Canonical structures pyridine

In contrast to pyridine, the unshared pair in canonical structure A in pyrrole is needed for the aromatic sextet. This is why pyrrole is a much weaker base than pyridine. 

Comparison of the bond lengths with those of benzene, naphthalene and pyridine suggests that whilst there is certainly aromatic character some bond fixation occurs in the naphthyridines. It is probable that canonical structure (1) (of, for example, 1,5-naphthyridine) and to a lesser extent structures (7) and (8) are the major contributors to the ground state, but the charged structures (9) and (10) must also be included in order to account for the chemical properties of the naphthyridines. 

To obtain a unique SMILES notation, computer programs such as the Toolkit include the CANGEN algorithm [1] which performs CANonicalization, resulting in unique enumeration of atoms, and then GENerates the unique SMILES notation for the canonical structure. In the case of pyridine, this is notation (III). Any molecular structure entered in the Toolkit is converted automatically into its unique representation. 

Pyridine A-oxide is considered to be a resonance hybrid of various canonical structures (la-e). Atom localization energies calculated... 

Pyrazine may be represented as a resonance hybrid of the canonical structure illustrated (21a -<+ 21d). The molecule is planar, and Pauling (114) states that it is stabilized by about 40kcal/mol as in benzene and pyridine, but resonance energies derived by different methods show considerable variation. Some of these resonance energies together with values for benzene and related heterocycles are summarized in Table 1.2 (115-117). More recent measurements of heats of hydrogenation are given in Section IV. 1B. 

The application of the HOSE model permitted estimation of aromaticity in the cases of very small changes in aromatic character of the molecule in question. This can be done by assuming equal or almost equal weights of basic canonical structures as encountered in benzene or pyridine. 

In what are called conjugate stems the concentrations of electrons, normally shown as double bonds, are delocalized (i.e. more diffused). By conjugate system is meant a molecule (or portion thereof) in which every second bond would conventionally be written as a double bond (examples benzene, butadiene, pyridine). The new electronic equilibrium is illustrated by benzene of which six of the valency electrons (one from each carbon atom) unite in forming the 7r-layer which completely covers both faces of the molecule. This condition is maintained by resonance. However, resonance can exist without conjugation, as in amides, which have a structure intermediate between 8.1) and 8.2), but not otherwise representable. The double-headed arrow is the accepted sign for resonance. Note that in the canonical forms [such as 8.1) and 8.2)] which make up the resonance hybrid, no atom ever shifts its position only electrons and charges are mobile. 

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