Acta Crystallographica Section B

Structural Science

Volume 60, Part 3 (June 2004)

Acta Cryst. (2004). B60, 300-314 [doi:10.1107/S0108768104009048]

A new polymorph of ortho-ethoxy-trans-cinnamic acid: single-to-single-crystal phase transformation and mechanism

M. A. Fernandes, D. C. Levendis and F. R. L. Schoening

Abstract: 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 P $\bar 1$ . 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 hydrogen-bonded pairs (via the -COOH groups), which are further connected by CHO 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.

Online 17 May 2004


	Structure factor file (CIF format) (246.7 kbytes) Contains datablock almer60

	Structure factor file (CIF format) (247.2 kbytes) Contains datablock almer80

	Structure factor file (CIF format) (269.3 kbytes) Contains datablock d6h_s

	Structure factor file (CIF format) (274.7 kbytes) Contains datablock d6hlt_s

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