摘要:

A combination of tandem mass spectrometry experiments (collisional activation and neutralization-reionization mass spectrometry) andab initiomolecular orbital calculations was used to provide evidence for the stability of thionitrosyl hydride, H-N=S, and to characterize it. Experimentally, HNS has been generated using three different precursors. The (HNS) potential energy surface has been explored in the lowest-lying singlet, triplet and doublet ionized states using (PU)MP4/6-311++G(2df,2pd) energies based on (U)MP2/6-31G(d,p) geometries. Predictions for several spectroscopic properties of NHS have been made on the basis of MO calculations and empirical corrections. These include geometry, rotational constants, vibrational wavenumbers, dipole moment,14N nuclear quadrupole coupling constant, triplet-singlet energy gap, electron and proton affinities, ionization and π* ← n transition energies.

ISSN:0026-8976    

DOI:10.1080/00268979300100111

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Thermodynamic properties of mixtures of dipolar with nonpolar linear molecules are obtained fromNVT-MD simulations and from a perturbation theory based on nonspherical reference systems. In the simulations, the interactions are all of the same two-centre Lennard-Jones (2CLJ) type, of elongationL= 0·505 and with an additional axial point dipole on the polar molecules. Simulation raw data are the pressures, internal energies and dipolar energies for a grid of dipole moments and compositions at the state pointT* = 3·078 and ρ* = 0·466. From these raw data, first the excess values of the internal energy and the Helmholtz energy for mixing at constant density are obtained. Then the excess values of the Gibbs energy, of the enthalpy and of the volume for mixing at constant pressure are derived. In perturbation theory, the equivalent Kihara interactions yield results which are in good agreement with the simulation data. Neither simulations nor theory, however, show a liquid-liquid phase separation for the state conditions and dipole moments considered. In addition, a reanalysis of previous simulation data for the Lennard-Jones-Stockmayer system does not necessarily confirm the claim that demixing has already been found there. Hence, since very recently demixing was found by RHNC theory for the hard sphere-dipolar hard sphere system, it remains a challenge to search for such a phase separation caused solely by the dipolar forces in mixtures with realistic interactions.

ISSN:0026-8976    

DOI:10.1080/00268979300100121

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

First-order perturbed hard sphere fluid expressions are fitted to Lennard-Jones fluid compressibility factor,Z, and internal energy,U, data taking the two Lennard-Jones constants as adjustable parameters. The results are used to distinguish the utility of alternative perturbative algorithms over a wide density and temperature range. First-order Barker-Henderson perturbative expressions are found to systematically underestimate temperature dependent changes in thermodynamic properties while the Weeks-Chandler-Andersen model is found to reasonably reproduce simulation results throughout the liquid and supercritical fluid domain, with only a few per cent adjustment of the effective Lennard-Jones parameters. Closed analytical expressions for the Barker-Henderson and Weeks-Chandler-Andersen hard sphere diameters as well as a hard fluid reference system radial distribution function model are presented. These are shown to accurately reproduce hard sphere diameters and thermodynamic properties of Lennard-Jones fluids. Efficient algorithms for determining effective Lennard-Jones parameters for real fluids are demonstrated using fits to supercritical fluid N2compressibility factor data.

ISSN:0026-8976    

DOI:10.1080/00268979300100131

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The coexistence of solid-like and liquid-like forms of a finite atomic cluster is examined analytically and numerically using model densities of states and partition functions. Within the harmonic normal mode approximation we can calculate the caloric curve, heat capacity and total energy probability distribution of a finite atomic cluster from a single molecular dynamics trajectory. The method requires data from a high energy simulation in the microcanonical ensemble, along with systematic quenching, to provide statistics about local minima. The results provide new insight into the coexistence of solid-like and liquid-like forms of such clusters, with a clear S-bend in the microcanonical caloric curve and a bimodal distribution of the total energy around the transition temperature for a 55 atom system.

ISSN:0026-8976    

DOI:10.1080/00268979300100141

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Results are reported of energy minimizations and molecular-dynamics calculations for chlorine physisorbed on the basal plane of graphite. The dispersion-repulsion component of the intermolecular potential is modelled by an anisotropic site-site potential and the permanent electrostatic potential is represented by distributed multipoles at the atomic nuclei. The molecule-surface interaction includes the anisotropic polarizability of the graphite, image-multipole interactions and substrate mediation of the dispersion interaction. The energy minimizations show that the adsorbed molecules are likely to favour a two-sublattice, herringbone structure; a four-sublattice pinwheel arrangement is also a possibility if image interactions play a sufficiently important role. Difficulties in reproducing the experimental X-ray scattering pattern are discussed. The molecular-dynamics calculations suggest that the low-symmetry structure postulated by Kleeet al., (1986,Surf. Sci.,171,103) is unstable, and melting of the simulated monolayers occurs at temperatures at least 100K below the claimed experimental value. There is a clear need for further thermodynamic and structural studies of this system.

ISSN:0026-8976    

DOI:10.1080/00268979300100151

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A near-equilibrium potential energy surface has been calculated for cyanoacetylene, HC3N, by means of the coupled electron pair approximation (CEPA) using a basis set of 118 contracted Gaussian-type orbitals, and from it vibration-rotation coupling constants αvand 1-type doubling constantsqtandqJthave been calculated for various isotopomers by standard perturbation theory. By the combination of experimentalB0and theoretical αvvalues for six different isotopomers we were able to determine an accurate equilibrium geometry:re(HC(1)) = 1·0624 Å,R1e(C(1)C(2)) = 1·2058 Å, R2e(C(2)C(3)) = 1·3764 Å andR3e(C(3)N) = 1·1605 Å. A conservative estimate of the error in the equilibrium bond lengths is 0·0005 Å.

ISSN:0026-8976    

DOI:10.1080/00268979300100161

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Using pairs of potential energy surfaces with common isotropic components for a series of nitrogen-rare gas mixtures, the sensitivity of thermal diffusion to changes in the anisotropy of intermolecular interaction potentials has been examined. The thermal diffusion factor has been shown to be sensitive to variations in the anisotropy of the repulsive wall; however each system has been found to exhibit a temperature range over which αTis relatively insensitive to details of the potential energy surface. Additionally, the contribution of angular momentum polarizations, rotational-translational coupling and the transport of rotational energy to the thermal diffusion factor have been examined for each of the nitrogen-rare gas systems. The relative importance of each of these contributions has been found to depend upon the mass ratio of the components of the binary mixture. It is seen that in general none of these terms can be neglected, and that for most systems, higher-order Chapman-Cowling approximations will be required in order that meaningful comparisons can be made with experiment.

ISSN:0026-8976    

DOI:10.1080/00268979300100171

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Methyl tunnelling has been measured in solid diacetyl at low temperatures as a function of pressure using low field NMR spectroscopy. In addition the associated thermally activated rotation at higher temperatures has been measured as a function of pressure by spin-lattice relaxation time (T1) analysis. The tunnel frequency decreases exponentially from 76 kHz at atmospheric pressure to 9 kHz at 6·25 kbar. TheT1data can be correlated with the tunnel frequency according to the methyl thermometer model, extending the model to lower tunnelling frequencies than previously demonstrated. A small sixfold symmetric term in the potential hindering rotation can be deduced from the data. The overall height of the hindering potential and its shape are compared with the potential calculated from the crystal structure.

ISSN:0026-8976    

DOI:10.1080/00268979300100181

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Simulations are reported for theNVEensemble for four states of a Lennard-Jones 12-6 sp2·5 fluid. The transport coefficients, thermal conductivity and shear, longitudinal and bulk viscosity are determined together with the second, fourth and sixth moments of the spectrum of the time autocorrelation function in each case. A comparison is made with the predictions of information theory using one, two, or all three moments. The sixth moment was not used in our previous paper on this topic nor were the longitudinal and bulk viscosities reported there. A first test is made of the well known macroscopic relation between the three viscosities. For these examples, the information theory predictions are, generally, better if the sixth moment isnotassumed known, contrary to expectation.

ISSN:0026-8976    

DOI:10.1080/00268979300100191

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Selfdiffusion coefficients determined by the NMR spin-echo technique are reported forn-octane (-25–75°C, 0·1–361 MPa) and toluene (-50–50°C, 0·1–369 MPa). In the case of toluene, these are supplemented at 25°C by14C radiotracer diaphragm cell measurements. The results for each substance are correlated with high pressure viscosity data. Those for toluene overlap the range of applicability of the rough hard sphere model, and this model is used to compare the transport properties of toluene with those of benzene and mesitylene.

ISSN:0026-8976    

DOI:10.1080/00268979300100201

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor