FF-EP Index: A Practical Index to Accurately Describe the Chemical Reactivity Together with its Calculating Program RAI Calculator

Abstract

Chemical reactivity is a foundational concept to elucidate the reaction mechanisms, which is widely employed in both experimental and theoretical researches. Nevertheless, traditional methodologies face challenges in reconciling computational precision with broad applicability and calculating efficiency. Herein, we propose two novel Reaction Activity Indexs for nucleophilic and electrophilic attacks (RAI+ and RAI−), which are derived through synergistic integration of Fukui function (FF) and electrostatic potential (EP). This is an ingenious combination of FF and EP, which can be regarded as a comprehensive indicator to reflect the chemical reaction tendency and physical adsorption strength. Validity tests on 21 typical reactions demonstrates that RAI⁺ and RAI⁻ indexes unambiguously delineate electrophilic/nucleophilic regioselectivity across diverse organic and organometallic systems. Crucially, RAI implementation necessitates only single-point energy and EP computations, thereby achieving high computational efficiency without sacrificing predicting fidelity. This work demonstrates the huge potential of the RAI algorithm as a quantitative tool for assessing chemical reactivity and provides new insights for predicting reaction mechanisms. At last but not least, we compiled the RAI program using FORTRAN language, which is also provided as exe files and source code.