4-Hydroxy-2-vinyl-2,3,4,5-tetrahydro-1-benzazepine and its 7-fluoro and 7-chloro analogues are isomorphous but not strictly isostructural.

4-Hydroxy-2-vinyl-2,3,4,5-tetrahydro-1-benzazepine, C(12)H(15)NO, (I), and its 7-fluoro and 7-chloro analogues, namely 7-fluoro-4-hydroxy-2-vinyl-2,3,4,5-tetrahydro-1-benzazepine, C(12)H(14)FNO, (II), and 7-chloro-4-hydroxy-2-vinyl-2,3,4,5-tetrahydro-1-benzazepine, C(12)H(14)ClNO, (III), are isomorphous, but with variations in the unit-cell dimensions which preclude in compound (III) one of the weaker intermolecular interactions found in compounds (I) and (II). Thus the compounds are not strictly isostructural in terms of the structurally significant intermolecular interactions, although the corresponding atomic coordinates are very similar. The azepine rings adopt chair conformations. The molecules are linked by a combination of N-H...O and O-H...N hydrogen bonds into chains of edge-fused R(3)(3)(10) rings, which in compounds (I) and (II) are further linked into sheets by a single C-H...pi(arene) hydrogen bond. The significance of this study lies in its observation of isomorphism in compounds (I)-(III), and its observation of a sufficient variation in one of the cell dimensions effectively to alter the range of significant hydrogen bonds present in the crystal structures.