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Crystals of chromous acetate are monoclinic with space group C2/c. The unit cell, of dimensions a = 13.15, b =-8.55, c = 13.94 A, fl ~--117 ° 0', contains four Cr2(CttaCOO)4.2H20 molecules. Projections of the structure on two crystallographic planes are given and from these it is inferred that chromous and cupric acetate are isostructural.The most striking feature of the structure is the close approach of 2.64 /~ between the two chromium atoms in a molecule, which, as in the case of the cupric acetate structure, suggests direct interaction between such atoms.

The alpha-polymorph of ortho-ethoxy-trans-cinnamic acid (OETCA) undergoes a reversible single-crystal-to-single-crystal phase transformation at 333 K. The new high-temperature polymorph (alpha'-OETCA) is stable between 333 and 393 K with three molecules in the asymmetric unit (Z' = 3), space group P1;. Unlike the other polymorphs (and solvate) of OETCA recently reported, two of the molecules in alpha'-OETCA deviate significantly from planarity. This conformational change results in the corrugated sheet-type structure of alpha'-OETCA. The sheets are made up of ribbons, each composed of R(2)(2)(8) hydrogen-bonded pairs (via the -COOH groups), which are further connected by CH.O interactions. When exposed to UV radiation the alpha'-OETCA polymorph can be stabilized below 333 K with ca 8% of the monomer converted into the photodimer. The crystal structures of alpha'-OETCA are reported at two temperatures above the phase transition point (at 345 and 375 K) as well as the stabilized forms at 173 and 293 K. A mechanism for the phase transition involving a cooperative conformational transformation coupled with a shift of layers of OETCA molecules is proposed. The alpha'-OETCA polymorph is also an example of a cinnamic acid derivative where two different potentially photoreactive environments exist in one crystal in which each unit cell has two non-centrosymmetric predimer sites and one centrosymmetric predimer site.