Solvate and polymorphs of ortho-ethoxy-trans-cinnamic acid:: the crystal and molecular structures

The 173K structures of the alpha, beta and gamma phases of o-ethoxy-trans-cinnamic acid (OETCA), which crystallize in P-(1) over bar, R-(3) over bar and C-2/e respectively are reported here. The common building block in all the structures is hydrogen bonded OETCA R-2(2)(8) centrosymmetric dimer pairs. The alpha polymorph forms a layered structure and the gamma polymorph a herringbone structure. Both of these phases are made up of hydrogen bonded OETCA dimers that are further assembled through C-H . . .O interactions to form layers or ribbons of OETCA molecules. The main difference between these structures is the 3-D assembly of the layers. The beta phase is actually a benzene solvate or host-guest complex (1:6 benzene: OETCA). Benzene. which occupies the (3) over bar site at (0,0,1/2) in the trigonal unit cell, is essential for the stabilization of this phase. It provides a template for the spiral arrangement of the OETCA molecules required for the formation of the beta -photo-dimerization product. (C) 2001 Elsevier Science Ltd. All rights reserved.