Chain structures zigzag chains

Pleated p sheet (Section 27.19) Type of protein secondary structure characterized by hydrogen bonds between NH and C=0 groups of adjacent parallel peptide chains. The individual chains are in an extended zigzag conformation. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

The principal compounds in this category are the monochalacogenides, which are formed by all three metals. It is a notable indication of the stability of tetrahedral coordination for the elements of Group 12 that, of the 12 compounds of this type, only CdO, HgO and HgS adopt a structure other than wurtzite or zinc blende (both of which involve tetrahedral coordination of the cation — see below). CdO adopts the 6-coordinate rock-salt structure HgO features zigzag chains of almost linear O-Hg-0 units and HgS exists in both a zinc-blende form and in a rock-salt form. 

HgO exists in a red and a yellow variety. The former is obtained by pyrolysis of Hg(N03)2 or by heating the metal in O2 at about 350°C the latter by cold methods such as precipitation from aqueous solutions of Hg" by addition of alkali (Hg(OH)2 is not known). The difference in colour is entirely due to particle size, both forms having the same structure which consists of zigzag chains of virtually linear O-Hg-O units with Hg-O 205pm and angle Hg-O-Hg 107°. The shortest Hg - O distance between chains is 282 pm. 

M(0H)2SOi, H2O where M=Zr (8), Hf (12) also have been determined and reveal the presence of almost planar zigzag chains of metal atoms joined by double hydroxide bridges. The single exception to this trend toward formation of double hydroxy-bridged metal dimers or chains is the compound which is best described as CeOSOif,H20 (17). However, even in this structure the cerium ions form chains which are linked by bridging oxide ions. 

Boron atoms in infinite boron zigzag chains form covalent B—B bonds at 165-190 pm with bond angles of 115°. The boron coordination is trigonal prismatic, giving rise to coordinated rows of face-connected trigonal metal prisms (Table 1). Most typical are the binary transition-metal monoborides such as the FeB, CrB and MoB structure types. Transformations between pairs by simple geometric shifts ... 

With decreasing My B ratio, two or three B zigzag chains are connected at bonding distances, thus forming double or triple chains (Fig. 1). This tendency toward two-dimensional boron net formation (i.e., AlBj type) is recognized from the structural series within the V-B system VnB +, = (n - 1) VB -I- VBj (n = 1,2,3,5 not known for n = 4). ... 

Fig. 5.7 View of the inter-cluster connectivity in the [(Zr6B)Clii xl2+x] structure c and the zigzag chains run horizontally). The Zr-Zr bonds are emphasized, and the inner halides are omitted for clarity.

Fig. 6.7 (a) Perspective of Ti2Nb5Cli404 structure in the [001] direction, (b) Cluster layer in Ti2Nb6Cli404. (c) Zigzag chain of edge-sharing [TiCl402] octahedra in Ti2Nb6Cli404. 

From the viewpoint of synthetic polymer chemistry, although the formation of stereospecific polymers (isotactic and syndiotactic) is very popular, the present polymer is the first example having a double syndiotactic structure. In ad tion, the polymer consists of an alternating zigzag-linear main chain structure. 

The zigzag chains of edge-sharing octahedra that occur among compounds MX4 can also be joined by common vertices, resulting in the a-Pb02 type (Fig. 16.12). This structure type is less frequent. 

Distorted variants, similar to the distorted variants of the NiAs type, are known for the Cdl2 type. For example, Zrl2 has a distorted Cdl2 structure in which the Zr atoms form zigzag chains. Therefore, every Zr atom is involved in two Zr-Zr bonds which is in accordance with the d2 configuration of divalent zirconium. 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

The stability constants of zinc complexes of 4,6-dimethyl-2-thiopyrimidine have been determined by potentiometric measurements. The crystal structure shows infinite zigzag chains of ZnL2 units with each zinc coordinated by an N3S2 donor set in a trigonal-bipyramidal geometry.853... 

Fig. 12 Displacement of M atom positions in MnP-type structure relative to NiAs-type structure, with formation of zigzag chains of M-M bonds...

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