text: As mentioned above the E and C parameters are obtained from enthalpies of adduct formation in which the bond between the acid and base is a σ interaction and adducts that have no steric repulsion between the acid and base.
  As a result, E and C parameters can be used to glean information about pi bonding.  When pi bonding contributes to the measured enthalpy, the enthalpy calculated from the E and C parameters will be less than the measured enthalpy and the difference provides a measure of the extent of the pi bonding contribution.
  The ᐃH calculated for the reaction of MeB with MeN is larger than the observed.  This discrepancy is attributed to steric repulsion between the methyl groups on the B and N.  The difference between the calculated and observed values can then be taken as the amount of the steric effect, a value otherwise not attainable. Steric effects have also been identified with (CH)SnCl and with Cu(HFacac).
  The use of E and C parameters have been extended to analyze spectroscopic changes occurring during adduct formation.  For example, the shift of the phenol OH stretching frequency, , that occurs upon adduct formation has been analyzed using the following equation:
  where asterisks on the E and C for phenol indicate that the acceptor is held constant and the frequency shift is measured as the base is varied. The asterisks also indicate that the phenol parameters are those for frequency shifts and not those for enthalpies. An analysis like this provides a basis for using E and C parameters as a reference scale of donor strengths for frequency shifts. This type analysis has also been applied to other spectroscopic shifts (NMR, EPR, UV-vis, IR, etc.) accompanying adduct formation. Any physicochemical property, , that is dominated by σ donor-acceptor interaction can be correlated with the enthalpy-derived E and C parameters.
  The ECW equations enables one to correlate and predict the enthalpies of adduct formation of neutral donor-acceptor interactions for which the electron-transfer is limited. For gas-phase reactions between cations and neutral donors, there is significant electron-transfer. The extension of the ECW model to cation-neutral Lewis base interactions has led to the ECT model. Others have concluded that the ECW model "is generally found helpful in many fields of solution chemistry and biochemistry".