Non-aromatic endocyclic angle at the ring fusion with the aromatic ring in benzocyclopentene and -hexene, revisited.

X-ray geometries of monocycloalkenobenzenes with a fused-ring size of r = 5, 6 were obtained by the use of the Cambridge Structural Database. In contrast to Allen's study [Acta Cryst. (1981), B37, 900-906], the data were not averaged over exact or imposed m(ad) symmetry (m(ad) is the mirror plane going through the midpoint of the aromatic ring fusion bond and the aromatic bond parallel to it). The distribution of the non-aromatic endocyclic angle epsilon at the ring fusion clearly shows two and three, respectively, distinct curves for r = 5 and r = 6. The respective ranges are 104.3-112.6 and 104.4-123.2 degrees. If one of the C atoms directly attached to the aromatic ring is sp(2) hybridized, the bond length e between the aromatic ring and this atom is shortened by some 0.044 (15) and 0.027 (12) A for r = 5 and r = 6, respectively. However, for r = 5, e is only shortened by 0.030 (17) A when both C atoms directly attached to the aromatic ring are sp(2) hybridized. For r = 5, the endocyclic angle epsilon is 1.2 (13) degrees smaller at the side involving the sp(2) hybridized C atom.