CHLORINE SUBSTITUTED ACETIC-ACIDS AND SALTS - EFFECT OF SALIFICATION ON CHLORINE-35 NQR

The NQR of a quadrupolar probe nucleus is often used to investigate th e effect of substituent in molecules. The inductive effect, based on a partial charge migration along the molecular skeleton is the only one present in saturated aliphatics, the conjugative effect appearing in conjugated molecules, especially aromatics. As the stepwise charge mig ration mechanism, formerly used to explain the inductive effect, is no w believed obsolete, we have wanted to reexamined the case of chlorine substituted acetic acids and salts. The data in literature was extend ed by observing resonances and determining NQR frequencies in several acids and salts. The present analysis of the salification of mono-, di - and tri-chloroacetic acids, which is equivalent to a deprotonation o r the substitution of the acid hydrogen by a negative unit charge, sho ws that a model based on the polarization of the chlorine atom(s) by t he carboxyle group is consistent with experimental results: the polari zation energy appears to be proportional to the NQR frequency shifts; experimental data show a correlation between the NQR frequency shifts accompanying salification and the variations of the intrinsic acidity measured in the gas phase; this, in turn shows that there is a proport ionality between the polarization energy and the variations in the aci d free enthalpy of dissociation. From the comparison between fluorine, chlorine, bromine and iodine, it also appears that an alternative mec hanism, the polarization of the carboxyl group by the halogen, would b e important only in the case of the fluoroacetic acid.