Linear solvation energy relationships. Part 32. A co-ordinate covalency parameter, ξ, which, in combination with the hydrogen bond acceptor basicity parameter, β, permits correlation of many properties of neutral oxygen and nitrogen bases (including aqueous pKa)

Abstract

Family-dependent (FD) basicity properties are defined as those which have a linear relationship with the hydrogen bond acceptor (HBA) basicity parameter, β, only when families of bases having similar HBA sites are considered separately. Family-independent (FI) properties are those which have a linear relationship with β when all bases are considered together. FD properties can be correlated and meaningfully related to FI properties if, in addition to the β parameter, an empirical co-ordinate covalency parameter, ξ, is used in equations of the form, XYZ=XYZ₀+bβ+eξ. Values of ξ are –0.20 for PO bases, 0.00 for CO and SO bases, 0.20 for single-bonded oxygen bases, 0.60 for pyridine bases, and 1.00 for sp³-hybridized amine bases. By means of the above equation proton transfer basicities (pK_a) are for the first time related to hydrogen bond basicities in a correlation involving all the above types of bonding sites.