Abstract
The chapter surveys mostly original work of the authors on the application of the electrostatic potential at nuclei (EPN) as a reactivity index in quantifying hydrogen bonding as well as different reactions of organic compounds. The EPN index was defined and introduced by E. B. Wilson (1962). However, it was first applied as a reactivity index much later in works from our laboratory (Bobadova-Parvanova & Galabov, 1998; Galabov & Bobadova-Parvanova, 1999; Dimitrova, Ilieva, & Galabov, 2002; Cheshmedzhieva, Ilieva, Hadjieva, Trayanova, & Galabov, 2009; Galabov, Cheshmedzhieva, Ilieva, & Hadjieva, 2004; Galabov, Ileiva, & Schaefer, 2006; Galabov, Nikolova, Wilke, Schaefer, & Allen, 2008; Galabov, Ilieva, Hadjieva, Atanasov, & Schaefer, 2008; Koleva, Galabov, Wu, Schaefer, & Schleyer, 2009). Numerous applications showed that the EPN index, an accurate quantum mechanical quantity, predicts with remarkable accuracy the energy shifts accompanying hydrogen bonding. The theoretically evaluated EPN descriptor correlates also excellently with experimental and theoretically evaluated kinetic parameters for a number of important organic reactions. Based on these findings an efficient computational approach for the evaluation of substituent constants was developed.
TopBackground
An important theoretical quantity employed in analyzing chemical reactivity is the molecular electrostatic potential (MEP) (Scrocco & Tomasi, 1973; Politzer & Truhlar, 1981; Murray & Politzer, 1988; Murray, Lans, Brinck, & Politzer, 1991; Tomasi, Bonaccorsi, & Cammi, 1990; Gadre, Kulkarni, & Srivastava, 1992; Murray & Sen, 1996; Naray-Szabo & Ferenczy, 1995; Suresh & Gadre, 1997; Suresh & Gadre, 2007). Minima and maxima of the surface MEP have been widely used in studying the structural factors determining the relative rates of various intermolecular interactions. Extensive surveys on the subject are available (Scrocco & Tomasi, 1973; Murray & Politzer, 1988; Murray & Sen, 1996; Naray-Szabo & Ferenczy, 1995; Politzer & Truhlar, 1981; Tomasi, Bonaccorsi, & Cammi, 1990). The great advantage of MEP over alternative theoretical approaches in analyzing reactivity is the fact that no additional approximations are introduced in deriving MEP. Thus, maps of MEP, typically at the Van der Waals surface of the molecule, have been successfully employed in determining the intramolecular factors governing reactivity (Scrocco & Tomasi, 1973; Murray & Politzer, 1988; Murray, Lans, Brinck, & Politzer, 1991; Gadre, Kulkarni, & Srivastava, 1992; Murray & Sen, 1996; Naray-Szabo & Ferenczy, 1995; Politzer & Truhlar, 1981; Suresh & Gadre, 1997; Suresh & Gadre, 2007; Tomasi, Bonaccorsi, & Cammi, 1990). MEP is defined as follows (in atomic units, bold font denotes vector quantities) (Politzer & Truhlar, 1981):