Tris-, structural notation

The systematic lUPAC nomenclature of compounds tries to characterize compounds by a unique name. The names are quite often not as compact as the trivial names, which are short and simple to memorize. In fact, the lUPAC name can be quite long and cumbersome. This is one reason why trivial names are still heavily used today. The basic aim of the lUPAC nomenclature is to describe particular parts of the structure (fi agments) in a systematic manner, with special expressions from a vocabulary of terms. Therefore, the systematic nomenclature can be, and is, used in database systems such as the Chemical Abstracts Service (see Section 5.4) as index for chemical structures. However, this notation does not directly allow the extraction of additional information about the molecule, such as bond orders or molecular weight. 

Figure 12.15 Gangliosides. The prefix mono-, di, tri- or tetra- denotes the number of sialic acid W-acetyl neuraminic acid, NANA) residues present in the molecule. The many different gangliosides have complex structures and for convenience shorthand notations are used. The two commonest were introduced by Svennerholm, who named the parent compound GM1, and Wiegnandt, who named it Ggnti. The latter system gives enough information for the structures to be worked out from the shorthand form once the symbols have been learnt. GalNAc represents N-acetyl galactosamine in the above structure.

Dithiolenes are best considered to be a resonance hybrid of the limiting structures (1)—(3). In both bis- and tris-dithiolenes the electron delocalization is not limited to the ligand, but includes the metals to give rise to cyclic delocalization ( aromaticity ). To symbolize this electron delocalization in dithiolenes, they can be represented, in a manner similar to that used for benzene, by formulas containing a ring inside the framework given by the metal, sulfur and carbon atoms. We will use this notation, shown in (4), throughout this chapter. 

Most methods to determine molecular structures try to compensate for or correct the effect of the vibration-rotation terms. As a result, there are now multitudes of methods available with as many special notations as methods. Their isotope dependence varies in practice from zero to full (for vq structures). Their error limits are sometimes difficult to determine. The goals of some of the newer methods are internal consistency and maximum relative errors (difference to Vg structure) of 0.1% or less. 

The next example (see Figure 13) is the tris(ethylenedia= mine)cobalt(III) complex. The site symmetry term is octahedral, six coordinate or OC-6, and the configuration number is 11. The nitrogen atoms are all equivalent, and the chelation defines a right-handed or A helix to give the complete notation 0C-6-11-A. I have indicated the primes on the chelating ligands in this structure to illustrate how to determine the... 

The structure type code was introduced by lUPAC (International Union of Pure and Applied Chemistry) to try to simplify the notation of zeolite structures. In the past several researchers had produced the same basic zeolite by different laboratory methods. The zeolite thus obtained was named by that researcher and resulted in a system where one zeolite structure was known by up to 20 different trade names This led to a great deal of confusion and so the three-letter code was introduced. So for example zeolite A (produced using sodium hydroxide, alumina and silica) and zeolite ZK4 (produced using tetramethylammonium hydroxide, alumina and silica) have the same structure, now denoted LTA (H). 

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