Favorable Direction in a Chemical Reaction Through the Maximum Hardness Principle

Abstract

Recently, an assessment regarding the validity of maximum hardness principle has been done taking 34 exothermic chemical reactions (Poater, J.; Swart, M.; Solà, M. J. Mex. Chem. Soc. 2012, 56, 311) in which only 46% and 53% of the total reactions have greater hardness for the products and the reactants than those for the reactants and the transition states, respectively. They have also mentioned that a larger set of reactions should be studied to draw a general conclusion regarding the validity of maximum hardness principle. We have noticed that the reactions having fewer number of reactants than that of products and / or very hard atoms like H, N, O, F or very hard molecules like H₂, N₂, HF, HCN, CH₄, etc. appearing in the reactant side, are more likely to disobey maximum hardness principle. In addition, dependence of hardness values on level of theory, basis sets, definitions, formulas, approximations should be kept in mind before criticising the validity of maximum hardness principle. Since these electronic structure principles are qualitative in nature, one should not expect them to be valid in all cases.