Structure-reactivity-spectra correlations for substituted 3,4-diphenylcyclobut-3-ene-1,2-diones

The infrared absorption bands of the C=O stretching vibration and the C-13 NMR chemical shifts were measured for a series of nine substituted 3,4-diph enyl-cyclobut-3-ene-1,2-diones in CHCl3, CDCl3, and partially in CCl4. The AM1 charge densities, bond orders, and valence angles were calculated for t he optimized geometry. Good to satisfactory mutual correlations were found between the arithmetic means of the wave numbers of symmetric and asymmetri c C=O stretching vibrations, Hammett a constants, rate coefficients, and en thalpies of activation of the base-catalyzed ring fission as well as the AM 1 oxygen charge densities and bond orders at the carbonyl groups. The resul ts show that the factors controlling the reactivity of these compounds and the structural and IR spectroscopic properties of the C=O groups must be th e same. The correlation between the wave numbers of symmetric and asymmetri c C=O stretching vibrations affords a slope of a=1.069 which points to the existence of a strong cross-conjugation effect in the cyclobutenedione ring . The correlation of the wave number separation of the absorption bands of symmetric and asymmetric C=O stretching vibrations (Delta nu(C=O)) for vari ous cyclic diketones enabled the prediction of the angle gamma=83 degrees b etween the bond vectors of the two carbonyl groups in the cyclobutenedione rings, which is in satisfactory agreement with the value calculated by the semiempirical AM1 method.