LONG-RANGE ELECTROSTATIC EFFECTS IN 4-SUBSTITUTED BICYCLOOCTANECARBOXYLIC ACIDS BY CNDO/2

Relative total energies, as calculated by the CNDO/2 method, successfully account for the order of dissociation constants of a number of 4-substituted bicyclooctanecarboxylic acids, provided a molecule of hydrogen-bonded water is included in the calculations on the 4-hydroxy derivative. Further solvent structuring for a number of members of the series improves the over-all correlation. The results are interpreted in terms of the electrostatic energy between the ionizable proton and the remainder of the molecule. It is further shown that the so-called “field effect” of the substituent is sufficient to account for differences in the calculated acidities of these compounds. This is done by comparing the CNDO/2 results to a calculation on a classical electrostatic model. The effective dielectric constant used in the latter calculation is based on representing the molecule as a dielectric material through which the charge or dipole substituent acts on the acid proton. CNDO/2 results are in good agreement with the result from electrostatic theory that such a dielectric material always increases the effect of the substituent over what it would have been in a vacuum, and that the increase is proportionally greater for a dipole than for a charged substituent. © 1969, American Chemical Society. All rights reserved.