摘要:

Pure fluids of hard heteronuclear dumbbells and linear symetric triatomics, and binary mixtures of these fluids with hard spheres at a hard structureless wall, are studied by Monte Carlo simulations. In addition to density profiles (angle-averaged wall/site pair correlation functions) of all atomic sites in each molecule, the average number density of sites in the first layer, and radial slices through the full one-particle correlation function of triatomics, are also calculated. Different patterns of packing of the dumbbells and triatomics in the mixtures are found, and their dependence on the state conditions is discussed. Finally, we derive the general expression relating the system pressure to the contact values of the wall/site correlation functions.

ISSN:0026-8976    

DOI:10.1080/00268979000102231

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A novel method has been developed to bias the insertion of structured particles into dense phases during grand canonical and Gibbs ensemble Monte Carlo simulations. The method biases the orientation rather than the position of a trial insertion. The method has been shown to increase the acceptance rate by a factor of two. It has been used in Gibbs ensemble simulations of TIPS2 and TIP4P water at and above ambient temperature, and points on a coexistence curve have been simulated. The TIP4P model has been shown to reproduce experimental data much more closely than the TIPS2 model.

ISSN:0026-8976    

DOI:10.1080/00268979000102241

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Using graphical expansion theory, we show that the pair distribution function (PDF) of a molecular fluid may be related in a simple way to the PDF of a simple fluid having the same density as the system under consideration. An approximation for the exact expansion is proposed. The numerical calculations required by the present treatment are very easy to perform and lead to results in good agreement with those obtained by Monte Carlo simulation.

ISSN:0026-8976    

DOI:10.1080/00268979000102251

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Multiple-quantum NMR has recently been shown to be an effective method for measuring the sizes of spin clusters in solids, with initial applications reported for hydrogenated amorphous silicon, zeolites and surface-bound species. The present analysis traces the time development of multiple-quantum coherence in polycrystalline materials containing clusters of two to five spin-½ nuclei. Numerical solutions to the Liouville-von Neumann equation are given for a series of different configurations in order to gauge the dependence of the coherence magnitudes on the geometry of the cluster, and to follow the simplification and damping of the macroscopic signal. Exact coherence magnitudes after prolonged excitation are compared with those predicted by statistical models, and found to differ significantly for most of the small polycrystalline systems.

ISSN:0026-8976    

DOI:10.1080/00268979000102261

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The theory of the evolution of aJ-coupled homonuclear system of two nuclei of spin 1/2 (in the liquid state) subjected to a spin-locking field is explicitly reformulated in terms of simultaneous differential equations. Quantities destroyed by the inhomogeneity of the spin-locking field are delineated; from these considerations it is found that only four quantities interfere: the two transverse magnetizations of concern (supposed to be in absorption), one observable mode corresponding to the difference of antiphase dispersive doublets and one mode (unobservable) involving zero/double quantum coherences. Transfers of Hartmann-Hahn type occur necessarily via the antiphase doublet mode. Analytical solutions could be derived for these transfers, which occur in an oscillatory fashion. The frequency of the Hartmann-Hahn oscillations is shown to depend not only uponJbut also upon the characteristics of the spin-locking field (amplitude and offset with respect to the resonance frequencies of the two nuclei). When relaxation phenomena are included, the resulting differential equations are no longer analytically solvable. Approximate treatments, as well as an exact numerical analysis, lead to the conclusion that Hartmann-Hahn oscillations always exist as long as theJcoupling is not too small with regard to relaxation parameters, their amplitude decreasing when the amplitude of the spin-locking field decreases. The numerical analysis allows extraction, from experimental data, of the relaxation parameters and particularly the transverse cross-relaxation term. Experimental one-dimensional results confirm the validity of the present theory. Implications for the intensities of cross-peaks in two-dimensional ROESY experiments are outlined.

ISSN:0026-8976    

DOI:10.1080/00268979000102271

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The ambiguities of the effective Hamiltonians for isolated doubly excited vibrational states of bending modes of tetrahedral molecules are investigated theoretically. Reduced forms of these Hamiltonians and invariant parameters are proposed. The theoretical conclusions are tested by actual fits of experimental data for the 2v2,vIn2 +v4and 2v4bands of12CH4. The experimental behaviours of fitted parameters are found to be in satisfactory agreement with theory. The results obtained, which represent one step of the comprehensive study of the so-called pentad, lead to significant improvements not only in the convergence of least squares but also in the accurate understanding of the physical meaning of specroscopic parameters. A generalization to states includingnquanta of thev1mode is outlined.

ISSN:0026-8976    

DOI:10.1080/00268979000102281

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Photofragment scalar pair correlations are determined following photodissociation of selected rovibrational levels of the third OH stretching overtone state of H2O2at λ ≈ 748 nm. An anticorrelation between the rotational populations of the two sibling OH fragments is observed that is insensitive to the selectedKalevel in the range 1–5 with ⟨J⟩ ≈ 7–14. Simulations of the OH pair correlations, and previously measuredv-jcorrelations and rotational distributions, indicate the breakdown ofKaas a good quantum number in the interrogated third OH stretching overtone state of H2O2and highlight the role ofa-axis rotation in promoting rotation-vibration coupling.

ISSN:0026-8976    

DOI:10.1080/00268979000102291

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Calculations are presented that estimate the energies ofallthe truly bound states of the N2-Ar van der Waals complex using a semi-empirical potential-energy surface. An effective dipole surface is constructed by comparison with the infrared spectra of McKellar. These spectra are reproduced by convoluting the 30 000 strongest transitions with suitable temperature- and pressure-dependent line profiles. These synthetic spectra reproduce the observed experimental features, supporting and extending McKellar's assignments. Some of the peaks are shifted to higher frequency, suggesting that the bending potential is slightly too stiff. The corresponding ‘pure’ (far-infrared) spectra of the van der Waals complex are also synthesized. These do not show the low-frequency features seen in the infrared spectra.

ISSN:0026-8976    

DOI:10.1080/00268979000102301

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A series of multireference (MRD-CI) calculations are presented for the electronic states of ArH correlating with the atomic limits up to Ar(1S) + H* (n= 3), in order to provide transition energies corresponding to the possible Rydberg spectra of ArH. The computed transition energies for several2Π → A2∑+transitions are found to be in good agreement with experimentalT0values, with discrepancies that in every case are smaller than 0·1 eV.

ISSN:0026-8976    

DOI:10.1080/00268979000102311

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The vibronic intensities of the vibronic origins due to the three odd-parity vibrational modes of the Γ8(2T2g) → Γ8(4A2g) electronic transition of the ReBr2-6ion are calculated using both crystal-field and ligand-polarization vibronic models. The crystal-field calculations is carried out using the closure approximation, and both models employ the double-group formalism. The vibronic intensity distribution is different for the two models, but by using reasonable values of the radial integrals and atomic charges, satisfactory agreement with experiment is achieved.

ISSN:0026-8976    

DOI:10.1080/00268979000102321

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor