<Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <P> In general, chiral molecules have point chirality at a single stereogenic atom, which has four different substituents . The two enantiomers of such compounds are said to have different absolute configurations at this center . This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism). The stereogenic atom is usually carbon, as in many biological molecules . However chirality can exist in any atom, including metals (as in many chiral coordination compounds), phosphorus, or sulfur . Chiral nitrogen is equally possible, although the effects of nitrogen inversion can make many of these compounds impossible to isolate . </P> <P> While the presence of a stereogenic atom describes the great majority of cases, many variations and exceptions exist . For instance it is not necessary for the chiral substance to have a stereogenic atom . Examples include 1 - bromo - 3 - chloro - 5 - fluoro adamantane, methylethylphenyl tetrahedrane, certain calixarenes and fullerenes, which have inherent chirality . The C - symmetric species 1, 1' - bi-2 - naphthol (BINOL), 1, 3 - dichloro - allene have axial chirality . (E) - cyclooctene and many ferrocenes have planar chirality . </P> <P> When the optical rotation for an enantiomer is too low for practical measurement, the species is said to exhibit cryptochirality . </P>

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