Z system

For olefins and related structures we use the same priority rules, but we divide the double bond in half and compare the two sides. For each carbon of an olefin, assign one ligand high priority and one low priority according to the rules above. If the two high priority ligands lie on the same side of the double bond, the system is Z (zusammen) if they are on opposite sides, the system is E (entgegen). If an H atom is on each carbon of the double bond, however, we can also use the traditional cis and trans descriptors. 

To name more complex carbohydrates or amino acids, one draws a similar Fischer projection where the CH2OH or R is on the bottom and the carbonyl group (aldehyde, ketone, or carboxylic acid) is on the top. The d descriptor is used when the OH or NH2 on the penultimate (second from the bottom) carbon points to the right, as in D-glyceraldehyde, and l is used when the OH or NH2 points to the left. See the following examples. 

---

The Rekker approach is still used with revised Z/ systems, e.g., in the software program Z/SYBYL [8]. Over recent decades various other substructure-based approaches have been developed that are mostly implemented and available as computer programs. 

The a, b, e, eoordinate system ean be formed by beginning with the original X, Y, Z system and sequentially ... 

Figure 9.38 Cavity ring-down absorption spectrum of the 0-0 band of the a Ag-X Z system of O2, where k is the reciprocal of the ring-down time. (Reproduced, with permission, from Newman, S. M., Lane, I. C., Orr-Ewing, A. J., Newnham, D. A. and Ballard, J., J. Chem. Phys., 110, 10749, 1999)...

A second unusual EPR spectrum was observed in the oxidized (as-isolated) protein (Fig. 3). This spectrum, which was assigned to an S = z system, was not reminiscent of any Fe-S cluster. Indeed, with g-values of 1.968, 1.953, and 1.903, it looked more like a molybdenum or tungsten spectrum. However, chemical analysis ruled out the possibility that this EPR spectrum arose from Mo or W, and the spectrum was assigned to an Fe-S center instead. The spin concentration, however, was sub stoichiometric and sample-dependent. Furthermore, when the as-isolated protein was oxidized with ferricyanide, it became EPR silent. This, together with the iron determination and the fingerprint of the reduced protein, led Hagen and colleagues to the... 

Two different chemical classes contribute to this sector. Initially it was entirely dominated by anthraquinone dyes typified by structure 7.102. The dye bases for attachment of haloheterocyclic (Z) systems are prepared by condensing bromamine acid (7.103) with various phenylenediamines. The outstandingly successful Cl Reactive Blue 19 (7.3 7) is the... 

It becomes difficult to decide whether, for example 1 is cis or trans isomer No. two groups are the same and the same is true for the compound II. The Chemical Abstracts Service has proposed an unambiguous system that has been adopted by IUPAC and is based on the preferences of groups. It is also called the E-Z system and applies to alkene diastereomers of all types. 

Phenylbenzoic acid is converted into benzo[c]coumarin in the Su z system, that is irradiation when mixed with trivalent iodine compounds and iodine the 2-iodo derivative may also be formed <99JCS(P1)1713>. 

To simplify this situation, the E/Z system is used for naming geometrical isomers. Z stands for German zusammen, which means the same side, and E for German entgegen, meaning on the opposite side. 

Now, let us have a look at the cyclic compounds. We can use this E) and (Z) system for a cyclic compound when two or more groups are attached to a ring. For example, if in the following substituted cyclopentane A and B are different groups, each C atom attached to A and B is a chiral carbon or stereocentre. 

Another form of behaviour exhibited by a number of chemical reactions, including the Belousov-Zhabotinskii system, is that of excitability. This concerns a mixture which is prepared under conditions outside the oscillatory range. The system sits at the stationary state, which is stable. Infinitesimal perturbations decay back to the stationary state, perhaps in- a damped oscillatory manner. The effect of finite, but possibly still quite small, perturbations can, however, be markedly different. The system ultimately returns to the same state, but only after a large excursion, resembling a single oscillatory pulse. Excitable B-Z systems are well known for this propensity for supporting spiral waves (see chapter 1). 

From a theoretical point of view, however, this arrangement is not so satisfactory. Even for the B-Z system, the bromide electrode record... 

Our convention is that a symmetry operation R changes the locations of points in space, while the coordinate axes remain fixed. In contrast in Section 1.2 we considered a change (proper or improper rotation) of coordinate axes, while, the points in space remained fixed Let x y z be a set of axes derived from the xyz axes by a proper or improper rotation. Consider a point fixed in space. We found that its coordinates in the x y z system are related to its coordinates in the xyz system by (1.120) or (2.29) ... 

If we attach a new reference frame (z) to the moving (stable planar) boundary, z = ,-V t. The transport equation (Fick s second law) reads in the z-system... 

This w-ay of naming double bonds is far superior to cis/trans nomenclature because you can use this E/Z system for any double bond, even if all four groups are different. Cis/trans nomenclature requires two groups to be the same. 

---