On the validity of the maximum hardness principle and the minimum electrophilicity principle during chemical reactions

Hardness and electrophilicity values for several molecules involved in different chemical reactions are calculated at various levels of theory and by using different basis sets. Effects of these aspects as well as different approximations to the calculation of those values vis-à-vis the validity of the maximum hardness and minimum electrophilicity principles are analyzed in the cases of some representative reactions. Among 101 studied exothermic reactions, 61.4% and 69.3% of the reactions are found to obey the maximum hardness and minimum electrophilicity principles, respectively, when hardness of products and reactants is expressed in terms of their geometric means. However, when we use arithmetic mean, the percentage reduces to some extent. When we express the hardness in terms of scaled hardness, the percentage obeying maximum hardness principle improves. We have observed that maximum hardness principle is more likely to fail in the cases of very hard species like F-, H2, CH4, N2, and OH appearing in the reactant side and in most cases of the association reactions. Most of the association reactions obey the minimum electrophilicity principle nicely. The best results (69.3%) for the maximum hardness and minimum electrophilicity principles reject the 50% null hypothesis at the 2% level of significanceWe thank Professor A. K. Nanda, IISER Kolkata, for help in statistical analysis. Financial assistance from CSIR, New Delhi, is gratefully acknowledged. P.K.C. would like to thank DST, New Delhi, for the J. C. Bose National Fellowship. M.S. thanks the following organizations for financial support: the Spanish MICINN (project CTQ2011-23156/BQU), the Catalan DIUE (projects 2009SGR637 and XRQTC), and the FEDER fund for the grant UNGI08-4E-00