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 th e unreactive gamma -polymorplis 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 gamma -form, the side groups deviate significantly from the plane of the benzene ring. Tile carboxylic groups form normal cyclic hydrogen bonds in the alpha -form and near-symmet ric hydrogen bonds in the gamma -form. Tile infrared spectrum of the alpha -form shows the characteristic features of the cyclic dimer, but that of th e gamma -form is entirely different, marked by the absence of the O-H stret ching band in the 3000 cm(-1) region. Charge density analysis throws some l ight on the structure and reactivity of the molecule in the two forms. The near-symmetric hydrogen bond in the gamma -form is ionic and appears to res trict conjugation by way of distorting the molecule. This unusual feature k eeps tile cinnamoyl double bonds away from each other, rendering it photoch emically unreactive. In the alpha -form, however, the double bonds have a c loser approach. The cyclobutyl ring of tile photodimer consists of weak sin gle bonds, with the new pair being slightly longer.