. We expect the general relationships to prove equally useful when applied to other experimental methods for studying time-correlation functions (e.g. absorption or scattering of electromagnetic radiation, relaxation phenomena).

ISSN:0026-8976    

DOI:10.1080/00268977800100241

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A theoretical study is presented of the ring-current and excess-charge effects on the chemical shifts of the conjugate bases of fluorene, 9,10-dihydroanthracene, xanthene, thioxanthene, dibenzo[a,d]cycloheptene and dibenz[b,f]azepine. Several approaches (SCF methods, coupled and uncoupled perturbation theories and ring-current models) have been compared.

ISSN:0026-8976    

DOI:10.1080/00268977800100251

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Klemperer has argued that rearrangements belonging to the same double cosetHxHgive rise to identical N.M.R. line shapes. This result rests on the assumption that, when a given effective spin hamiltonian is associated with each molecular configuration, rearrangements corresponding to identical switches of effective spin hamiltonians must give rise to identical line shapes.

ISSN:0026-8976    

DOI:10.1080/00268977800100261

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

We relate the mean first-passage time for thermal decomposition to the steady-state and equilibrium rate constants using a general separable transition probability model. These relations are used to assess the relative merits of various approximate rate constants.

ISSN:0026-8976    

DOI:10.1080/00268977800100271

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The Monte Carlo method has been used to study a model system of equimolar mixtures of 256 hard diatomic molecules, each consisting of two fused hard spheres having the same diameter σ with the elongations of the componentsla* andlb* (l*=l/σ). Mixtures withla*/lb*=0·2/0·4, and 0·2/0·6 were studied at densities typical of the liquid state. The orientational structure of dense hard diatomic fluids has been studied by calculating up to 13 terms in the expansion of the total pair distribution functiong(r12θ1θ2) in spherical harmonics. The coefficientsgll'mij(r12) of the series have been calculated as ensemble averages in the simulation. At short distances the system exhibits a high degree of angular correlation, which increases with density and the elongation of the longer component, the correlation is relatively short-ranged, with no significant angular structure beyond twice the major diameter of the larger component. A zero-order blip function perturbation theory is presented for mixtures of non-spherical molecules, and the liquid structure predicted by this theory is compared with the simulation results. The virial pressure of the mixtures is also calculated as a function of the density.

ISSN:0026-8976    

DOI:10.1080/00268977800100281

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Existing published E.P.R., nuclear spin lattice relaxation, dynamic nuclear polarization and electron-electron double-resonance data on the plastic phase of cyclohexane doped with a nitroxide free radical are re-examined and analysed in a consistent way. It is shown that (i) the local concentration of free radicals is higher than foreseen, assuming a uniform distribution and (ii) the molecular motions near the radical are slower than in the pure system. These results suggest that this system should be pictured as a heterogeneous solid solution where the local properties around the radicals are significantly different from those far from the radicals.

ISSN:0026-8976    

DOI:10.1080/00268977800100291

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Dans ce travail sont établies les formules permettant de relier, dans le modèle de la cavité moléculaire ellipsoïdale, les polarisabilités optiques principales et le moment dipolaire de la molécule aux valeurs classiques mesurées en solution dans le tétrachlorure de carbone (polarisabilité optique moyenne, moment dipolaire, anisotropie optique moléculaire et constante de Kerr). L'application de ces formules sur des dérivés du benzène conduit à un accord satisfaisant entre les valeurs calculées et les valeurs expérimentales de la plupart des composés. Les causes des écarts observés sur certains composés sont ensuite discutées.

ISSN:0026-8976    

DOI:10.1080/00268977800100301

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The effect of an external magnetic field on the propagation of sound in a paramagnetic liquid is considered. It is shown that because of the interaction of molecular spins with the magnetic field and the interaction of the spins with the molecular orientation the resonant singularity appears in the spectra of the molecular orientation fluctuations. Due to this and to the coupling between orientational fluctuations and high-frequency shear waves in the liquid a number of peculiarities in the sound propagation arise. In particular, the reflection of sound is followed by rotation of the polarization plane and by changes of reflection coefficient.

ISSN:0026-8976    

DOI:10.1080/00268977800100311

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

An initioSCF perturbation theory in the formulation of Ditchfield and Snyder has been applied to the calculation of the N.M.R. spin-spin coupling constants in CH4, NH3, H2O and HF using large gaussian basis sets, and including all second-order contributions (Fermi contact, orbital and dipolar). In accordance with most other work, the first-order term in the orbital contribution has been neglected, however. The sensitivity of the coupling constants to variations in the basis set has been studied, and the reliability of the method is discussed in the light of these results. It is shown that the Fermi contact contribution provides the largest component of the calculated coupling constants, with the dipolar term being generally negligible. However, the orbital contribution is found to be non-negligible, particularly in the case of the directly bonded couplings in HF and H2O, and is shown to be the primary reason, at the level of calculation adopted, for variations in the HH geminal couplings in the series CH4, NH3and H2O. We have shown how convergence of the iterative procedure for the computation of the first-order wavefunction may be assured by the use of ‘damp factors’ and ‘level shifters’.

ISSN:0026-8976    

DOI:10.1080/00268977800100321

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor