Crystal and molecular structures of 2-(p-methylbenzyl)- and 2-(p-chlorobenzyl)-5-(p-bromobenzylidene)cyclopentanone. Influence of chloro and methyl substitution on solid-state reactivity

The crystal structures of the title compounds have been determined as part of an investigation into the factors which may influence the solid-state reactivity of benzylidene cyclopentanones. Thep-methyl derivative, which is photoactive in the crystalline state, in monoclinic:P2₁/c,a=18.381(2),b=11.209(2),c=8.285(2) Å,=94.46(12)°, andz=4. The chloro derivative, which is photostable, is also monoclinic:P2₁/c,a=17.522(4),b=7.906(2),c=11.888(2) Å,=91.19(14)°, andz=4. The structures were solved and developed via the heavy-atom method and refined by least-squares to finalR values of 0.062 and 0.061, respectively, using 1902 and 2356 observed reflections measured using an automatic diffractometer. The conformations of the two molecules in their crystal structures are significantly different and lead to dissimilar packing arrangements, which clearly account for the different photochemical behaviors.