摘要:

In this paper we employ the tensor operator formalism to obtian the NQR responses to the application of one, two and three radiofrequency (RF) pulses in the case of an ensemble of non-interacting nuclear spins withI= 3/2 case in a single crystal. The cases of axial and non-axial field gradients are considered separately. The evolution of various tensor operators in the above cases as well as under the influence of the RF Hamiltonian, including the phase of the RF carrier, is also described. These results can be used to analyse, in the absence of relaxation effects, pulse responses of spinI= 3/2 nuclei in a variety of situations. The advantages of this formalism as applied to pure NQR case are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979100100381

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Application of tensor operator formalism is extended for the pulse response calculations in the NQR of spinI= 3/2 nuclei in the presence of a weak Zeeman field. The merits and drawbacks of the tensor operator formalism as applied to the above case are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979100100391

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We calculate the interaction nuclear magnetic shielding tensor for the3Σu+state of H2, using the electron gas theory that we have presented previously. We confirm the finite direct contribution of the exchange energy, and our general results agree qualitatively with a simple Heitler-London calculation. Comparison with experimentally derived results for129Xe suggest that an electron gas calculation on that system would be valuable.

ISSN:0026-8976    

DOI:10.1080/00268979100100401

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The low-lying singlets and triplets of biphenyl are obtained exactly within the PPP model using the diagrammatic valence bond method. The energy gaps within the singlet manifold as well as the lowest singlet-triplet gap are found to be in good agreement with experimental results. The two weak absorptions between 4·1 and 4·2 eV reported experimentally are attributed to the two states lying below the optical gap that become weakly allowed on breaking electron-hole and inversion symmetries. The observed blue shift of the spectral lines, attributed to a change in dihedral angle, on going from crystalline to solution to vapour phase is also well reproduced within the PPP model. The bond orders show that the ground singlet state is benzenoidal while the dipole excited state as well as the lowest triplet state are quinonoidal and planar. Comparison with the experimental spin densities and the fine structure constantsDandEin the triplet state point to slightly weaker correlations than assumed by the PPP model. The introduction of a 1–8 bond to mimic poly(paraphenylene)s gives an optical gap that is in good agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268979100100411

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Polarized Mandelstam-Brillouin scattering data in molten Ca(NO3)2. 4·17 H2O and in its mixture with K(NO3) are presented. The measurements are performed in the deeply supercooled region. Hypersonic velocity and absorption exhibit a relaxational phenomenon to which both the structural and shear mechanisms contribute. The dynamic responses of these hydrated molten salts are interpreted in terms of existing viscoelastic models. The mean relaxation time τrand the relaxation modulusMrhave non-Arrhenius behaviour, supporting that this class of glass-forming liquids can be viewed as ‘fragile’ liquids. In addition the local order around the cations is determined by means of EXAFS measurements carried out at the PULS facility. The EXAFS data give evidence of the existence of inner shell water complexation for the cations Ca2+and K+that leads to a close-packed structure in which the H2O molecules are shared among the ions.

ISSN:0026-8976    

DOI:10.1080/00268979100100421

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Nuclear magnetic shielding tensors and spin rotation constants are calculated for CO and HCN using both the coupled Hartree-Fock (CHF) and the second-order polarization propagator approximation (SOPPA). For CO we find that SOPPA recovers about half of the difference between CHF and experiments for both nuclei. For HCN there is no effect of SOPPA for H, little for C, but a large effect for N, where the resulting isotropic shielding, σn= -17 ppm, is in good agreement with the most recent experiment. We have also analysed some earlier correlated calculations of shieldings of CO. Isotope effects on the shieldings are reported for CO.

ISSN:0026-8976    

DOI:10.1080/00268979100100431

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The Fourier transform infrared spectrum of monoisotopic H328Si79Br (fwhm ≈ 0·0035 cm-1) is investigated in the region of thev1andv4fundamental bands, from 2100 to 2330 cm-1. Anharmonic resonances involvingv4,v±15+ 2v06,v∓15+ 2v±26,v2+ 2v∓26and 3v3+v5are observed, yielding information on this complex perpendicular polyad. The fundamentalv1is anharmonically coupled tov2+ 2v06. Several strong vibration-rotation perturbations are also observed, especially in the low-Kregion, so that several anomalous patterns in the rotational structure of the spectrum are caused by different concomitant and even conflicting mechanisms. The determination of the values of vibration-rotation parameters requires different series of least squares iterative calculations based on different appropriate selections of the avcailable data. The total overall standard deviation σ (in units of 10-3cm-1) of the fit of the observed wave-numbers ofv1,v4and perturber lines made detectable near resonance is 8·8, with σ = 6·7 forv4and its perturbers and σ = 12·8 forv1. Disregarding the most heavily perturbed spectral region, using only data withK″ ⩾ 3, standard deviations are 5·9 forv4and its perturbers and 7·6 forv1. The diagonal energy parameters ofv4are determined from transition wavenumbers withK″ ⩾ 6 and σ = 2·4 × 10-3cm-1.

ISSN:0026-8976    

DOI:10.1080/00268979100100441

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Molecular dynamics computer simulations have been carried out on a system consisting of cylindrically symmetric molecules with length-to-breadth ratiok= 3 and well depth ratiok′ = 5 interacting through the Gay-Berne potential. For this system we have located the coexistence points corresponding to the isotropic-nematic transition by calculating the absolute free energy of each phase. Two temperatures,T* = 1·25 and 0·95, have been studied. In each case a weak first-order phase transition has been found, with a density change close to 2·5%. The isotropic-nematic coexistence densities are found to increase with increasing temperature.

ISSN:0026-8976    

DOI:10.1080/00268979100100451

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The total torsional potential heightVand the average jump time (T→ ∞) τ∞for the hindered methyl rotation in the 5-yl radicals of 5-methylbarbituric acid (· MBS) and thymidine (· TH) are determined by studying the temperature dependence of ENDOR intensities. Single crystals, powder and glass samples are examined. The results obtained from the glass samples differ significantly from those obtained from single crystals of the same substance. It is found thatV= 2·0 ± 0·4 kJ mol-1, τ∞= 0·4 ± 0·1 ps for · MBS single crystals,V= 5·5 ± 0·6 kJ mol-1, τ∞= 0·11 ± 0·02 ps for · TH single crystals andV= 1·8 ± 0·4 kJ mol-1, τ∞= 0·8 ± 0·2 ps for · TH glass. To obtain the dynamical parameters for the methyl group motion in low hindering potentials, a theoretical description of the temperature dependence of ENDOR intensities is developed that is based on a general diffusion model for the methyl group rotation. It is shown that the theoretical result for the enhancement is nearly independent of the shape of the hindering potential.

ISSN:0026-8976    

DOI:10.1080/00268979100100461

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The thermodynamics of fluids composed of tangent diatomics that interact via site-site square-well potentials is studied. Two different routes to the equation of state are investigated: integral equations and perturbation theory. Reference interaction site model (RISM) theory with the mean spherical approximation (MSA) is used to obtain an equation of state via integral equations. Perturbation theory is based on the hard dimer reference fluid, where the local structure of the reference fluid is obtained from Monte Carlo simulation. The theories are tested against Monte Carlo simulations for the pressure, internal energy, coexistence curve, and the second and third virial coefficients. Monte Carlo simulation data for the pressure are obtained from simulations of the fluid confined between hard walls, where the value of the density at the wall yields the bulk pressure. Second and third virial coefficients are calculated from Monte Carol evaluation of the cluster integrals, and the coexistence curve is obtained from Gibbs ensemble simulations. The theories are in good agreement with the simulation data. The RISM/MSA theory is particularly accurate, and its performance (when compared to simulations) is better than the perturbation theory.

ISSN:0026-8976    

DOI:10.1080/00268979100100471

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor