摘要:

We use the finite size scaling method to study the critical points, points of non-analyticity, of the ground state energy as a function of the coupling parameters in the Hamiltonian. In this approach, the finite size corresponds to the number of elements in a complete basis set used to expand the exact eigenfunction of a given molecular Hamiltonian. To illustrate this approach, we give detailed calculations for systems of one electron and two nuclear centres,Z+e−Z+. Within the Born-Oppenheimer approximation, there is no critical point, but without the approximation the system exhibits a critical point at Z = Zc= 1.228279 when the nuclear charge, Z, varies. We show also that the dissociation occurs in a first-order phase transition and calculate the various related critical exponents. The possibility of generalizing this approach to larger molecular systems using Gaussian basis sets is discussed.

ISSN:0026-8976    

DOI:10.1080/00268970009483354

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The rotational spectrum of tricarbonyl(methylcyclopentadienyl) manganese has been recorded in the range from 1–23 GHz using an FTMW spectrometer working on a molecular beam. This spectrum is assigned for the first time, yielding the rotational constants of the molecule and quartic centrifugal distortion constants. The fine structure due to the quadrupole interaction of the55Mn nucleus could be solved and gave the quadrupole coupling tensor (which is compared with that of tricarbonyl(cyclopentadienyl)manganese, and the spin-rotation constants. An additional splitting of a few kHz, which could be observed for some transitions, is interpreted as arising from the internal rotation of the methyl group.

ISSN:0026-8976    

DOI:10.1080/00268970009483355

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The effects of short and long range interactions on the structure of water, both spatial and orientational, has been studied in detail by computing the full pair correlation function, site-site correlation functions, 2-dimensional site-site correlation functions in the (rOO,rOH) and (rOO,rHH) planes, dipole-dipole correlation function, radial Kirkwoodgfactor, and the dielectric constant. Two model potentials, the T1P4P and ST2, and their short range versions have been considered at ambient and elevated temperatures and under supercritical conditions. The Ewald summation under different conditions has been used to investigate also their effect on results. An analysis of the results shows that although all site-site correlation functions for the short and long range systems are similar, the orientational ordering in systems of different range may be considerably different, this evidence being provided mainly by the dipole-dipole correlation function and the radial Kirkwood factor. The orientational ordering is only short range in long range systems, whereas in short range systems the hydrogen bonding gives rise to a damped long range regular pattern of alignment. Nonetheless, the resulting dielectric constants for the short and long range systems coincide within the combined error bars. All findings are more pronounced at low temperatures but otherwise they are only marginally temperature and density dependent.

ISSN:0026-8976    

DOI:10.1080/00268970009483356

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The porosity and pore geometry of disordered materials can be influenced by employing a removable template during synthesis. A theoretical and simulation study is reported of the configurational effects of template size and density on the adsorption isotherms of templated porous materials. To isolate the configurational (entropic) contributions, the adsorbate, matrix and template components are modelled as hard spheres. The replica Ornstein-Zernike equations proposed by Zhang, L. and Van Tassel, P. R. (2000)J. chem Phys.,112, 3006 are used within the Percus-Yevick approximation to calculate adsorption isotherms for differently sized adsorbate and template components. These theoretical results are compared with results from Monte Carlo simulation. It is found that adsorption is most enhanced whenever the size of the template is equal to or slightly larger than that of the adsorbate. Also, for systems of constant matrix density or constant matrix plus template density, increasing the density of template enhances the adsorption.

ISSN:0026-8976    

DOI:10.1080/00268970009483357

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The applicability of an extended Förster theory (EFT) is explored with respect to donor-donor energy migration within a pair of identical fluorophores. The EFT accounts for reorienting motions and orientational restrictions of donor groups, which are attached to a macromolecule. Because EFT involves averaging over stochastic functions, it is inappropriate for the conventional methods used for analysing fluorescence depolarization experiments. For this reason approximations of the EFT were derived. To examine the validity of these different approximations, depolarization data were generated and re-analysed. To create the depolarization data, the EFT was used together with a Brownian dynamics simulation. Limitations of the approximate EFT are ascribed to the handling of secondarily excited donors (i.e. donors excited through energy migration). Finally on the basis of the EFT, a simulation-deconvolution method is presented which enables one to analyse the fluorescence anisotropy, without introducing any approximation.

ISSN:0026-8976    

DOI:10.1080/00268970009483358

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The static dielectric constant of pentanol diluted in decane, cyclohexane and carbon tetrachloride was measured as a function of temperature, at different concentrations. The experimental points were adjusted using the Onsager equation to which a term reducing the molecular interaction was added. Such an empirical equation has already been published for decane solutions. Good agreement between experimental and fitted values of the permittivity was found. The results obtained for the different solvents used are compared and analysed, together with the parameters resulting from the fittings.

ISSN:0026-8976    

DOI:10.1080/00268970009483359

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Numerical simulations of powder spectra produced by a two-pulse sequence applied to a quadrupolar nuclei system of half-integer spin number make it possible to propose simple experimental instructions to record static echo spectra with minimum lineshape distortion. Calculations take into account quadrupolar (first and second orders), shielding, scalar and inhomogeneous dipolar interactions as well as the radiofrequency pulse specifications (strength, duration, phase). The suggested instructions have been checked experimentally in different cases: for a large spin number system with Li93NbO3, when two interactions are present with a63/65Cu complex, and in the two-site system of87Rb2SO4.

ISSN:0026-8976    

DOI:10.1080/00268970009483360

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A study is reported on the diffusion process of a solute molecule in a Lennard-Jones-like liquid near the triple point by a molecular dynamics simulation. Systematic changes were made to the strength of the solute-solvent or solvent-solvent attractive interaction in order to elucidate its effects on the diffusion coefficient. When the solute-solvent attractive interaction is enhanced, the diffusion coefficient of the solute becomes much smaller than that predicted by the Stokes-Einstein relationship with a stick boundary condition. The generalized friction coefficient on the solute molecule was investigated, and the attractive force between solute and solvent is found to be the main cause for the enhancement of the friction. When the attractive interaction between solvent molecules is weakened, the diffusion coefficient of a solute does not change, whereas that of a solvent does. Compared with the shear viscosity of the solvent, the diffusion coefficient of the solute breaks the Stokes-Einstein relationship, whereas the Stokes-Einstein relationship appears to hold in the case of the solvent molecule.

ISSN:0026-8976    

DOI:10.1080/00268970009483361

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Ab initiocalculations have been carried out at the CCSD(T)/aug-cc-pVTZ level on di-nitrogen and neon interacting with a calcium (II) cation. The interaction of nitrogen with the Ca2+cation is examined for end-on and side-on approaches. The two adjustable parameters in the TrAZ potential (ACa2+-Ne,bCa2+-Ne) are fitted to the Ca2+-Ne potential energy curve and combination rules are then applied in reverse to getACa2+-Ca2+,bCa2+-Ca2+. Combining these with values of ANN, 6NNenables the Ca2+-N2potential energy curves to be predicteda priori.Agreement with theab initiodata is found to improve dramatically when the anisotropy of the nitrogen polarizability is included in the induced interaction energy. Other combination rules are tried but are not found to have an impact on the agreement between thea prioripotential and theab initiodata

ISSN:0026-8976    

DOI:10.1080/00268970009483362

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Rotational spectra of the Kr-H2O van der Waals complex were measured in the frequency range 4–19 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The isotopomers studied include those of H2O, HDO, D2O, H217O and H218O with the six most abundant isotopes of Kr. A tunnelling splitting due to a large amplitude internal motion of the H2O subunit that exchanges bonded and non-bonded hydrogen atoms was observed. Nuclear quadrupole hyperfine structure was resolved and measured for the complexes containing83Kr, D, andl7O, and the corresponding nuclear quadrupole coupling constants were determined. These were used to estimate structural parameters and to derive information about the intermolecular dynamics. The results, and in some instances the spectroscopic constants themselves, were compared with experimental and theoretical data previously reported for Ar-H2O.

ISSN:0026-8976    

DOI:10.1080/00268970009483363

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor