The nature of field effects and their fall-off with distance: The acidity of substituted quinuclidinium and bicyclooctylammonium ions

Gas-phase acidities are reported for series of substituted quinuclidinium and bicyclo [2.2.2] octylammonium ions. Ab initio calculations are also reported for these equilibria at the STO-3G and 3-21G (for the quinuclidines) bases and can be used to extend the results. pK a values obtained for the bicyclooctylammonium ions are compared with previously reported results for the quinuclidinium ions. The results, as a whole, are considered together with similar values obtained for other saturated systems, to look at the varying transmission of field effects with distance. The fall-of observed between the two series in the gas phase is considerably less than expected from simple electrostatics. In contradication with a recent claim, it is also shown that field effects, as measured by the parameter σ F , give a good measure of the effect of the substituents in each series compared with a combination of resonance and electronegativity effects