Studies in molecular polarisation

• Main Author: MacQueen, Jean
• Published: Royal Holloway, University of London 1955
• Subjects: 535.5; Molecular Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.703754

A number of inner-complex compounds of beryllium, aluminium, iron, copper and nickel have been prepared, and measurements of the molecular polarisation have been made on those which were sufficiently soluble in either benzene or dioxan. A heterodyne beat apparatus was used for dielectric constant measurements. The appropriate molecular refractions have been either measured or calculated, and, on the assumption that the molecules are symmetrical, the atomic polarisations have been estimated. These values of the atomic polarisations are discussed in relation to the force constants for the bending vibrations of the chelate rings and the effective dipole moments of the rings. It was found that the large atomic polarisations observed by previous workers for the acetylacetonates are apparently general for all inner-complex compounds in which the chelate ring contains six atoms. Exceptions may occur when the ring is held rigidly by some means, as in the salicylaldoxime complexes, where hydrogen bonding between the two oxime residues has been postulated to explain their unusually low atomic polarisations. For the other six-membered ring complexes, atomic polarisation values of between 29 and 67 c.c. were found for complexes containing two chelate rings in the molecule, and between 45 and 94 c.c. when three chelate rings are present, the variations being due to changes in the ring moment and the force constant. It was found that the force constants were considerably affected by changes in the strain of the chelate ring, since an increase in the strain of the ring increased its rigidity. Complexes of copper and nickel which contained five-membered rings have quite small atomic polarisations, this being attributed to the large force constants for the vibration of the rings which in these compounds are greatly strained.