Phantom atom

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

When a stereogenic center is tricoordinate, as is the case for sulfoxides, sulfbnium salts, and phosphines, then a phantom atom of atomic number zero is taken to occupy... 

If X or Y is a pyramidal trigonal center, then a phantom atom, in the direction of a lone pair, is added and the above rules are applied (example 4). 

These phantom atoms fill out the valences of the atoms involved in the multiple bonds and are considered to have zero atomic number and zero mass. They are not considered in establishing priorities. 

Groups containing multiple bonds are assigned priority as if both atoms were doubled or tripled. Thus a vinyl group is equivalent to a 2-butyl group by so-called phantom atoms. The phantom atoms do not include the requisite number of hydrogen atoms to complete file valences. 

According to the most recent IUPAC nomenclature, the R/S nomenclature may also be used with trigonal pyramidal TPY-3 structures. For TPY-3 structures, the vacant coordination site may be considered to be occupied by a lone pair of electrons or a phantom atom and given lowest priority. In Figure 5.4, we illustrate the use of this nomenclature for ethylmethyphenyltelluronium ion.18... 

N, O), phantom atoms are assigned an atomic number of zero and therefore rank the lowest. 

Phantom atom Used in the CIP notation to terminate the valency of all atoms except hydrogen, when the atom in question is involved in a multiple bond. 

All atoms, except hydrogen, are formally given a valency of four. Where the actual valency is less than four, phantom atoms are added as necessary, with assigned atomic numbers of 0, which are represented by subscripted zeros. 

All multiple bonds are split into single bonds, and phantom atoms are used to terminate the bonds. 

Molecules containing a trigonal pyramidal centre (TPY-3) may exist as a pair of stereoisomers. The configuration of this centre can be described in a similar way to that of a tetrahedral centre. This is achieved through notional placement of a phantom atom of low priority in the coordination site that would create a tetrahedral centre from a trigonal pyramidal centre. The centre can then be identified as R or S by the methods described above. 

The use of some bonding theories leads to the placement of a lone pair on a trigonal pyramidal centre. If this is done, the absolute configuration of the centre is also described by the R/S convention, in this case by placing the phantom atom in the site that is occupied by the lone pair. Examples of this practice may be found in the description of absolute configurations for sulfoxides in which the alkyl substituents are different. 

Ligancv complementation All atoms other than hydrogen are complemented to quadriligancy by providing one or two duplicate representations of any ligands which are doubly or triply bonded, respectively, and then adding the necessary number of phantom atoms of atomic number zero [85]. For example, the representation of a carbonyl is expanded as follows ... 

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