The Claude Bernard Lecture, 1989 - Deterministic chaos: the science and the fiction

The crystal structure of $\alpha$-benzylidene-$\gamma$-butyrolactone 2, can be determined with the aid of atom-atom pairwise energy evaluation procedures, because its (previously reported) solid state photoreactivity coupled with topochemical principles, greatly restricts the number of possible orientations of the molecule in the unit cell. Crystals of lactone 2 are monoclinic with space group P2$_1$/n and with Z = 4, a = 11.014(2), b = 5.959(1), c = 14.286(5), $\beta$ = 108.05(2). Refinement on 846 non-zero reflections led to an R (reliability) of 0.046. In contrast, the isoelectronic ketone 2-benzylidenecyclopentanone (3) is photostable, and crystallizes in the same space group with Z = 4, a = 7.466(4), b = 6.821(4), c = 19.005(1), $\beta$ = 94.14(1). The structure of 3 was solved by direct methods and refined on 1037 non-zero reflections to an R of 0.036. The difference between the two structures can be rationalized in terms of intramolecular conformation and weak C-H$\cdots$O hydrogen bonding. Differences in the solid state photoreactivities of the two compounds can be related to the extent of orbital overlap between `potentially reactive' double bonds on nearest neighbour molecules that are related by inversion. Compound 2 reacts in the solid state topochemically but not topotactically showing directional preference, while 3, which has reduced orbital overlap, is photostable.