Definition of extended Hammett equation

1. This term applies in a general way to any multiparametric extension of the Hammett equation. It is sometimes used specifically for a form of dual substituent-parameter equation in which the actual value of the correlated property P under the influence of the substituent X is used, rather than the value relative to that for X = H. An intercept term h corresponding to the value of P for X = H is introduced, e.g.

   P = I + R + h

   The equation may be applied to systems for which the inclusion of further terms to represent other effects, e.g. steric, is appropriate. CHAPMAN and SHORTER (1972, 1978); CHARTON (1987).