Inter-ring dihedral angles in polychlorinated biphenyls from photoelectron spectroscopy

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Abstract
A proposal that the physiological activity of a chlorinated biphenyl is related to its degree of molecular planarity is explored through experimental measurements of inter-ring dihedral angles in the gas phase. A known correlation between the difference between fourth and first ionization energies of substituted biphenyls (via photoelectron spectroscopy), and the thermally averaged inter-ring dihedral angle as established by electron diffraction techniques, is confirmed and exploited for the present purpose. The expectation, based on both qualitative and quantitative theoretical considerations, that increased ortho substitution causes an increase in inter-ring angle, is confirmed. However, the observation that, for 2,2′-substituted halobiphenyls the only observed conformation is the near-syn form, is not easy to explain, and is discussed in terms of a non-bonding attractive force between halogen atoms.

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