Dipole moments and electronic absorption spectra of aryltricyanoethylenes

The dipole moments of several aryltricyanoethylenes have been measured and found to be higher than those calculated by vector addition. This deviation is probably due to partial charge separation, facilitated by the pronounced ability of the tricyanovinyl group to spread the negative charge. This view is supported by the linear relation between the dipole moment μ and Hammett's σ constant and by the fact that μ_meas-μ_calc is proportional to Brown's σ⁺ value and to the energy of the longest wavelength transition in the electronic spectra.