摘要:

The effect of high initial vibrational (v= 0-25) and rotational (j= 0-100) excitation of the reactant molecule on the cross-section σRfor the reaction Li2+ Na → LiNa + Li at a relative translational energy (Etrans) of 0·5 eV has been investigated using the three-dimensional quasiclassical trajectory method on the recently proposed Varandas-Morais-Pais (VMP) potential. σRincreases almost linearly with increase inv, but it remains unaltered, within the statistical error, with increase injforj= 0-30. For the higherjstates, there is an increase in σRwith increase inj, and the rotational enhancement gets larger with increase inv, becoming comparable to the vibrational enhancement atv= 20. The dependence of σRonEtransover the range 0·18 - 0·85 eV has been examined forv= 0,j= 0. ForEtrans⩽ 0·25 eV, all trajectories get trapped. For higherEtrans, σRrises to a maximum and then levels off. Comparison of our results with those on the Whitehead-Grice (WG) potential reveals that the reactivity is larger on the VMP potential than on the WG for the lowvwhile the opposite is true for the higherv.

ISSN:0026-8976    

DOI:10.1080/00268979300100681

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have used time-resolved electron-paramagnetic resonance (EPR) spectroscopy to investigate the triplet energy transfer in various covalently linked porphyrin dimers. For all these dimers the fine structure parameters are rather similar to the monomer values; thus according to the field-swept EPR spectra the triplet state appears to be localized on one monomeric subunit. However, for particular conformations, the time evolution of the EPR spectra reveals dynamic properties specific to a dimer. For these cases the hopping rates of the triplet exciton can be determined from the measured spin-lattice relaxation times. The values obtained for the different dimers can be related to structural parameters of the dimers and to the overlap of their electronic wavefunctions. Such overlap is essential for energy transfer via the exchange mechanism.

ISSN:0026-8976    

DOI:10.1080/00268979300100691

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Neutron diffraction experiments have been performed at 220 K on liquid carbon dioxide (CO2), deuterated ethylene (C2D4), deuterated ethane (C2D6) and the equimolar liquid mixtures 0·5 CO2+ 0·5 C2D4and 0·5 CO2+ 0·5 C2D6in order to investigate the effect of quadrupole forces on the structure of the mixtures. Comparison of the structure of the mixtures with the mean of those of their pure components reveals structural features which are present only in the mixtures. Analogous to thermodynamic excess functions, excess intermolecular structure functions,iEm,0·5CO2+0·5C2D4(Q) andiEm,0·5CO2+0·5C2D6(Q) and excess intermolecular pair correlation functions,GEm,0·5CO2+0·5C2D4(r) andGEm,0·5CO2+0·5C2D6(r) have been obtained. The differences in the corresponding excess functions for the two liquid mixtures are discussed in terms of the nature and extent of the quadrupole coupling between the constituent unlike molecules. It is concluded that electrostatic quadrupole interactions play a significant role in determining the local structure of the two liquid mixtures.

ISSN:0026-8976    

DOI:10.1080/00268979300100701

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Fluctuations of enthalpy and density in the isostress-isostrain ensemble are employed to accumulate evidence for shear strain induced melting (shear melting) to occur as a second order liquid-solid phase transition. Shear melting of a solid-like monolayer film in a model slit-pore (vicinal phase) is studied by means of an isostress-isostrain ensemble Monte Carlo (MC) method. Quantities of prime interest are isostress heat capacityc3, isothermal compressibility χ3and isostress expansivity γ3where the subscript ‘3’ references the component of stress normal to the pore walls. If studied as functions of laterally applied strain α, these quantities diverge at the shear melting point αcaccording to theoretical considerations. Theory also predicts a power law decay ofc3, χ3and γ3proportional to (α - αc)vfor α > αcwherevis a critical exponent. Fitting the power law to MC data forc3yields αc= 0·28 in very good agreement with previous structural and dynamic results for the same thermodynamic state. Our results do not show any significant system size dependence. Average film thickness depends solely on thermodynamic conditions and not on enlarging the pore cross-section in the (x,y) plane. The same can be expected for fluctuations around this average value. Although fluctuations of enthalpy and density diverge at αc, enthalpy and density themselves remain continuous functions of applied strain everywhere. Thus, shear melting occurs without release of latent heat, which furnishes further evidence for a liquid-solid phase transition of second order.

ISSN:0026-8976    

DOI:10.1080/00268979300100711

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A virgin sample of polycrystalline (C10H21NH3)2ZnCl4shows two phase transitions, at 284 K and 363 K. In the low-temperature phase threefold reorientations of methyl and NH3groups dominate the nuclear magnetic resonance relaxation results. The motion of the NH3groups can be described as a three-fold jump among three potential minima of which one is shallower than the other two. In the intermediate temperature phase a defect motion of chain ends has been identified. In the high-temperature phase the spacing between adjacent layers of ZnCl4tetrahedra is increased by ∼ 16% due to the non-intercalation of the chains. In this phase the chains experience a high degree of mobility, which is partly due to the diffusion of chain defects along the chain. On cooling the sample from the high-temperature phase, it transforms to the intermediate phase, but the transition to the low-temperature phase is absent.

ISSN:0026-8976    

DOI:10.1080/00268979300100721

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Expressions for the heat had momentum transport around the state of uniform shear flow are given for a dilute gas. The results are obtained from a kinetic model recently proposed: the Liu model. This model improves some insufficiencies of the well-known Bhatnagar, Gross and Krook (BGK) equation. It is shown that the coexistence between both velocity and temperature gradients is only possible for interaction models with uniform collision frequency. In this case, the heat flux verifies a generalized Fourier's law where the thermal conductivity tensor depends on the shear rate. When considering systems where the collision frequency is not uniform, a perturbation method around the shear flow state is proposed. The irreversible fluxes are evaluated explicitly up to first order in the expansion for Maxwell molecules. The transport coefficients obtained are highly nonlinear functions of the shear rate. The results are compared with those previously obtained from the BGK and Boltzmann equations.

ISSN:0026-8976    

DOI:10.1080/00268979300100731

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

From the general expression for the electrostatic intermolecular potential in a two-centre spherical waves development, a technique which facilitates calculations at short range and a tensorial expression of angular dependencies are proposed.

ISSN:0026-8976    

DOI:10.1080/00268979300100741

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have used a form of umbrella sampling with molecular dynamics to calculate the dielectric constant for a periodically replicated cubic system of the simple point charge (SPC) model for water. The dielectric constant is related to the average of the square of the total electric dipole. With standard sampling, the average is slowly converging due to slow diffusion of the dipole. Comparison between standard sampling and umbrella sampling is made to determine the improvement in convergence that can be obtained. Probability distributions are calculated as well as averages. The distributions are used to study the shape of the pertinent free energy surface and the anisotropy of a system of periodically replicated cubic boxes. For a system of 250 water molecules in the primary cell, there is substantial anisotropy that must be accounted for in an accurate calculation of the dielectric constant.

ISSN:0026-8976    

DOI:10.1080/00268979300100751

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The quantum characteristics of liquid neon at four state points have been studied by means of Monte Carlo simulations involving two effective pair potentials arising from the path-integral formalism, namely the quadratic (QFH) and the Gaussian (GFH) Feynman-Hibbs (FH) potentials. Results include mechanical, thermal and structural properties for the FH and the classical Lennard-Jones models. The procedure to compute thermal quantities has been based on Widom's insertion technique. The data reported in the paper are also compared, where possible, with previous path-integral simulation results and with available experimental information.

ISSN:0026-8976    

DOI:10.1080/00268979300100761

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A 78–118 GHz synthesizer-driven backward-wave oscillator is used together with klystron sources and frequency doublers to measure the electric-resonance optothermal spectrum of theKa= 1 ← 0 rotation-tunnelling subband of (D2O)2. Transitions are observed originating from each of the six tunnelling states, A+1, B+1, E+1, A-2, B-2, and E-2, allowing an estimate of the largest tunnelling matrix elementh4v, characterizing the separation of the A+1, B+1, and E+1states from the A-2, B-2, and E-2states. We find the average ofh4vfor theKa= 0 and 1 states to be ∼ -8 943 MHz. A comparison of theKa= 1 ← 0 band origins for the A/B states with the band origin for their E partner givesh2v∼ -6·9 MHz, whereh2vis the tunnelling matrix element responsible for the displacement of the E symmetry levels from the center of their interconversion split A/B partners. Values found for theArotational constant,A∼ 124 923 MHz, andh2vare in good agreement with those obtained from the submillimetre measurements of Zwart, E., ter Meulen, J. J., and Meerts, W. L., 1990,Chem. Phys. Lett.,173,115, on theKa= 2 ← 1 band of the complex. Estimates are presented for the potential barriers to the 1 → 2, 1 → 5 and 1 → 7 tunnelling processes.

ISSN:0026-8976    

DOI:10.1080/00268979300100771

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor