摘要:

Monte Carlo calculations have been used to study the properties of a molecular monolayer self-assembled on a metallic substrate. The surface density and interaction parameters were chosen to represent CH3(CH2)15SH (hexadecyl mercaptan) molecules chemisorbed on the (111) surface of gold. The system studied consisted of 90 alkyl thiol molecules, each modelled by 17 pseudo-atoms and periodically replicated in the plane of the surface. The conformational states of the molecules were sampled by the recently developed configurational-bias Monte Carlo method (Siepmann, J. I., and Frenkel, D., 1992,Molec. Phys.,75, 59). Two different approaches were used to treat the flexibility of the adsorbed molecules. The structures obtained were in both cases ones in which the molecules are preferentially aligned and tilted with respect to the surface normal, but the assumed degree of flexibility of the carbon backbones has a major effect on the degree of translational, orientational and conformational order within the monolayer. Comparison is made with results both from experiments and from molecular dynamics calculations for the same physical system, and the relative merits of the two methods of simulation are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300101371

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The emissive triplet states ofp-chlorobenzaldehyde andp-chlorobenzaldehyde-d1as guests inp-dimethoxybenzene host crystals are investigated by optical Zeeman and low field optically detected magnetic resonance (ODMR) studies of their phosphorescence at ⩽4·2 K. The active spin level, the sign of the principal fine structure splitting constant, its magnitude and the approximate location of the fine structure axes are deduced from high-field Zeeman perturbations of the phosphorescence origins of the guests. ODMR measurements confirm the conclusions and yield better values for the constants and locations of the principal axes of the fine structure tensor. The results for the perprotonated guest show thatD≡ -3Z/2 = -0·58290 andE≡ (Y-X)/2 = -0·00714 cm-1; the fine structure axes are oriented relative to the host with theyaxis nearly normal to the molecular plane and thezaxis rotated 40° from the long molecular axis; and the phosphorescence arises predominantly fromTz. The d1-isotope of the guest has a larger magnitude for |D| (≠ 11%), but the other properties of its triplet state are similar to those of the perprotonated molecule. The results are interpreted in terms of a substitutional guest with a3nπ* assignment for the phosphorescent triplet state.

ISSN:0026-8976    

DOI:10.1080/00268979300101381

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Lineshape data have been obtained for the isotropic Raman spectra of HD and para H2diluted in liquid and hypercritical argon. The observed profiles near 2ρcare interpreted in terms of pure vibrational dephasing and fast modulation process. In the critical region an inhomogeneous broadening is observed; it is superimposed in the HD case, on a rotational contribution. A detailed analysis of theQspectra obtained for the normal H2-argon system along the critical isochore provides evidence for the role of density fluctuations near the liquid-gas critical point.

ISSN:0026-8976    

DOI:10.1080/00268979300101391

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We present numerical computations of the third dielectric virial coefficientC*εand the third pressure virial coefficientC*pof a fluid of hard sphere dipoles, over the range 0 < μ*2< 4. The requisite 9-dimensional quadratures are performed using an adaptive Monte Carlo algorithm. We also compute terms in the high temperature expansions of the virial coefficients up to order μ*20(C*ε) and μ*24(C*p). Our work complements and extends earlier investigations by Joslin and by Rushbrooke and Shrubsall. Our results agree well with available experimental data, especially considering the relative crudeness of the molecular model. We also compute the critical point of the fluid.

ISSN:0026-8976    

DOI:10.1080/00268979300101401

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper we compare two different sampling methods for probing the distribution of conformations of a flexible molecule. The first method makes use of a form of umbrella sampling which effectively enhances the crossing of energy barriers. The second method, the so-called blue moon ensemble, is based on the use of an holonomic constraint which fixes the reaction coordinate. We apply these methods to the evaluation of the configurational free energy associated with the dihedral angle of one n-butane molecule in CCl4and in gas phase. We analyse the nature and the influence, in the blue moon approach, of the compensating term which arises from constraining the reaction coordinate and setting its velocity to zero.

ISSN:0026-8976    

DOI:10.1080/00268979300101411

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

It is well known that the numerical solution of the hypernetted chain (HNC) equations yields satisfactory results for the pair correlation function of the primitive model of electrolytes and similar models of ionic particles over a considerable range of thermodynamic states. Despite this, it has become apparent that for low densities (or low ionic concentration in electrolytes) the numerical solution breaks down for temperatures well above the expected coexistence region between gas and liquid phases. Here we study the situation by analytic means, comparing it to a similar problem for sticky hard spheres in the Percus-Yevick (PY) approximation. On the basis of our analysis we conclude that the failure of the HNC is of the same nature and is connected to the existence of two possible solutions for low densities. When the temperature is lowered these solutions will merge into one at a particular temperature, below which a real solution is no longer possible. By extending our analysis to systems like the monoatomic Lennard-Jones fluid and comparing with previous results of Gallerani, Lo Vecchio and Reatto for two-Yukawa and Lennard-Jones systems in the PY approximation, we conclude that these are general common features of the HNC and PY approximations in the low-density regime. Numerically they appear to persist in the HNC closure at high densities as well (in contrast to the behaviour of the PY approximation) although our analysis is silent in this regard. Our analysis is consistent with the results of a recent comprehensive numerical study by Belloni.

ISSN:0026-8976    

DOI:10.1080/00268979300101421

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structure of a Stockmayer fluid confined to a slit-pore, that is between two parallel f.c.c. (100) planes of rigidly fixed Lennard-Jones (12, 6) atoms (walls), is studied by means of the grand-canonical ensemble Monte Carlo method. The pore fluid is in equilibrium with its bulk phase counterpart, which is liquid. The local density and the cylindrical pair-correlation function in planes parallel to the walls, indicate that the structure of the pore fluid depends strongly on the distancehseparating the walls and on their lateral alignment (registry). The pore phase may be liquid, solid or gaseous depending on the registry,h, and the strength |μ| of the dipole moment. The additional degree of freedom provided by |μ| of the Stockmayer fluid strongly influences the structure of the pore fluid. As |μ| increases, the degree of order of the pore fluid decreases.

ISSN:0026-8976    

DOI:10.1080/00268979300101431

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We use the experimental values of the polarizabilities of the water molecule in conjunction with atomic trajectories obtained from molecular dynamics (MD) simulations to calculate numerically the Raman and infrared (IR) spectra for hexagonal ice. We compare the results when the MD trajectories are generated using an effective two-body potential and a polarizableab-initiopotential. We find that, for both potentials, the IR and the Raman spectra, in the region between 0 and 400 cm-1, are dominated by induced dipole contributions. The polarizableab-initiopotential reproduces fairly well the experimental spectra, while the results for the two-body empirical potential are less satisfactory.

ISSN:0026-8976    

DOI:10.1080/00268979300101441

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In the low-temperature (T< 304K) phase both CH3and NH3groups execute threefold reorientations, the former in a symmetric potential well and the latter in an asymmetric potential well. The relaxation results in the rotating frame reveal a defect motion in this phase. Additional defect motions, superimposed on fourfold reorientations of chains, are present in phase II (317 K <T< 369K). In phase I (T> 369 K), melting of chains occurs. Although hydrogen bonding is stronger in (C13H27NH3)2ZnCl4than in (C10H21NH3)2ZnCl4, fast fourfold motions of chains are present at lower temperatures in the former compound. Melting of chains occurs at approximately the same temperature in these compounds.

ISSN:0026-8976    

DOI:10.1080/00268979300101451

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The rotational spectrum of NiD in the lowest three vibrational levels (v= 0, 1, 2) of its ground2Δ state has been studied by far-infrared laser magnetic resonance (LMR). Transitions have been detected for all five isotopes of nickel and within both the lower2Δ5/2and upper2Δ3/2spin components. The nuclear hyperfine splittings for the61Ni isotope (I= 3/2) have been resolved. An effective Hamiltonian has been used to fit the data which include the known vibration-rotational transitions within the2Δ5/2(v= 1 ←v= 0) subsystem and a fine structure transition in thev= 0 level of the2Δ ground state, recently measured by CO2LMR. From a parity splitting of the rotational transition within the X2Δ3/2spin component we have determined the first Λ-type doubling parameter for the X2Δ ground state. A set of molecular parameters for NiD in the2Δ ground state of NiD is given which allows a comparison with the parameter set for the well known NiH radical.

ISSN:0026-8976    

DOI:10.1080/00268979300101461

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor