MOLECULAR COCRYSTALS OF CARBOXYLIC-ACIDS .17. SPECTRAL CHARACTERIZATION OF THE ADDUCTS OF TRIPHENYLPHOSPHINE OXIDE WITH SUBSTITUTED PHENOXYACETIC ACIDS AND THE CRYSTAL-STRUCTURE OF THE 1-1 ADDUCT WITH (4-CHLORO-2-METHYLPHENOXY)ACETIC ACID

A series of 1:1 molecular adducts of triphenylphosphine oxide (tppo) w ith substituted phenoxyacetic acids has been prepared and characterize d by using infrared, solid-state H-1 and P-31 n.m.r. spectroscopy, and X-ray powder diffraction methods. The crystal structure of one of the se, with the herbicidally active (4-chloro-2-methylphenoxy)acetic acid (mcpa), [(tppo)(mcpa)], has been determined by X-ray diffraction meth ods. This compound is isomorphous with the tppo adduct of (2,4-dichlor ophenoxy)acetic acid. The title compound has two independent and diffe rent molecular adducts in the asymmetric unit each with strong hydroge n-bonding interactions between the carboxylic acid proton and the phos phoryl oxygen of tppo [O ... O, 2.579, 2.647(9) angstrom].