Azepines are seven-membered nitrogen-containing unsaturated heterocycles having the general molecular formula C₆H₇N for the parent compound. The nitrogen atom is part of a seven-membered ring containing three carbon–carbon double bonds. Because the hydrogen attached to nitrogen can migrate to different carbon atoms, azepines can exist in several tautomeric forms. These forms differ in the position of the N–H bond and the arrangement of the carbon–carbon double bonds. The principal tautomeric forms of azepine may be represented as 1H-, 2H-, 3H- and 4H-azepines. Owing to the symmetry of the seven-membered ring, the corresponding 5H-, 6H- and 7H-forms are equivalent to 4H-, 3H- and 2H-forms, respectively. The structures can be represented schematically as follows: ```text 1H-Azepine NH / \ C=C C=C | | C-----------C | C=C ``` In 1H-azepine, the hydrogen is attached to nitrogen and the three double bonds are distributed around the carbon framework. ```text 2H-Azepine N / \ C–H C || || C C \ / C = C ``` In 2H-azepine, one of the carbon atoms adjacent to nitrogen bears the hydrogen, while nitrogen is involved in a different double-bond arrangement. Similarly, hydrogen migration can produce 3H- and 4H-azepines: ```text 3H-Azepine 4H-Azepine H on C-3 H on C-4 ↓ ↓ N–C–C=C–C=C–C N–C–C=C–C=C–C ``` These representations indicate the different positions of the mobile hydrogen and the corresponding displacement of the double bonds. The relative stability of the tautomeric forms is governed mainly by the degree of conjugation, the position of the double bonds, steric effects, and the electronic properties of the nitrogen atom. The 1H form generally receives considerable stabilization from conjugation because the three carbon–carbon double bonds form an extended unsaturated system. However, unlike a six-membered aromatic ring, the seven-membered azepine ring does not remain completely planar. The ring adopts a non-planar or boat-like conformation, which reduces unfavorable electronic interactions. The 2H, 3H and 4H forms have different distributions of π electrons and different degrees of conjugation. Tautomers having a more continuous conjugated system are generally more stable than those containing isolated double bonds.