The Drago–Wayland Equation

The electrostatic, covalent, constant W (ECW) model can predict the enthalpies (sigma bond strengths) of the formation of neutral Lewis acid–base adducts in the gas phase or in weakly solvating solvents, whereas the electrostatic, covalent, transfer of electron density (ECT) model calculates the gas-phase enthalpies of (1) cationic Lewis acids reacting with neutral Lewis bases, (2) anionic Lewis bases reacting with neutral Lewis acids, and (3) cationic Lewis acids reacting with anionic Lewis bases. The differences between experimental and calculated enthalpies indicate the presence of bonding contributions other than normal sigma bonding in the adduct. For example, the presence of steric effects and π-back bonding can be easily determined. Physicochemical measurements involving reactivity (equilibrium constants, rate constants, etc.) or spectrochemical properties (spectral shifts in NMR (nuclear magnetic resonance), EPR (electron paramagnetic resonance), UV-vis, IR, etc.) can be determined using these models if Lewis acid–base interactions control the reactivity or spectrochemical properties.