ISSN:0026-8976    

DOI:10.1080/00268979100102181

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is demonstrated by theory, simulation, and experiment that it is possible to optimize heteronuclear cross-polarization experiments in liquids to such an extent that it becomes possible to transfer polarization uniformly through heteronuclear one-bond and long-range couplings. It is found that cross polarization is competitive with pulsed transfers, such as INEPT or DEPT, for the preparation of directly detected low-γ nuclei. However for proton detection after two-way transfer, experiments employing heteronuclear multiple quantum coherence are usually more sensitive than cross-polarization experiments.

ISSN:0026-8976    

DOI:10.1080/00268979100102191

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The Percus-Yevick (PY), HNC, nonspherical bridge function (NSB), and VM integral equation theories are used to study the structure and thermodynamics of the hard linear homonuclear triatomic fluid. Results for the spherical harmonic coefficients of the pair distribution function and for the compressibility factor are compared with computer simulation data for a range of triatomic densities and elongations up to 1·6. The VM theory gives excellent results for the harmonic coefficients in the site-site and centre-site molecular coordinate frames, the PY theory results are fair, and the NSB and HNC theory results are poor at low distances. None of the theories satisfactorily describes theg000harmonic coefficient in the centre-centre frame in the vicinity of contact at high densities. The VM compressibility factors agree quantitatively with those of the simulations. The NSB equation of state results are also excellent. The PY values are too low and the HNC values are too high. The thermodynamic consistency of the VM theory is excellent, that of the NSB theory very good, and that of the PY and HNC theories poor, especially at high densities.

ISSN:0026-8976    

DOI:10.1080/00268979100102201

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The PY, HNC and VM integral equation theories are used to study the structure and thermodynamics of the hard prolate spherocylinder fluid. Results for the spherical harmonic coefficients of the pair distribution function and for the compressibility factor are compared with computer simulation data, including new results reported in this work, for length-to-breadth ratios up to γ = 6. For γ = 2 the VM structural results agree quantitatively with those of the simulations. For γ = 3, the results are mixed. For γ = 6 the HNC results are the best. The PY results are the worst at any value of γ considered. The VM compressibility factors agree excellently with those of the simulations for all length-to-breadth ratios, while the HNC values are too high and the PY values too low. Finally, we propose a geometrical argument explaining the accuracy of the HNC for very elongated molecules.

ISSN:0026-8976    

DOI:10.1080/00268979100102211

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

By analysing the results of a previous experimental work it has been found that the transition changes from a liquid-gas type at low pressure to a liquid-liquid one at high pressure. However, not much experimental data are available in the literature at high Xe concentrations, and some criticism can be applied to them. Therefore, new measurements of the isobaric coexistence curves for He-Xe mixtures at three pressures (80, 132, 200 atm) have been performed, with the use of a more precise technique, based on energy resolved neutron transmission measurements. A qualitative agreement with the previous data has been found, while a disagreement is present in the behaviour ofTcversuspc, thus confirming our picture of a change in the nature of the transition as the pressure increases.

ISSN:0026-8976    

DOI:10.1080/00268979100102221

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The molecular structure of cyclobutane has been analysed from its previously measured dipolar couplings. Assuming that the cyclobutane molecule oscillates between two equivalent bent conformers, the C-H bond length decreases from an anomalous value of 1·144 Å to 1·097 Å when harmonic vibration corrections are applied to observed dipolar couplings. The optimized values for the remaining geometric parameters are: 0·26 Å for the puckering amplitude, 109.8° for the HCH bond angle and 3.5° for the methylene rocking coordinate. The inversion barrier (ΔV) value cannot be determined because the RMS deviation between observed and calculated direct couplings remains practically unaltered for ΔVvalues greater than 4 kJ mol-1.

ISSN:0026-8976    

DOI:10.1080/00268979100102231

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The self-complementary oligodeoxynucleotide sequences d(CG)8and d(m5CG)8have been synthesized and characterized by COSY and NOESY experiments. In the respective B conformations, spectra are obtained with a resolution that is significantly higher than the one observed for the corresponding polydeoxynucleotides, d(CG)n. As a consequence, the signals from terminal bases can be resolved and identified for our systems. For the Z conformation, the lines are broader due to the higher viscosity of the solutions, and as a consequence of partial aggregation processes. The B → Z transition, induced by the addition of NaCl or MgCl2, respectively, can be monitored unambiguously in the NOESY spectra. In the B conformation, which is characterized by a 2′-endo sugar pucker and an anti-conformation of the bases with respect to the sugar ring, small interproton distances and hence strong NOE cross peaks are found for the proton pairs GH8/H2′ and CH6/H2′. In contrast, the bases are adopting a syn-conformation with respect to the ribose in the Z conformation, such that the GH8/H1′ cross peak is dominant. NOE build-up rates have been determined for three proton pairs in d(CG)8(Z conformation), by varying the NOE mixing time. Results are analyzed in terms of the relevant interproton distances, as derived from molecular modelling. As a consequence of spin diffusion, magnetization transfer between distant protons is faster than predicted by a simpler-6dependence.

ISSN:0026-8976    

DOI:10.1080/00268979100102241

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A triplet correlation function for Lennard-Jones fluids is calculated with the inhomogeneous integral equation method recently introduced by Attard for hard core fluids. The results are compared with those of molecular dynamics simulations. A simple extension of the mixed closure for the bulk fluids successfully reproduces the correlation function for the fluids near the triple point.

ISSN:0026-8976    

DOI:10.1080/00268979100102251

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Simulation results for a square-well (SW) fluid of range λ = 3 are presented and used to test the long-range (LR) theory recently developed for SW systems. The discontinuous molecular dynamics methods was used to obtain values of internal energies, pressures and orthobaric densities. The predictions of the second-order LR theory are in excellent agreement with the simulated results, showing that the LR approximation is highly accurate for ranges λ ⩾ 3. The accuracy of the van der Waals, mean-field andLRapproximations is established in terms of the temperature of the system and the range of the well.

ISSN:0026-8976    

DOI:10.1080/00268979100102261

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Geometry optimization using analytical gradient methods, followed by either analytical second derivative or numerical force constant calculations, have been carried out on a number of structures and low-lying electronic states of the complex anions, O2-. H2O and O2-. CO2. RHF, UHF, UMP2 and PUMP2 calculations were performed with a 6–31 + G** basis set.

ISSN:0026-8976    

DOI:10.1080/00268979100102271

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor