摘要:

A method that combines the best elements of thermodynamic integration and the Gibbs ensemble technique is proposed for the direct evaluation of phase equilibria by molecular simulation. Given the conditions of coexistence at a single state point, simultaneous but independentNPTsimulations of each phase are performed in succession along the saturation line. In each simulation, the pressure is adjusted to satisfy chemical potential equality according to the Gibbs-Duhem equation. Each coexistence point is determined by just one simulation, and particle insertions are never performed or attempted. Vapourliquid coexistence for the Lennard-Jones model is evaluated, and extensions are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300100881

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A theory is presented of the angular momentum distribution of diatomic fragments ejected in the photodissociation of triatomic molecules. The theory is based on semi-classical dynamics and treats dissociation from an ensemble of nuclear configurations reflecting the zero-point oscillations of the parent molecule. The mean angular momentum of the diatomic is determined by the range parameter of the repulsive potential, while the width of its distribution is determined by the gradient parameters of the potential surface as expressed in the Franck-Condon spatial and angular coordinates. The odd moments of the asymmetrical distribution are related to the nonlinear part of the potential energy surface.

ISSN:0026-8976    

DOI:10.1080/00268979300100891

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Ab initiomolecular orbital theory has been used to study the binary complex formed between the benzene and ClF molecules. The 4–31G and 6–31G** basis sets were employed and also electron correlation effects were assessed by second order Møller-Plesset perturbation theory with the 6–31G** basis set in singlepoint calculations at the SCF optimized geometry. Basis set superposition errors have been evaluated by the counterpoise correction method. It was found that the Cl-F subunit interacts with the π electrons of the benzene ring through the Cl end, and also that there is a significant deviation from theC6vsymmetry expected.

ISSN:0026-8976    

DOI:10.1080/00268979300100901

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A reinterpretation of the experimental NMR proton spin-lattice dispersion curve of the Ni2+(H2O)6complex is presented within a general slow-motion theory. The extended pseudo rotation (PR) model developed allows for cross-correlation effects between the nuclear spin-electron spin dipole-dipole and zero field splitting (ZFS) interaction. It is shown that the decomposition approach, treating the electron spin relaxation and the reorientational dynamic of the dipole-dipole correlation function as independent processes is not generally valid. For the Ni-hexa-aquo complex the transient ZFS interaction and the reorientational correlation time change by about 20 per cent due to the correlation effects. Molecular dynamics (MD) simulation of a divalent ion in water provided the timescale of the dynamics present in the PR and the Smoluchowsky models. The structure and dynamics of the octahedral complex is described. The transient ZFS interaction generated by the low frequency vibration modes n(M ↔ OH2) is characterized by a correlation time τv= 0·2 ps and the timescale of the orientational motion of the PR model is in the range τv= 1-10 ps. The fast E- and A-symmetric vibrations cause partial averaging of the ZFS interaction. The timescale of the ligand orientational modes, wag, twist and rock modes are comparable with the electron spin dynamics. The reinterpretation of the NMRD curve suggests that the dynamics described by the single exponential reorientational correlation time, τRreflects the wag, twist and rock modes rather than the overall reorientation of the whole metal-aquo complex. The simulations also suggest that the relatively short proton-metal ion distance obtained from the interpretation of the NMRD curve reflects neglect of outer sphere contributions rather than oversimplification of electron spin dynamics.

ISSN:0026-8976    

DOI:10.1080/00268979300100911

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The effect of radiofrequency pulses in nuclear quadrupole resonance (NQR) spectroscopy is examined. The description is different from that needed in nuclear magnetic resonance (NMR) spectroscopy. In particular, both rotating and counter-rotating radiofrequency field components are retained. Pulses in NQR spectroscopy are selective and cause transitions between two pairs of levels (±M) → ±(M+ 1), other transitions are not normally excited. The formulation of pulses is then described for any spinIby two 2 × 2 rotation matrices, one causing anM→M+ 1 transition and the other causing a - (M+ 1) → -Mtransition. Calculations on resonance for spins with an axially symmetric nuclear quadrupole for up to three pulses are presented for spinsI= 1 andI= 3/2. The results agree with the work of Reddy, R. and Narasimhan, P. T., 1991,Molec. Phys.,72,491, in the appropriate limits.

ISSN:0026-8976    

DOI:10.1080/00268979300100921

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Empirical potential functions comprising two-body and three-body terms have been derived for the alkali metals by fitting parameters to the phonon frequencies, elastic constants, lattice energies and lattice distances of the body-centred-cubic (b.c.c.) solids. These potentials give, in all cases, very similar energies for the b.c.c., face-centred-cubic (f.c.c.) and hexagonal closest packing (h.c.p.) structures, with simple cubic (s.c.), diamond and various two-dimensional structures being much less stable. The lithium potential has been used to predict structures and stabilities of neutral Linmicroclusters. No ‘magic number’ stabilities have been found. Forn≥ 6 the structures can, in the main, be described as face-fused tetrahedra, andab initiocalculations support these structures in some important cases (e.g.n= 6, 7).

ISSN:0026-8976    

DOI:10.1080/00268979300100931

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The electronic emission, from a radiofrequency discharge, of the Ã2Σ+-[Xtilde]2Πitransition of the free radical NCO has been recorded using high resolution Fourier transform spectroscopy. Some weaker bands were also observed in laser induced fluorescence experiments. The rotational structure of seven bands have been analysed and molecular constants have been obtained. Some of these bands involve the two fundamental vibrationsv1andv3and (0 2 0)2Φ of the electronic ground state. Several perturbations involving the lower vibronic levels are reported. Improved parameters for the three interacting2Π levels (1 0 0) and (0 2 0) κ and μ have been determined using the model of Woodwardet al.

ISSN:0026-8976    

DOI:10.1080/00268979300100941

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The π-coordinated interaction of a palladium atom with acetylene and ethylene is investigated through theoretical calculations, including electron correlation effects. The interaction is analysed as a van der Waals one and it is concluded that the ground state of PdC2H2and PdC2H4molecules are bound by 48·93 kJ mol-1and 85·85 kJ mol-1relative to the ground state of the Pd (1S) atom, C2H2(1Σg) and C2H4(1Ag) molecules. A comparison with experimental results for this system is presented.

ISSN:0026-8976    

DOI:10.1080/00268979300100951

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structure of a hard sphere fluid inside a planar slit pore is studied using integral equation and computer simulation approaches. A robust and efficient method of numerically solving the one-particle Ornstein-Zernike (OZ) equation for the density profile in conjunction with an arbitrary closure is described, which is an approximate Newton-Raphson iteration in Fourier space. A new form of reference hypernetted chain theory is proposed that uses the bridge function of a fluid of hard spheres near a hard wall as a reference system. This is tested against new grand canonical ensemble Monte Carlo computer simulation data obtained herein and the results of other OZ equation based theories, including the Percus-Yevick hypernetted chain, Martynov-Sarkisov and modified Verlet theory, at reduced bulk fluid number densities up to 0·7 and for pore sizes varying from 1·25 to 4·0. The proposed theory gives accurate density profiles and pressures for the entire range of slit sizes, and is superior to the other theories considered.

ISSN:0026-8976    

DOI:10.1080/00268979300100961

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The orientational order in the fluid and liquid crystal phases of a simple model of dipolar discotic molecules (cut-spheres) is determined by Monte Carlo simulations. Our results are mainly directed to a qualitative study of this ordering in view of the long relaxation times of metastable states in the dense liquid crystal phases. In the fluid and nematic phases, short columns of five molecules are found to be the most probable arrangements of neighbouring molecules. In the columnar phase the dipole-dipole interactions can induce the polarization of the columns which are arranged with antiferroelectric order. When the orientation of the dipole moment breaks the axial symmetry of the molecules, a rectangular columnar phase with molecules tilted with respect to the columnar axis appears to be the stable dense liquid crystal phase.

ISSN:0026-8976    

DOI:10.1080/00268979300100971

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor