Off-atom sites

Off = atomically adsorbed oxygen o = vacant catalyst site... 

Quantitative determination of the local adsorption structure of carbonate on Ag(llO) has been done by Kittel etal. [110]. They have found that the carbonate species is essentially planar and adsorbs almost parallel to the surface at the off-atop site with respect to the outermost layer Ag atom. The C—Ag layer spacing was 0.264 0.009 nm, with a well-defined azimuthal orientation. This geometry is understood best in terms of the added-row model proposed by Guo and Madix. This model assumes that additional Ag atoms lie adjacent to the carbonate, such that the... 

A display of every atom in a protein is often forbidding and incomprehensible. Viewers are interested in some particular aspect of the structure, such as the active site or the path of the backbone chain, and may want to delete irrelevant parts of the model from the display. Display commands allow viewers to turn atoms on and off. Atoms not on display continue to be affected by rotation and translation, so they are in their proper places when redisplayed. Viewers might eliminate specific atoms by pointing to them and clicking a mouse, or they might eliminate whole blocks of sequence by entering residue numbers. They may display only alpha carbons to show the folding of the protein backbone (refer to Plate 5), or only the backbone and certain side chains to pinpoint specific types of interactions. 

Schematic illustration of ordering of Na(2) ions in Na-A zeolite (a) Fm3c symmetry. The four equivalent off-center sites associated with an 8-ring are shown as shaded circles. The 0(1) and 0(2) atoms of the ring are shown as open circles joined by the cross-hatched circles. The sites represented by open circles are now unoccupied. Examples of complete ordering are shown by the full circles in...

The PPD and shell models are nearly equivalent in this sense, because they model the electrostatic potential via static point charges and polarizable dipoles (of either zero or very small extent). Accuracy can be improved by extending the expansion to include polarizable quadrupoles or higher order terms.The added computational expense and difficulty in parameterizing these higher order methods has prevented them from being used widely. The accuracy of the ESP for dipole-based methods can also be improved by adding off-atom dipolar sites. 

The displacement of atoms just discussed is due to causes outside the affected crystal. But in the early days of radioactivity the destruction of radium salts by self-irradiation was recognised. Indeed the process occurs more or less in all minerals and compounds containing radioactive atoms. Recently D Eye (1957) calculated that 2 x 10 atoms in 1 mg of a polonium compound are knocked off their sites, mainly by recoil nuclei, in a day. As there are only about 10 atoms in this weight of the compound, every atom suffered, on the average, one displacement a day. By X-ray powder photography, with the special precautions necessitated by the polonium activity... 

This equation tells us how to determine the density of the electron gas at the site of interest which we denote pi in terms of the geometric disposition of the neighboring atoms. In particular, f(R) is a pairwise function that characterizes the decay of electronic density from a given site. Hence, if we wish to find the contribution of atom j to the density at site i, the function f(R) evaluated at the distance R = Rj — R tells us exactly how much electronic density bleeds off from site j onto its neighbors. 

An atomic site in a rare-gas matrix is, of course, of microscopic size. However, atoms in a beam can be excited by laser spectroscopy to extremely high n values if such a beam is pointed at a metallic grating made up of micrometre-size slits, then a cut-off will be observed in the maximum n value of the atoms which can fly through the apertures unimpeded. Such an experiment has been reported by Fabre [37] and coworkers in Paris. While it is experimentally very challenging to realise, the principle of this... 

For the samples after high-fluence irradiation (state O2), the nature of the defects is different in He- and Ar-irradiated samples because and resistivities showed different behavior in spite of the close agreement of the Ao variations. However, the main origin of the Ao decrease is the same in both samples and is thought to be due to vacancies that did not recombine with Nb interstitials because these agglomerate into clusters or precipitates. This is suggested because of the continuous increase of the concentration of off-lattice-site Nb atoms in spite of an Ao decrease during the irradiation up to this irradiation state. 

W2 atomic adsorption on relaxed unreconstructed substrate 1 S at center, S at long-bridge 2 S at off-center sites off-center site S shift by 0.39A Ifom center 1 st-layer Rh atoms forming the long-bridge site are buckled outward by 0.20A and laterally shift by 0.27A to open up the site... 

The basis functions are localized at the atomic sites (one per site) and only the diagonal elements vary from site to site. The off-diagonal terms therefore define, by themselves, a periodic system. It remains to assign values to S . For present purposes let us suppose that it can take two values, for A atoms, Sg for B atoms, with probabilities x and (1 — x), respectively. The parameter x is therefore the fractional concentration of A atoms. It is assumed that there is no correlation between sites. 

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