Disorder in crystal structure

C. Paul As you probably know, many spurious effects in crystal structure work are often incorporated into the temperature factors Be values). In particular, positional disorder of atoms is often hidden in large temperature factors. Does this type of situation cause you any trouble If you are able to detect positional disorder of some atoms or groups, could your approach be a general method for sorting out such disorder in crystal structure work ... 

MM has been used to predict metal complex structures in order to resolve disorder in crystal structures. " Disorder may arise from the presence of a mirror plane in the molecule, or from the presence of multiple conformers. In each of these cases, the structures were fully resolved after application of MM. 

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

Conformational disorder is a common phenomenon in crystal structure analyses of metal complexes of chelate ligands. For instance, en (ethane-1,2-diamine) rings can adopt two conformations, X or 5 (see Fig. 7.1 and 7.2), and frequently both, or a smeared average of the two, are observed. If there is more than one such ring disordered, then it is often not possible to determine what overall conformations are adopted. For instance, in the case of [Fe(tra s-diammac)]3+ (Fig. 7.6), there are two five-membered chelate rings, trans to one another, and in the crystal structures, both show evidence of conformational disorder1182,207,217]. Thus, it is... 

In crystal structure analyses where the hydrogen atoms are not directly observed, as in macromolecules and structures with disordered water molecules, the signature of the three-center hydrogen bond is a triangle of potential donor or acceptor nonhydrogen atoms at distances in the range observed for weak hydrogenbonding interactions, i.e., 2.8 to 3.5 A. 

Disordered O-H 0 intramolecular hydrogen bonds are not uncommon in crystal structures of molecules having cis-enol configurations, but without evidence for an order-disorder transition, they do not necessarily imply that proton transfer takes place in the crystalline state. 

The first structure of yMIPS was solved with only partial occupancy of NAD+ in the active site. Residues 352-409 were not built into the model and were thought to be disordered in the structure (Stein and Geiger, 2002). This encompasses al3, al4, al5, and (315. Subsequent structures were determined both of the apo enzyme and the fully occupied NAD+-bound enzyme. yMIPS bound to NAD+ has been crystallized and its structure determined from three crystal forms, a C2 form with two independent molecules in the asymmetric unit, a P2j2j2 crystal form with four molecules in the asymmetric unit, and a P2j form, also with four monomers per asymmetric unit (Jin and Geiger, 2003, Kniewel et al., 2002). 

Here we should note that the dispersion curve calculation has provided all the information required to obtain the response from a single crystal sample aligned along a specific direction in Q. Indeed, if such an experiment were realistically feasible it would be the preferred technique. This is because the dispersion curves would be measured directly and the detailed information about the force field could be extracted. However, this is often not practical, at least for the exotic phases of ice and powdered samples were used. For ice Ih, single crystals are widely available (but a large crystal of ice Ic has not been obtained), after many attempts [49,55], reliable dispersion curves have yet to be obtained. This is due to the proton disordering in the structure of ice Ih and hence all the optic modes are localised. 

Disorder (atomic and molecular) Lack of regularity. In crystal structures it implies that there is not exact register of the contents of one unit cell with those from all others. The atoms or molecules in the crystal structure pack randomly (nonperiodically) in alternative ways in different unit cells. Such disorder may cause some diffuse scattering around intense Bragg reflections. See also the definition in Chapter 2. 

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