Zigzag chirality

For the inherently chiral (R,R)- and (S, S)-TA, a zigzag distortion has been confirmed experimentally via XPD [25]. DFT calculations, however, predicted also for achiral (R,S)-TA and SU on Cu(110) a chiral zigzag conformation after deprotonation of both carboxyl groups (Fig. 11) [26]. Indeed, observations of long-range chiral patterns suggest this geometry [27-29]. 

Fig.11 Top-view ofball and stick models of (R,S) tartaricacid (a) and succinic acid (b) in chiral zigzag arrangements on Cu(110). The interaction with the surface atoms causes deprotonation of the carboxyl groups in first place and forces both species into chiral conformations. For (R, S)-TA, a stabilizing intramolecular hydrogen bond is indicated as a...

As shown in Scheme 12, jc-stacked polymers (/ p)-41 and (5p)-41, in which the 7C-electron systems partially overlapped to form chiral zigzag structures, were synthesized from (/ p)-40 and (5p)-40 [97], respectively, by Sonogashira-Hagihara polymerization. The appropriate combination of a palladium complex and ligand was important for successful polymerization. In this case, the Pd2(dba)3/P(f-Bu)3 (dba=dibenzylideneacetone) catalytic system gave the best result, and the polymers were obtained in better yields and with higher Mp than those obtained through... 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Fig. 1. The 2D graphene sheet is shown along with the vector which specifies the chiral nanotube. The chiral vector OA or Cf, = nOf + tnoi defined on the honeycomb lattice by unit vectors a, and 02 and the chiral angle 6 is defined with respect to the zigzag axis. Along the zigzag axis 6 = 0°. Also shown are the lattice vector OB = T of the ID tubule unit cell, and the rotation angle 4/ and the translation r which constitute the basic symmetry operation R = (i/ r). The diagram is constructed for n,m) = (4,2).

Fig. 3. The 2D graphene sheet is shown along with the vector which specifies the chiral nanotube. The pairs of integers ( , ) in the figure specify chiral vectors Cy, (see Table I) for carbon nanotubes, including zigzag, armchair, and chiral tubules. Below each pair of integers (n,m) is listed the number of distinct caps that can be joined continuously to the cylindrical carbon tubule denoted by (n,wi)[6]. The circled dots denote metallic tubules and the small dots are for semiconducting tubules.

Fig. 2. By rolling up a graphene sheet (a single layer of ear-bon atoms from a 3D graphite erystal) as a cylinder and capping each end of the eyiinder with half of a fullerene molecule, a fullerene-derived tubule, one layer in thickness, is formed. Shown here is a schematic theoretical model for a single-wall carbon tubule with the tubule axis OB (see Fig. 1) normal to (a) the 6 = 30° direction (an armchair tubule), (b) the 6 = 0° direction (a zigzag tubule), and (c) a general direction B with 0 < 6 < 30° (a chiral tubule). The actual tubules shown in the figure correspond to (n,m) values of (a) (5,5), (b) (9,0), and (c) (10,5).

Fig. 8. Diffraction space according to the "disordered stacking model" (a) achiral (zigzag) tube (b) chiral tube. The parallel circles represent the inner rims of diffuse coronae, generated by streaked reflexions. The oo.l nodes generate sharp circles. In (a) two symmetry related 10.0 type nodes generate one circle. In the chiral case (b) each node generates a separate corona [9].

When we compare the calculated Raman intensities for armchair, zigzag and chiral CNTs of similar diameters, we do not see large differences in the lower frequency Raman modes. This is because the lower frequency modes have a long... 

Isotactic polymer (Section 7.15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain. 

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

The structure of carbon nanotubes depends upon the orientation of the hexagons in the cylinder with respect to the tubule axis. The limiting orientations are zigzag and arm chair forms, Fig. 8B. In between there are a number of chiral forms in which the carbon hexagons are oriented along a screw axis, Fig. 8B. The formal topology of these nanotube structures has been described [89]. Carbon nanotubes have attracted a lot of interest because they are essentially onedimensional periodic structures with electronic properties (metallic or semiconducting) that depend upon their diameter and chirality [90,91]. (Note. After this section was written a book devoted to carbon nanotubes has been published [92], see also [58].)... 

Fig. 16. (a) The chiral vector OA or Ch = nhi + md2 is defined on the honeycomb lattice of carbon atoms by unit vectors ai and a of a graphene layer and the chiral angle with respect to the zigzag axis (9 = 0°). Also shown are the lattice vector... 

If m = 0, the nanotubes are called zigzag nanotubes, if n = m, the nanotubes are defined as armchair nanotubes, and all other orientations are called chiral . The deviation of Cn from a is expressed by the inclination angle 0 and ranges from 0° ( armchair ) to 30° ( zigzag ) [17]. 

Fig. 1.1 (a) Schematic of unrolled SWCNT showing chiral vector Cn and the effect of m and n on the electronic properties of SWCNTs. (b, c, d) The direction of the chiral vector affects the appearance of the nanotube showing (b) (4,4) armchair, (c) (6,0) zigzag and (d) (5,3) exemplary chiral shape. With kind permission from [18],... 

Another complication in CNT applicability arises from the way the graphene sheet is rolled up to create the cylindrical structures, which is usually called helicity . Depending on the angle of the wrapping, three different structures (different helicities) can result (1) armchair, (2) chiral or (3) zigzag (Fig. 3.3). Such structures exhibit differ-... 

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