BOND MOMENT AND ELECTRONEGATIVITY' CHARLES P. SMYTH Department of Chemistry, Princeton University, Princeton, New Jersey Received October 18, 1036
One of the fondest hopes of the early investigators of dipole moments was that they would give a quantitative means of evaluating the electronegativities of atoms in molecules so often used to explain chemical behavior. Difference in the values of dipole moments associated with chemical bonds A-B and A-C would seem to measure the difference in the electronegativities of B and C. If, however, the molecule containing the bond is polyatomic, the assignment of a constant value to the moment of the bond may be uncertain or impossible. Although values, more or less incorrect but not without usefulness, have been calculated for the moments associated with a number of bonds in polyatomic molecules, it has been shown (17) that a bond moment is merely a measure of the electrical asymmetry of a certain section of a molecule and is affected by the environment of the section. If the moment found for a bond in one environment is used for the same bond in a different environment, quite erroneous conclusions may be drawn. A group attached to a benzene ring may give a moment markedly different from that produced by its attachment to an aliphatic group, and the differences in moment thus arising have been correlated with the orienting influences of the groups upon substitution in the benzene ring (21). The large mutual inductive effects and consequent lowering of moments of two or more dipoles attached to the same atom in the substituted methanes have been pointed out (18). The moment of a group which is the resultant of a number of moments of varying size, one or two usually predominating, may, through the insulation of its component dipoIes, be less dependent upon the rest of the molecule than that of a single large dipole like that occasioned, for example, by the presence of a halogen atom. The effect of extramolecular environment upon the weakly screened halogen bond moments of diatomic molecules is interestingly shown by the much higher values found for the hydrogen 1 Presented a t the Symposium on Alolecular Structure, held at Princeton University, Princeton, S e w Jersey, December 31, 1936 to January 2, 1937, under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society.
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halides ( 5 ) and iodine monochloride (6) in solution than in the vapor state (9). TABLE 1 Group moments* ( ~ 1 0 1 8 )
1
GROUP
