A Remark on the Dipole Moment of Cyclohexane-1,4-dione in Relation to Its Flexible Molecular Conformation

A critical remark on the interpretation of the polarity of cyclohexane-1,4-dione molecule has been made taking into consideration a possible contribution of atomic polarization estimated from the electric permittivity of the compound in the solid state. The value 9.6 cc has been assigned as the minimum for atomic polarization which gives dipole moments, 1.16D (28.6°C), 1.20D (42.9°C), and 1.22D (60.8°C) obtained in benzene solutions. A theoretical calculation of the dipole moment has been made also with an assumed potential barrier of the form, V(θ)=V0. [(π–θ)−2β]⁄2α, in relation to a flexing intramolecular movement of the molecule, where θ is the angle between two carbonyl groups substituted at the opposite sites of the molecule, V0 is the potential for a boat form A (with two carbonyl groups at the bow and the stern), and α, β are constants to be determined by applying boundary conditions: V(θ)=0 for θ=180°–137°, and V(91°)=V0. By taking 2.46D as the bond moment of carbonyl group, the best fit has been realized between the theoretical and experimental dipole moments when V0 is assumed to be close to 6000 cal/mol, i. e., twice as large as the hindering potential for the internal rotation of ethane molecule. Mention has also been made of the mechanism of dielectric absorption by this molecule in benzene solution with particular reference to the flexing motion suggested above.