The adsorption of N2 gas on the LiF(001) surface is studied by canonical Monte Carlo (CMC) computer simulation. These results show that N2 forms an ordered structure where the molecules are arranged in a unit cell of p(22×2)R45° symmetry at temperatures below 23K with 50% coverage. The nitrogen molecules are tilted by 53° from the surface normal and have the same azimuthal orientation along diagonals, with diagonals alternating their orientation. Beyond 23K, the molecules become azimuthally disordered but with residual short-range order. No change in the position of the peak of the polar (tilt) angle distribution was observed above the transition temperature. This transition is purely of the order–disorder type.

In response to recent helium atom scattering (HAS) and neutron scattering results, Monte Carlo simulations and perturbation theory calculations have been performed for D2 on MgO(001). Monte Carlo simulations predict that D2 molecules form a series of interesting structures, p(2×2)→p(4×2)→p(6×2), with coverages Θ=0.5, 0.75, 0.83 respectively, and followed by a formation of a top layer of p(6×2) unit cell symmetry. The three types of mono-layers are stable up to 13K, whereas the top layer still exists up to 10K. This is in partial agreement with the neutron scattering and HAS results that report c(2×2), c(4×2) and c(6×2); they agree in terms of coverage and stability, but disagree in terms of symmetry. A quantum mechanical examination of the D2 molecules’ rotational motion shows the molecular axes are azimuthally delocalized and hence the simulated structures are c-type rather than p-type. These calculations also indicate that ortho-D2 and helicoptering para-D2 prefer cationic sites, while cartwheeling para-D2 prefers anionic sites.

Monte Carlo (MC) simulations of D-2 molecules on the MgO(001) surface are reported and show that a series of interesting structures form with increasing coverage, viz. p (2 X 2)-> p (4 X 2) -> p (6 X 2), with coverages Theta = 0.5, 0.75, and 0.83 respectively, and are stable up to 13 K. The p (2 X 2) structures contain two D-2 molecules per unit cell, with each molecule lying parallel to the plane of the surface (theta = 90 degrees) directly above every other Mg2+ site. The molecules adopt a "T" configuration with respect to their nearest neighbors. The p (4 X 2) and p (6 X 2) structures, have two kinds of adsorption sites: a parallel site, as in the case of p (2 X 2), and a tilted site, where the D-2 molecules sit between cationic and anionic sites with the molecular axis directed towards the anionic site, with theta approximate to 60 degrees. These structures are consistent with recent neutron scattering results in terms of coverage and stability, but disagree in terms of symmetry; the neutron scattering work found "c" type structures whereas the MC simulations (without quantum considerations) yield a "p" type structures. To reconcile the results of the simulations and experiments, the quantum mechanical rotational motion of the adsorbed D-2 molecules was studied using perturbation theory. These calculations show that the adsorbed D-2 molecules are azimuthally delocalized and hence the structures are indeed "c" type rather than "p" type.

Perturbation theory calculations of the rotational motion of a hydrogen molecule rotates at the center of a p-type unit cell are reported. These calculations show that the molecular axis of H2 is azimuthally delocalized and hence it can be tunneled into classical forbidden region where the structures are indeed “c” type rather than “p” type. Thus estimating the quantum effects of those species will change the monolayer configurations into c-type structures and hence match the recently reported Helium Atom Scattering (HAS) results.

Monte Carlo simulations show that, at one monolayer coverage, H2 molecules adsorbed on a NaCl(001) surface occupy all Na+ sites and form a commensurate c(2×2) structure. If the Cl− sites are occupied as well, a bi-layer p(2×1) structure forms. An examination of the H2 molecules’ rotational motion shows the molecular axes are azimuthally delocalized and so both of the structures acquire (1×1) symmetry in accord with experimental observations. These calculations also show that helicoptering o-H2 (J=1, m=±1) prefer to sit on top of Na+ sites, while cartwheeling o-H2 (J=1, m=0) prefers to locate over Cl− sites, in agreement with other work.

In response to recent helium atom scattering (HAS) results, Monte Carlo (MC) simulations and perturbation theory have been performed for H2 on LiF(001). MC simulations predict that H2 molecules form a series of structures, p(2×2), p(8×2), p(4×2) with coverages Θ=0.5, 0.625 and 0.75, respectively, that are stable up to 8K. This is in partial agreement with the HAS results that report c(2×2) and c(8×2) structures; they agree in terms of coverage and stability, but disagree in terms of symmetry. To reconcile the results of the simulations and experiments, the orientation of the adsorbed H2 molecules was studied using perturbation theory. These calculations show that the adsorbed H2 molecules are azimuthally delocalized and that the structures are c-type rather then p-type. The calculations also indicate that p-H2 and helicoptering o-H2 prefer cationic sites, while cartwheeling o-H2 prefers anionic sites.

The adsorption of CO2 gas on the MgO (100) crystal surface is investigated using grand canonical Monte Carlo simulations. This allows us to obtain adsorption isotherms that can be compared with experiment, as well as to explore the possible formation of monolayers of different densities. Our model calculations agree reasonably well with the available experimental results. We find a "low-density" adsorbed monolayer where each CO2 molecule is bound to two Mg2+ ions on the MgO substrate. We also observe the formation of monolayers of higher density, where some of the CO2 molecules have rotated and tilted to expose additional binding sites. Low-temperature simulations of both the low- and high-density monolayers reveal that these states are very close in energy, with binding energies of approximately 7 kcal/mol at T=5 K. The high-density monolayer given by our model has a density that is significantly less than the reported experimental value. We discuss this discrepancy and offer suggestions for resolving it.

The Rosin Rammler Sperling Weibull distribution and its use in the analysis of complex data is explained with reference to metoprolol and acebutolol AUC values and isoniazid plasma concentrations. The technique is then applied to sparteine and debrisoquine data to resolve populations into distinct sub-groups. Goodness of fit is measured by applying the X2 test to the untransformed data. The method is simple to use and sub-groups can be identified rapidly. Each sub-group can be characterised by a simple exponential equation.

The AM1-BCC method quickly and efficiently generates high-quality atomic charges for use in condensed-phase simulations. The underlying features of the electron distribution including formal charge and delocalization are first captured by AM1 atomic charges for the individual molecule. Bond charge corrections (BCCs), which have been parameterized against the HF/6-31G* electrostatic potential (ESP) of a training set of compounds containing relevant functional groups, are then added using a formalism identical to the consensus BCI (bond charge increment) approach. As a proof of the concept, we fit BCCs simultaneously to 45 compounds including O-, N-, and S-containing functionalities, aromatics, and heteroaromatics, using only 41 BCC parameters. AM1-BCC yields charge sets of comparable quality to HF/6-31G* ESP-derived charges in a fraction of the time while reducing instabilities in the atomic charges compared to direct ESP-fit methods. We then apply the BCC parameters to a small “test set” consisting of aspirin, d-glucose, and eryodictyol; the AM1-BCC model again provides atomic charges of quality comparable with HF/6-31G* RESP charges, as judged by an increase of only 0.01 to 0.02 atomic units in the root-mean-square (RMS) error in ESP. Based on these encouraging results, we intend to parameterize the AM1-BCC model to provide a consistent charge model for any organic or biological molecule. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 132–146, 2000

Monte Carlo simulations of CO/MgO(001) show that below 41 K the CO molecules form a c(4×2) structure with six molecules per unit cell distributed into two kinds of adsorption sites: a perpendicular site and a tilted site (polar angle of 31°). Both sites are localized near Mg2+ ions. The occupancy of perpendicular sites to tilted sites occurs in the ratio of 1:2. At 41 K the c(4×2) phase undergoes a phase transition into a less dense, disordered phase accompanied by the expulsion of some molecules to form a partial second layer. The density of the remaining disordered layer is the same as for a p(3×2) phase and portions of the disordered layer show regions of short range ordering with either the c(4×2) or p(3×2) structures. The p(3×2) phase contains four molecules per unit cell and also consists of perpendicular and tilted sites, but in the ratio of 1:1. This structure was found to be stable up to 50 K after which the expulsion of some molecules and disordering of the layer occurred. A model to test the relative stability of these two phases by examining the difference in Gibbs free energy is constructed and shows that below 41 K the c(4×2) phase is the most stable but above 41 K the p(3×2) phase is the most stable. However, at low pressures the model suggests that the p(3×2) phase will not be observed and the layer will instead transform from the c(4×2) phase to a disordered phase at 41 K. This result reconciles the findings of low-energy electron diffraction (LEED) experiments [p(3×2) phase observed] with those of helium atom scattering (HAS) and polarization infrared spectroscopy (PIRS) experiments (disordered phase observed). It is proposed that the c(4×2)→p(3×2) transition is part of an infinite sequence of transitions involving (n×2)-type structures which, under suitable conditions of temperature and pressure, constitutes an example of the devil’s staircase phenomenon. Such a phenomenon has been suggested by previous LEED experiments.

Monte Carlo simulations of N-2 molecules physisorbed on a NaCl(001) surface show that at low temperature a monolayer forms an ordered p(2 x 1) structure which, upon heating, undergoes a continuous order-disorder phase transition around 25 K. This transition is characterized by a logarithmically divergent heat capacity and the formation of pairs of counter rotating vortices.

Monte Carlo simulations of CO physisorbed on a LiF(001) surface show that a monolayer of CO molecules forms an ordered p(2√×√)R45 herringbone structure which undergoes an order–disorder phase transition around 30 K. The CO molecules sit near the Li+ sites (C atom down) with a tilt of ∼40° from the surface normal.

We report a Monte Carlo simulation of the p(2×1)→p(1×1) phase transition in a monolayer of CO molecules adsorbed on a NaCl(001) surface. In the p(2×1) phase the CO molecules are tilted by 24° from the surface normal and have preferred azimuthal orientations whereas in the p(1×1) phase the molecules are oriented perpendicular to the surface with no preferred azimuthal orientation. The transition temperature is estimated to lie in the range 30–35 K. Multiple domains and phase coexistence are also observed.

Results of a Monte Carlo simulation of monolayer and multilayers of CO2 molecules adsorbed on the (001) surface of NaCl at temperature T=55 K are presented. It is found that the monolayer adopts a (2×1) structure; bilayers adopt either a (2×1) or a c(2×2) structure with the latter configuration energetically favored; and trilayers adopt a c(2×2) structure owing to the instability of the (2×1) structure. These results are in accord with recent helium scattering work.

This paper reports a classical trajectory study of surface-aligned photochemistry in HBr(ad)/(LiF(001) using the detailed interaction potential developed in an earlier study (Polanyi et al. J. Chem. Phys. 1991, 94, 978. Exchange reaction, H + H'Br --> HBr + H', and the minor pathway of abstraction reaction, H + H'Br --> HH' + Br, were incorporated in the trajectories by means of a London-Eyring-Polanyi-Sato function. The energy of the reactant H was, in general, E(xs) = 2.6 eV. Marked changes in angular and energy distributions of the scattered H and H' were found with increasing coverage in the range 0.2-1.0 ML (monolayer). This was related to altered dynamics, especially to the increasing importance of collisions with coadsorbate at higher coverage resulting in energy loss and in scattering nearer to the surface normal (termed ''channeling''). It was predicted that at low coverages the products of exchange reaction induced by polarized light would show an angular anisotropy that mirrored the collision geometries: bent or collinear. Reaction probability was found to increase with coverage, saturating at approximately 0.5 ML. This saturation was found to be due to a marked 2D ''surface-aligned caging'' effect (SAC). Reducing the surface temperature increased the reaction probability at low coverage due to increased alignment. By contrast, reducing the surface temperature at higher coverage (greater-than-or-equal-to 0.5 ML) decreased the reaction probability due to more pronounced SAC. Exchange reaction exhibited a higher threshold energy in the adsorbed state as compared with gas, due to the less than optimal surface-aligned collision geometry in the present system.

SUMMARYThe impact of cimetidine, ranitidine and placebo on the pharmacokinetics of metoprolol, given either as a single dose (100 mg) or for 7 days (100 mg b.d.), has been evaluated in two separate studies. The doses used were 800 mg cimetidine daily and 300 mg ranitidine daily. The subjects were all young, healthy volunteers. In the single dose study, cimetidine produced a marked increase in the peak plasma concentration of metoprolol and in the area under the plasma concentration‐time curve; ranitidine had less effect, though the area under the curve was significantly greater than placebo. In the chronic dosing study, the area under the curve for metoprolol was also significantly higher on cimetidine (1796 ng h/ml;P< 0.001) whereas the area under the curve on ranitidine (1258 ng h/ml) was comparable to that on placebo (1183 ng h/ml). Despite these drug‐induced changes in plasma metoprolol concentration, neither cimetidine nor ranitidine altered the change in exercise‐induced heart rate during dosing with metoprolol.

Using reversed-phase thin-layer chromatography, with octan-1-ol as stationary phase and phosphate buffer (pH 7.4) as mobile phase, the behaviour of different drugs at 37 degrees C was studied. Three classes of drug were examined: beta-adrenoceptor antagonists, non-steroidal anti-inflammatory agents and dihydropyridine calcium antagonists. As well as ranking these compounds in terms of their distribution coefficients, an attempt was also made to assign a quantitative value to each. For the beta-adrenoceptor antagonists this was done by using a series of published values obtained using the shake-flask technique: for the non-steroidal anti-inflammatory agents a series of standard compounds was used. No good calibration data were available for the dihydropyridine calcium antagonists, but approximate values were assigned. The results obtained were compared with other published data and the applicability of the method discussed.