Single crystal experiments

An alternative, possibly parallel, mechanism for cis-trans isomerization122 has been resurrected by single crystal experiments.123 During studies of double... 

The voltammograms shown here (Fig. 15 - 17) agree well with published data. It is proved that the obtained surfaces were well-defined as the intended single crystal surfaces and the system was clean enough to perform the single crystal experiments. 

Scheme 3 The solid yielded by mechanical mixing of the reactants can be used to seed crystal growth from solution to obtain crystals for single-crystal X-ray diffraction experiments. This procedure allows one to compare the X-ray powder diffraction pattern measured on the mechanochemical sample with that calculated on the basis of the single-crystal experiment for the solids obtained by crystallization via seeding of a solution of the ground powder of the adduct...

By flipping the sample between the evolution and detection periods (so that the axial symmetry is broken), the correlation spectrum of two sample orientations can be established, as shown in the early single-crystal experiment by Carter et al 254,255 "phis technique was employed by Henrichs256 to propose a unique type of correlation spectrum in solid-state NMR by which the orientation correlation is established and the orientation distribution of ordered samples can be determined. 

Although most solid NMR experiments use powder samples, single-crystal experiments remain useful and new techniques continue to appear.358,359... 

A particularly difficult problem appeared to be the systems of two active metals [27,28]. While, in several cases [27], the product patterns of the catalytic reaction show the presence of both active metals (Pt-Re, Pt-Co, Pt-Ir, Pd-Ni) in the surface, the chemisorption data, such as e.g. IR spectra of adsorbed CO, are less definite on this point. Recently Joyner and Shipiro [28] even speculated that — at least with Pt alloys — it is only Pt which forms the surface. Important information on the last mentioned problem has been supplied by single-crystal experiments, in which one metal (B) is covered by one, two or more monolayers of the second metal (A). It appeared [29] that, to see the bulk properties of a metal A, with regard to XPS and/or CO chemisorption, at least two or three layers of A should be laid down on metal B. This means that an ensemble of three or four contiguous surface A atoms must also have the A atoms underneath (atoms in the next layer, filling the holes of the first layer), to behave like corresponding ensembles of A in bulk metal A. This could be one of the reasons why the size of the necessary ensemble formally derived from the overall kinetic and the topmost layer composition is sometimes unreasonably large. 

Ground-state triplet dicarbenes in photochemical systems have also been extensively studied by ESR. Dowd and coworkers (304) examined the interesting, frequently postulated ground triplet trimethylenemethane obtained by photolysis of either 4-methylene-A -pyrazoline or a single crystal of 3-methylene-cyclobutanone. This triplet molecule is axially symmetrical and the proton hyperfine splittings of 8.9 gauss observed in single-crystal experiments indicate that all protons in the molecule are equivalent when the axis perpendicular to the plane... 

For single-crystal experiments, data collection times (1 day to 2 weeks) are several orders of magnitude longer than X-ray data collection times. 

In cases where it is difficult to obtain large crystals, chemists may logically conclude that powder diffraction experiments offer a suitable alternative. However, powder diffraction experiments " are usually restricted to large samples of smaller unit cells than those commonly involved in a typical organometallic structure. In addition, because the nature of the diffraction data is one-dimensional (rather than three-dimensional in a single-crystal experiment), peak overlap is a serious problem. This difficulty is largely alleviated by the use of the Rietveld method, " in which the overall profile of the diffraction pattern is essentially scanned stepwise to yield hundreds of individual intensity measnrements. Fnr-thermore, deuteration is almost always necessary in a powder... 

Finally, in a single crystal experiment it is possible to extract all five independent elements of the traceless NQI tensor, usnaUy chosen to be the orientation of the EFG principal axis system, in addition to the two parameters accessible for randomly oriented samples (jj and vq). This, of course, requires that it is possible to make a single crystal incorporating the radioactive mother nucleus, and is therefore limited to cases where the preparation can be completed within roughly the lifetime of the mother nucleus, for example, see Ref 138. 

In a single crystal experiment weaker reflections may be measured more easily and the form factor for Ni2+ in NiO was one of the first ionic form factors to be accurately determined (27). 

Single crystal experiments are essential in the detailed investigation of spin density distributions. The anisotropy in the form factor at a given x can be measured and if a fairly extensive set of information is collected the spin density may be determined by Fourier transformation techniques. 

The conclusive proof came only recently and was based on MgO single crystal experiments. A single crystal surface is expected to have a small number (almost negligible) of defects. On a UHV cleaved MgO single crystal [85,86], the CO thermal desorption spectra (TDS) showed a peak at T 57 K and a broad feature at 76 K. The adsorption energy, calculated via the Redhead equation... 

The above practice is rather straightforward for a single-crystal experiment, but often provides doubtful results when only powder diffraction data are available. The basic reason is that the powder diffraction pattern is onedimensional, owing to the collapse of the reciprocal lattice of the individual crystallites onto the 26 axis. Consequently, reflections with the same r/ / modulus (i.e. with the same interplanar spacing d ki- indeed dhki= l/ rM /l) will overlap on the 29 axis. For convenience, we quote in Table 7.2 the algebraic expressions of d ki for the various crystal systems. 

The actual pattern resulting from powdered samples is similar to the single crystal experiment, in which the data are generated from the particular arrangement of atoms within the unit cell. 

Fig. 15. (a) Schematic of the orientation relations in the Mb single crystal experiments. (b) Projection of the heme on the a b plane. Heme nitrogens are denoted by nos. 1 to 4. q> is the angle of rotation around the crystallographic b axis... 

Fig. 16. Schematic of the occupation probabilities in phase II of TCTMB and DCTMB. From left to right molecules under study, three well potential, averaged quadrupolar interaction and asymmetry parameters measured from powders spectra, occupations probabilities calculated from single-crystal experiments.

Fig. 17. Schematic of the nonadecane/urea-inclusion compound. From left to right single-crystal experiment by rotation around the channel axis. Critical inhomogeneous linebroadening on approaching the transition temperature from above, with a lineshape invariant by rotation around c hole burning in the last spectrum proves the inhomogeneous nature of the broadening. Schematic of rotation patterns that gives the chain orientation in the low-temperature phase.

In essence, a powder diffractogram contains as much information as a single crystal experiment. When the intensity and the positions of the diffraction pattern are taken into account, the pattern is unique for a single substance. The X-ray diffraction pattern of a substance can be likened to a fingerprint. In effect, the pattern of lines on the powder diffraction pattern of a single phase is virtually unique, and mixtures of different crystals can be analysed if a reference set of patterns is consulted. 

Assignment of the observed hydroxide bands is trivial, as far as only one type of OH ion is present in the structure, but it is highly complicated in the case of three or more crystallographically different ions. There are several convenient procedures (see below). Assignment to the various unit-cell group modes is usually established by Raman and infrared single-crystal experiments. However, such investigations are experimentally difficult and, hence, they are rarely carried out. Respective Raman studies are, for example, reported in Refs. [54,60,65-69], IR reflection and transmission measurements in Refs. [54,55,68,70]. 

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