Chemical concepts such as electronegativity, hardness, electrophilicity, etc and the associated hard-soft acids and bases (HSAB) principle have been widely used and reviewed to explain chemical bonding and reactivity patterns. In this work, we investigate the physical basis, performance and validity of an electronic structure principle related with hardness (η) called Maximum Hardness Principle, MHP, postulated by Pearson as “there seems to be the rule of nature that molecules arrange themselves to be as hard as possible”. [1] The effect of the level of theory, inclusion of solvent effects and approximations used to evaluate ionization potential and electron affinity has been thoroughly examined for a diverse range of chemical space. In addition, for the reactions having two reactants or products, the applicability of MHP has been tested by computing the hardness as the difference of highest HOMO and lowest LUMO of the isolated reactants & products as well as by using the geometric and arithmetic means to obtain an average value of η. This includes examination of the expected ηProduct > ηReactant > ηTS order for high-quality and chemically diverse existing reaction transition state (TS) datasets. Our study involves the use of a recent [3+2] cycloaddition reaction dataset of ~ 5000 reactions by Stuyver et al. [2] We further plan to test the applicability of MHP on one of the largest and chemically diverse TS RGD1 dataset of 1,76,992 reactions by Zhao et al. [3]
Ashima Bajaj