ISSN:0026-8976    

DOI:10.1080/002689797171689

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The spectra of the fundamental, and first and second overtones (Δ ν = 1,2,3) around the C-H stretch of methane in liquid argon solutions have been measured at approximately 95 K. Combination bands involving one quantum of a bending mode in the region 3500–5000 cm-1and three quanta of bending modes in the region 6500–7600 cm-1were also measured. The near-infrared (IR) spectra of pure liquid methane was recorded between 6500 and 12 000 cm-1at 94 K. Measurements were made using a cryostat, a low-temperature cell, and a Fourier transform IR and near-IR spectrometer. Visible spectra of the fourth (Δ ν = 5) and fifth (Δ ν = 6) overtones of liquid methane and methane in liquid argon solutions were measured at 95 K. Visible spectra were recorded with a photoacoustic spectrometer employing resonant continuous-wave excitation and piezoelectric detection. Spectra in solutions are greatly simplified compared with the gas phase. This simplification is attributed to a narrowing of the rotational distribution at low temperatures and partial hindering of the rotational motion of sample molecules. Peak positions are systematically red shifted with respect to the gas phase, and the magnitude of the shift increases with the vibrational quantum number. The harmonic frequency and anharmonicity were obtained from a Birge–Sponer fit of the C–H vibrations. Previously reported assignments in normal-mode notation and local-mode calculations of energy levels of methane were used to assign the vibrational transitions observed in our experiments. The local mode notation is based on the harmonically coupled anharmonic oscillator model. Linewidths of the pure overtones increase proportionally as a function of the vibrational quantum number ν. There is indication that, for transitions with Δ ν = 5 and 6, the bandwidths are due to a dephasing mechanism in the slow modulation limit.

ISSN:0026-8976    

DOI:10.1080/002689797171698

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

NPTMonte Carlo simulations of racemic mixtures were performed on a simplified model for the enantiomers. The molecules were spheres of equal size, interacting via a modified LennardJones potential. The interaction strength and the optimum distance between heterochiral molecules (D and L) differed (by a factoreand a factors, respectively) from the homochiral values (D and D or L and L). Chemical potentials, enthalpies and orientational bond order parametersQlwere calculated for a lot of racemic liquid mixtures. Weak heterochiral interactions (e< 1) showed a tendency to phase separation. Extremely strong interactions (e≈ 5) gave rise to a glassy phase. A correlation analysis of the measuredQlvalues fore≠ / 1 ands= 1 showed no similarity of the molecular scale structure with that of binary crystals. Mixtures withs< 1 and withs> 1 both had a negative excess entropy, showing ordering. This effect was outweighed by a large negative excess enthalpy, and in total they had a negative excess Gibbs energy. In these mixtures the molecular scale structure was similar to that of certain binary crystal structures. By comparing the measuredQlvalues with those for ideal reference crystals an NaCl-like ordering was found for smallsand various layered closepacked crystal structures for largers.

ISSN:0026-8976    

DOI:10.1080/002689797171706

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Classical trajectory calculations of diffusion, viscosity, thermal conductivity and thermal diffusion in first-order kinetic theory have been performed using the MKWAab initiopotential energy surface of Moszynskiet al. [3] for temperatures in the range 77–800 K, and compared with available experimental data and previous calculations. For diffusion and viscosity the results are consistent with experiment. The ≈5% disagreement between the present results and the measurements for thermal conductivity could stem in part from the omission of higherorder terms in the kinetic theory expansion. The results for thermal diffusion suggest that the repulsive part of the potential energy surface may be too anisotropic. While there are no measurements for DPR and viscomagnetic effect cross-sections, comparison of the present results with previous calculations by Gianturcoet al. [10, 14] suggests that in the region sampled in room temperature collisions the anisotropy of the MKWA potential energy surface by Moszynskiet al. [3] is significantly less than that of a surface reported earlier by Thomaset al. [12].

ISSN:0026-8976    

DOI:10.1080/002689797171715

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Whereas model constraints (e.g., frozen bonds, rigidified atomic groups, etc.) are often resorted to for the calculation of the potential energy surfaces of polyatomic molecules, the derivation of exact expressions for the corresponding kinetic energy operators is a rather new topic. Indeed, the dimensions of the configuration spaces being altered, the differential operators should be modified accordingly, but not the multiplicative operators. In addition, the physical condition ℱ =0(where ℱ is the total angular momentum vector) which, from a mathematical viewpoint, is a set of three constraints, also is often considered. The particular question raised is: in the derivation of the kinetic energy operator, is the order in which we apply the model constraints on the one hand and the dynamic constraints ℱ =0on the other hand, immaterial? The answer is yes.

ISSN:0026-8976    

DOI:10.1080/002689797171724

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A simple analytical model is presented of the equation of state of the hard sphere Yukawa fluid with interaction polydispersity. On the basis of the model, the effects of polydispersity on the isotherm behaviour and the critical point of the phase transition are investigated. The isotherm shows characteristic behaviour due to the effect of polydispersity.

ISSN:0026-8976    

DOI:10.1080/002689797171733

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The Percus–Yevick (PY), hypernetted chain (HNC), and Martynov–Sarkisov (MS) closures for the Ornstein–Zernike equation are used to calculate the pair distribution functions of binary additive hard-sphere mixtures. The theoretical results are compared with new precise Monte Carlo simulation data described herein. Generally, the agreement of the MS closure with the data is the best and that of the HNC closure the worst. At some state points deviations from the simulation data are several times larger than those for pure hard spheres at the same packing fractions. An unusual behaviour pattern of the distribution functions has been found for systems with a hard sphere diameter ratio of 0·3, at low concentrations of the larger spheres. Unexpected minima and maxima ingij(r) appear at distancesr∼ σij+ σ22, where σ22denotes the diameter of smaller spheres. The phenomenon seems to be related to the predominance of certain geometrical arrangements of the component spheres.

ISSN:0026-8976    

DOI:10.1080/002689797171742

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The response of a spin 7/2 system subject to a first-order quadrupolar interaction and excited by two in-phase pulses separated by time delay τ is calculated ignoring high frequency terms. The results obtained are valid for any ratio of the quadrupolar coupling to the amplitude of an RF pulse. It is shown here that single- and multiple-quantum coherences developed during the first pulse can be detected at the end of the second pulse. Other pulse sequences with various phase cycling are also discussed. The results are applied to51V nuclei in a single ferroelastic crystal of BiVO4.

ISSN:0026-8976    

DOI:10.1080/002689797171751

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Optical emissions from the CO (a3Πr) and N2(B3Πg) electronically excited products of the dissociative electron recombination of vibrationally relaxed HCO+and N2H+and their deuterated analogues have been detected from flowing afterglow plasmas. From the CO (a → X1Σ+) and N2(B→ A3Σ+u) emissions, vibrational population distributions were determined for the CO a (ν' = 0-6) and N2B (ν' = 2-9) states. The changes in the vibrational populations with ν', and somewhat unexpectedly the changes in the overall distributions on deuteration, were smaller than suggested by the ‘impulse’ theory of Bates, which predicts a marked decrease with increasing ν' in both cases. For CO, the observed variations are more compatible with the Bates' ‘relic’ mechanism, if the restriction of zero overlap for the CO components of the wavefunction in the ion and the free diatomic is relaxed. For N2, in the change in the distribution on deuteration, a population change is seen around the vibrational level (ν' = 6) which is almost resonant with the energy of the recombining ion. Here the populations in the hydrogenated system are enhanced, suggesting the involvement of a tunnelling mechanism. The variations are qualitatively consistent with a simple theoretical model for tunnelling.

ISSN:0026-8976    

DOI:10.1080/002689797171760

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Computer simulation results for the angular dependence of spatial correlation functions in liquids are given. The results were obtained using smoothing filters applied to the molecular trajectories of liquids. The purpose of this procedure is to enhance the local structure by attenuating thermal vibrations, and to study low frequency motions. New spatial correlation functions for positions and velocities are measured. These depend not only on distance but also on the angle between the distance vector and the velocity of the reference particle. The results suggest that cooperative self-diffusion is mainly a longitudinal process. An alternative recursive filter is introduced so that the smoothing procedure can be obtained on-line, while the simulation is proceeding. Possible applications include molecular dynamics simulations of solid-liquid interfaces and real-time measurements of local geometry.

ISSN:0026-8976    

DOI:10.1080/002689797171779

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor