Interatomic contacts

The structure of iodine at four different pressures. The outlined face-centered unit cell in the 30-Gpa figure corresponds to that of a (distorted) cubic closest-packing of spheres. At 24.6 GPa four unit cells of the face-centered approximant structure are shown the structure is incommensurately modulated, the atomic positions follow a sine wave with a wave length of 3.89 x c. The amplitude of the wave is exaggerated by a factor of two. Lower left Dependence of the twelve interatomic contact distances on pressure... 

Restricted group rotations caused by either short interatomic contact or strong interactions... 

In any case, it is worthwhile to point out that the observation of such unusually prolonged 13C T, values or displacement of the 13C chemical shifts can be utilized as a characteristic means to be able to locate a possible site for the presence of restricted or ceased group rotations for either methyl or hydroxymethyl groups caused by short interatomic contact or strong interactions. 

A reduced strain parameter is then defined with reference to an arbitrarily selected set of contacts. With reference to the dx distances the strain parameter may be defined as . S = (Dx — dx)/DY. This parameter gives an indication of the atomic dimension compression. It is computed, as a function of the ratio e = DX/DY = Rx/Ry, for the different kinds of interatomic contact. 

Figure 4.24. Near-neighbour diagrams for binary phases calculated according to Pearson (1972) for a few structural types. The lines calculated for the different interatomic contacts are shown. The numbers of contacts X-Y, Y-X, X-X, etc. are indicated. The experimental values determined for the various compounds are contained, for each structure type, within the hatched fields, (a) XY3 compounds belonging to the cP4-AuCu3 structural type (b) XY2 Taves phases of the cF24-Cu2Mg type (c) XY compounds of the cF8-ZnS structural type. Notice the importance of the high coordination contacts in the more metallic phases, whereas in the ZnS-type compounds the role of the chemical bond factor is clearly relevant.

This type of interatomic contact is sometimes found in halogen or chalcogen gold derivatives, and the vast majority of the examples described in the literature are of the type X- X, that is, they contain the same element in both extremes of the interaction. Of these, the most frequent are I- -I or S- -S interactions, although there are other non-metal- non-metal contacts, such as F- F, Br- -Br, Se- -Se or - - . 

Lower left Dependence of the twelve interatomic contact distances on pressure... 

The presence of the four minima in the conformational map reported by Mark and Goodman for poly(W-methyl-L-alanine) (Biopolymers 1967, 5, 809) Is confirmed. The conformational map reported by Tanaka and Nakajlma for polysarcosine (Polymer J. 1970, 1, 71 see also Polymer J. 1971, 2, 717), however, Is found to be incorrect due to their faillure to consider several crucial interatomic contacts. The conformational maps for both polysarcosine and polyW-methyk-alanlnel are found to be sensitive to the orientation selected for the methyl groups. 

In an environment of atoms in collision, interatomic contacts consist of interacting negative charge clouds. This environment for an atom is approximated by a uniform electrostatic held, which has a well-defined effect on the phases of wave functions for the electrons of the atom. It amounts to a complex phase (or gauge) transformation of the wave function ... 

Using a computer and matrix multiplication techniques the first and second peptide units can be rotated through all possible combinations of values of phi and psi. For each possible set of these dihedral angles, the distances between all pairs of nonbonded atoms can be compared using a set of minimum interatomic contact distances (Table 2.3) if the distance between any set of atoms is smaller than the minimum contact distance then that conformation is not allowed. This boils down to the fact that two atoms cannot be closer than the sum of the van der Waals radii of each atom or electron repulsion occurs. Therefore, by determining the values of phi and... 

The average of these converged structures is taken as the protein structure, whose precision can be assessed by the deviations of the individual structures from the average. The quality of the final structure can be described in terms of this root mean square deviation, for both the peptide backbone and side chains, and to some extent by the extent to which it conforms to limitations of dihedral bond angles and interatomic contacts anticipated from thousands of previously known structures (the Ramachandran plot ). By all criteria, NMR structures of proteins that are determined in this way are comparable to structures determined by x-ray crystallography. In addition, NMR methods can be applied to evaluate the... 

The connection principle leads to the occurrence of atom groups with the shortest interatomic contacts referred to as connections according to Laves Owing to the ability of chemical bonds to be coordinated in certain directions, these connections may be finite formations which are called groups and islands, respectively (see Sect. 3). Onedimensional, two-dimensional and three-dimensional infinite connections are called chains, layers and skeletons, respectively. 

Equation (2.2.12) may be directly obtained from minimizing the elastic free energy under the constraint that the mean-square radius of gyration has a fixed value [see Eqs. (2.1.63) and (2.1.39), C q) -+ ot q)C q) [10]. The physical meaning of this result is that under chain compression the free energy due to the interatomic contacts is basically a function of only, no matter what are the individual values of the a (q). As a consequence, all the mean-square distances (r (k)) may be expressed under a general form [53]. Defining... 

At room temperature, solid [Pt(bph)(CO)2] 26 (H2bph=biphenyl) displays an intense and broad emission at 726 nm [33]. As described in earlier report [34], [Pt(bph)(CO)2] packs in a columnar structure with Pt-Pt distances of 3.24 A. Such close interatomic contacts in the chain packing of square planar platinum(II) complexes always impart red-shifts in their emission spectra. The emission maximum is red-shifted to 791 nm upon decreasing the temperature from 296 to 77 K, which is consistent with a decrease in the Pt-Pt separation observed by Connick [35] and Yersin [3a]. 

In solid state chemistry where there are often a number of equal short interatomic contacts, the simplest definition of coordination number is the number of nearest neighbours. In coordination chemistry (solid, solntion and gas phases) a more practical definition of coordination number is the number of atoms or ligating groups (such as a C=C double bond) bound to a metal. Thus in [CoCl4] the coordination nnmber is four. The majority of coordination complexes are either 4- or 6-coordinate. For... 

Fig. 5.03. (a) Clinographic projection of the unit cell of cubic close packing. The atoms occupy the comers and face centres of the cubic cell, (b) Clinographic projection of 27 unit cells of cubic close packing, showing interatomic contacts. Some atoms have been omitted to reveal the close-packed layers perpendicular to one of the cube diagonals. 

In Appendix A published tables of standard interatomic distances [4] are reproduced in slightly abbreviated form. We have limited ourselves to a selection of the more reliable standards, those based on at least four observed distances for the type of interatomic contact in question for the others the reader is referred to the original tables [4]. Appendix B gives database identification codes (refcodes) to crys-... 

Crystallographic literature is full of information on the so-called close interatomic contacts, which in the case of distances between H-atoms and electron donating atoms are used as a criterion of H-bond existence. If this distance is shorter than the sum of van der Waals radii - then the H-bond type interaction is assumed... 

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