Validation of Hammett's Linear Free Energy Relationship Through an Unconventional Approach

The present study tries to validate Hammett's linear free energy relationship through an unconventional approach based on the density functional reactivity theory (DFRT). A kinetic energy component [ΔE_B(A)], derived from the DFRT-based comprehensive decomposition analysis of stabilization energy scheme, is used to verify the linear nature of Hammett's log(k_X/k_H) versus σ plot. The study shows that the [Formula: see text] versus σ plot (where -X is the atom or group substituted in place of -H) is linear in nature (with reasonably high correlation coefficient values) for different series of reactions. The slopes of the plots also reveal the electrophilic or nucleophilic nature of the transition states as is obtained from the conventional log(k_X/k_H) versus σ plot. The study thus establishes that the DFRT-based energy component ΔE_B(A) (which is very easy to compute) can be used, instead of k-values, obtained either from the experiment or from computationally intensive conventional thermochemistry calculations to generate reliable Hammett's plot.