An investigation of the photo-reactive and unreactive polymorphs of o-ethoxy cinnamic acid and of its photodimer

Detailed X-ray crystallographic investigation of the reactive alpha- and the unreactive gamma-polymorphs of o-ethoxy cinnamic acid has been carried out along with that of the photodimer, alpha-truxillic acid. The molecule is quite planar in the alpha-form, but in the g-form, the side groups deviate significantly from the plane of the benzene ring. The carboxylic groups form normal cyclic hydrogen bonds in the alpha-form and near-symmetric hydrogen bonds in the g-form. The infrared spectrum of the alpha-form shows the characteristic features of the cyclic dimer, but that of the gamma-form is entirely different, marked by the absence of the O–H stretching band in the 3000 cm–1 region. Charge density analysis throws some light on the structure and reactivity of the molecule in the two forms. The near-symmetric hydrogen bond in the g-form is ionic and appears to restrict conjugation by way of distorting the molecule. This unusual feature keeps the cinnamoyl double bonds away from each other, rendering it photochemically unreactive. In the a-form, however, the double bonds have a closer approach. The cyclobutyl ring of the photodimer consists of weak single bonds, with the new pair being slightly longer.