DIPOLE MOMENTS OF PYRIDYL KETONES

In a previous work the electronic absorption spectra of phenyl-pyridyl and dipyridy1 ketones were studied [l J . Results of calculations performed by the MIM method gave a satisfactory interpretation of the spectra, but were not able to give valuable information concerning the preferred conformation of the molecules. A contribution presented to this problem is the dipole moment measurements and calculations. Experimental dipole moments were obtained by Guggenheim’s method [Z] . Calculated dipole moments for all possible isomers were obtained by the vector composition of the group moments and mesomeric-n-moments through the fragments of the molecule. Mesomeric moments were calculated from the ground state wave functions computed by the MIM method utilizing the procedure and parametrization previously described [l] . As group moments we took the pyridine moment (2.21 D) and the formaldehyde moment (2.30 D). Since the difference in rr-electron energy between the various isomers with respect to the most stable one results negligible, we assume that the isomeric distribution in pyridyl ketones is determined mainly by the dipolar interaction energy, which was evaluated by the equation