A from C3 on the ( 2 1 x 3 axis (Figure 4). The crystallographic analyses of parabanic acid and alloxan show that the C.-O-C angles corresponding to the closest intermolecular distances are 157.4 and 154.7O, re~pectively;~.~' the corresponding projected positions of the oxygens on the molecular planes of XI1 and XI11 are 1.06 A from C2 and C3 for the former and 1.19 A from C3 for the latter. Thus these oxygens, which are negative ( Figure 4), are near the most positive regions of the surface potentials, and the resulting electrostatic interactions can accordingly account for the short internuclear distances.Finally, it should be noted that the calculated atomic charges listed in Table I bear little or no relationship to the Vs,c values.For example, the dinitro derivative IX, which has the largest V S ,~ in the table, has only an average acyl carbon charge. Even more striking are the ordering and range of magnitudes of the acyl carbon charges in parabanic acid and alloxan, which do not show at all the same trends as do the surface potentials. In alloxan, the carbon with the largest Vs,c is C3, which has the lowest charge in all of Table I. These observations are consistent with the absence of a reliable correlation between calculated atomic charges and chemical r e a c t i~i t y ? *~~*~~ SummaryWe have investigated the susceptibilities toward nucleophilic attack, e.g. hydrolysis, of a group of cyclic ureides. ,We examined the surface electrostatic potentials of these molecules, focusing upon the regions above the acyl carbons, to determine the effects of various chemical and structural modifications, including the presence of NO2 and/or NF2 substituents. The relative hydrolytic stabilities of these systems have been predicted. For the polycarbonyl molecules parabanic acid and alloxan, the magnitudes and locations of the maxima in their surface potentials are fully consistent with observed unusually short intermolecular distances in their crystalline forms.Acknowledgment. We thank Dr. Jorge M. Seminario and Mrs.Monica Concha for computational assistance. We greatly appreciate the support of this work by the Office of Naval Research through Contract No. N00014-85-K-0217. Molecular dynamics simulations were performed in order to study the influence of the zeolite structure on molecular migration in the zeolitic void space. The migration of methane in various zeolitic environments was examined by using the all-silica polymorphs of zeolite EU-I , mordenite, and silicalite. Furthermore, diffusion simulations of ethane and propane in silicalite were carried out and computed diffusion characteristics were compared with experimental data.
Anomalous dispersion of X-ray diffraction at wavelengths near the X-ray K-absorption edge of sulfur at wavelengths around 5 A has been applied to single crystals of trypsin obtained from an ammonium sulfate solution. The multiwavelength anomalous-dispersion method based on 775 unique reflections (+183 Bijvoet mates) measured at three wavelengths near the K-absorption edge of sulfur in trypsin (two methionines and disulfide bridges of six cystines) reproduces the known features of the trypsin structure of a resolution of 4 A. It appears that there is anisotropic anomalous scattering from the disulfide bridges of cystine. The multiwavelength anomalous solvent contrast shows up at wavelengths near the K-absorption edge of the sulfate ions, which is shifted by 10 eV to higher energies with respect to that of sulfur in trypsin. The influence of the complex contrast of trypsin in 2.5 M ammonium sulfate on the dispersion of a low-order reflection is analyzed. The measurement of anomalous dispersion of X-ray diffraction at long wavelengths beyond 5 A requires a special diffractometer, the features of which are presented. An outstanding one is a detector system consisting of four multiwire proportional counters. Its efficiency is compared with that of imaging plates. The influence of radiation damage with soft X-ray diffraction from single crystals of trypsin is presented and possible remedies are discussed.
Results from a molecular dynamics simulation of xenon in silicalite at 298 K and 4 atoms per unit cell (A&, = -26.9 kJ/mol, D = 1.86 X m2/s) are in good agreement with the experimental value of -24.5 kJ/mol and the diffusion coefficient derived from the NMR pulsed field-gradient method (4.00 X m2/s). The diffusivity is predicted to be negligible at temperatures around 77 K and then increases over the investigated range to D = 3.25 X m2/s at 450 K, yielding an activation energy of 5.5 kJ/mol. Increasing the concentration from 4 to 16 atoms per unit cell results in a decreased internal energy of adsorption (-28.1 kJ/mol) and a decreased diffusion coefficient (D = 0.37 X lo4 m2/s). The anisotropy of diffusion is also examined. IntroductionComputer simulations of zeolite/adsorbate systems are currently of increasing importance in the context of understanding the catalytic properties of zeolites. They are used in conjunction with experimental techniques but possess the significant advantage that a wide range of conditions may be simulated, conditions which may be difficult to obtain in an experimental setup. Early based upon molecular mechanics procedures, employed simple atom-atom potentials to calculate heats of adsorption and adsorption isotherms and to estimate diffusion coefficients. More recently, adsorption sites and potential energy maps were calculated for xenon in zeolite rho,3 pyridine in zeolite L,4 and benzene in silicalite and zeolite theta-1.5 Monte Carlo simulations have been employed as a useful means of probing the potential energy space of zeolite/adsorbate systems as a function of temperature and sorbate concentration. Yashonath et al. examined the temperature dependence of the behavior of methane in zeolite Y6 and Smit and den Ouden the adsorption of methane in mordenite with variable zeolite composition.' More recently the molecular dynamics (MD) technique has been applied to such systems with a view to obtaining time-dependent data such as diffusion coefficients for methane and benzene in zeolite NaY8s9 and water in ferrierite.I0 These studies have been based upon the approximation of a rigid zeolitic framework, but MD framework simulations of natrolite" and zeolite AIz have been reported recently by Demontis et al.The emergence of '29Xe N M R as a means of probing the internal structure of zeolitesI3 and of exploring xenon as an adsorbate14 prompted us to investigate the silicalite/xenon system using the MD technique. There is also the added value that the diameter of xenon is close to that of methane, a molecule of increasing importance in heterogeneous catalysis. We developed three separate computer programs which were first used to investigate the test system, xenon in silicalite, at 298 K and a concentration of 4 xenon atoms per unit cell, followed by the use of the separate programs for varying the system temperature from 77 to 450 K and the adsorbate concentration from 4 to 16 ad-
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