The section of density functional theory that studies the quantum descriptors for handling the chemical reactivity of chemical components is well known as “chemical reactivity density functional theory” (CR-DFT). Some of such descriptors are electronic chemical potential [1], global and local chemical hardness [2] and softness, electrophilicity index [3] and etc. The main problem in determining these indexes for chemical compounds is the lack of explicit function for electronic energy function in terms of the number of electrons and the external potential. Therefore some operational definitions of electronic chemical potential and chemical absolute hardness were reported in literatures. Recently Noorizadeh and Parsa [4] proposed the new Morse-like function for the electronic energy function of atomic and molecular system and then calculated the electronic chemical potential and molecular chemical hardness indexes as the first and second derivatives of the electronic energy function with respect to electron numbers at constant external potential respectively. In the present work the Morse-like function applied to calculate these reactivity indexes for investigating of the regioselectivity for some Diels-Alder cycloadition reactions to determine and predict the main product of these reactions. The results show that in many cases the global hardnesses and global electrophilicities which derived from this new approach, with maximum hardness principle (MHP) [5] and minimum electrophilicity principle (MEP)[6] can predict the major regioisomer products of Diels-Alder reactions correctly.
