Studies in crystal engineering: topochemical photodimerization and structure of p-chlorobenzylidene-DL-piperitone

3-(p-Chlorophenylvinyl)-6-isopropyl-2-cyclohexen-1-one, C17H19CIO, Mr=274.7, triclinic, P1, a=6.636 (1), b=10.537 (2), c=10.811 (3) Å, α=95.31 (2), β=92.60 (2), γ=100.66 (2)°, V=738.2 (2) Å3, Z=2, Dm= 1.23, Dx=1.236 Mg m-3, Cu Kα, λ=1.5418 Å, μ=2.09 mm-l, F(000)=292, T=298 K, R=0-050 for 2220 observed reflections. The title compound undergoes a topochemical photodimerization giving an anti head-to-tail dimmer instead of the syn head-to-head dimer expected on the basis of the ability of the chloro group to steer β-type packing. There are no significant C1…C1 intermolecular interactions. Possible reasons for the observed α -type packing of benzylidenepiperitones are advanced. The low dimer yield is also rationalized