Double crystal structures

One of the structural implications of the delocalization of the negative charge, the identity of the two C—O bond lengths, intermediate between those of single and double bonds, has been verified by many crystal structure determinations. 

The NMR spectrum of the syn isomer shows evidence of a diamagnetic ring current, based on both the relatively low-field position of the vinylic hydrogens and the upfield shift of the methylene hydrogens. The anti isomer shows much less pronounced shifts. The X-ray crystal structure of the syn isomer shows a moderate level of bond alternation, ranging ftom 1.36 to 1.45 A (Fig. 9.4A). In the anti isomer, bond alternation is more pronounced, vith the double bond in the center ring being essentially a localized double bond (Fig. 9.4B). 

AMJnsubstituted compounds are, in general, too unstable to be isolated. The X-ray crystal structure of 1-tosyl-l //-1,2-diazepine shows that the molecule adopts a boat conformation with localized double bonds.69... 

LaNis has the CaCu, structure, space group P6/mmm [26] the hexagonal metal lattice is shown in Fig. 4. The crystal structure of LaNi5D7 has been determined 127, 28] and is illustrated in Fig. 5. There are three types of interstitial D sites La2Ni4 tetrahedra, and NiA tetrahedra. The unit cell is doubled along the oaxis because of the formation of a superlattice which is a consequence of long-range correlations between occupied and unoccupied... 

The hexamethylbenzene complex could be similarly deprotonated [54, 57] but the red complex obtained is then stable and its x-ray crystal structure could be recorded [55], showing a dihedral angle of 32° between the cyclohexadienyl plane and the exocyclic double bond (Fig. 5). This complex can also be cleanly obtained by the reaction of dioxygen with the 19e wostructural complex Fe CpfCgMeg) as shown in the preceding section. 

Figure 1. The crystal structure of Zr2(0H)2(SO4)3(H2O)4> reprinted with permission from Ref. 5, copyright 1966, American Chemical Society. Zirconium atoms are shown as solid circles, oxygen atoms as open circles. The Pu compound is isomorphous, Zr being replaced by Pu. la shows the manner in which the bridging sulfates link Pu atoms to form layers, lb shows the manner in which layers are linked through the double hydroxide bridges.

In situ Fourier transform infrared and in situ infrared reflection spectroscopies have been used to study the electrical double layer structure and adsorption of various species at low-index single-crystal faces of Au, Pt, and other electrodes.206"210 It has been shown that if the ions in the solution have vibrational bands, it is possible to relate their excess density to the experimentally observed surface. 

K. The electrical double-layer structure at Ag single-crystal faces has been studied extensively.6,10,i5,22,24,32,61,63,75,ss, 149-151,177a, 188,250-... 

CO adsorption on electrochemically facetted (Clavilier), 135 Hamm etal, 134 surfaces (Hamm etal), 134 Platinum group metals in aqueous solutions, 132 and Frumkin s work on the potential of zero charge thereon, 129 Iwasita and Xia, 133 and non-aqueous solutions, 137 potentials of zero charge, 132, 137 preparation of platinum single crystals (Iwasita and Xia), 133 Platinum-DMSO interfaces, double layer structure, 141 Polarization time, 328 Polarons, 310... 

Revised Values of Double-Bond Covalent Radii.—This investigation has led to the value 1.34 A. for the carbon-carbon double-bond distance, 0.04 A. less than the value provided by the table of covalent radii.111 4 Five years ago, when this table was extended to multiple bonds, there were few reliable experimental data on which the selected values for double-bond and triple-bond radii could be based. The single-bond radii were obtained -from the study of a large number of interatomic distances found experimentally by crystal-structure and spectroscopic methods. The spectroscopic value of the triple-bond radius of nitrogen (in N2) was found to bear the ratio 0.79 to the single-bond radius, and this ratio was as-... 

The atomic structure of this subunit and its complexes with substrate analogs revealed the enzymatic activity of the rRNA backbone. Thus, the ribosome is in fact a ribozyme P Nissen, J Hansen, N Ban, PB Moore, TA Steitz. Science 289 920-930, 2000. Atomic structure of the ribosome s small 30S subunit, resolved at 5 A WM Clemons Jr, JL May, BT Wimberly, JP McCutcheon, MS Capel, V Ramakrishnan. Nature 400 833-840, 1999. The 8-A crystal structure of the 70S ribosome reveals a double-helical RNA bridge between the 50S and the 30S subunit GM Culver, JH Cate, GZ Yusupova, MM Yusupov, HF Noller. Science 285 2133-2136, 1999. 

The even-numbered degree of polymerization behaviour of 2 OEt can be interpreted from its crystal structure (Fig. 9), in which the molecule is related to its neighbouring molecules by two different inversion centres to make a plane-to-plane stack (Maekawa et al., 1991a). The ethylenic double... 

---