Chemical Applications of Atomic and Molecular Electrostatic Potentials

Abstract

A substituent attached to a benzene nucleus is capable of perturbing the electronic distribution within that nucleus. Substituent constants, first introduced by Ham-mett, 1 give a measure of these electronic perturbations. The most widely used substituent constants are σ m and σ p which are obtained from the dissociation constant data of the meta-and para-substituted benzoic acids, respectively. The success of the Hammett parameters for the study and interpretation of thousands of organic reactions and mechanisms 2 underlines the consistency of a substituent to perturb the electronic environment of the benzene nucleus in different chemical systems. An organic chemist explains these perturbations classically in terms of inductive and resonance effects. These electronic perturbations were well-studied by correlating them with computed quantities such as total energy, atomic charges, electrostatic potentials, etc., derived from either ab-initio quantum chemical or semiempirical methods. 3 Very recently, Haeberlein and Brinck 4 have analyzed the substituent effects in para-substituted phenoxide ions and found a close, linear relation between the minima of the electrostatic potential, V min , observed near the phenoxide oxygen and the gas phase acidities. However, they have not considered the electronic perturbations occurring over the aromatic ring due to a substituent. In the present work, we propose a method for directly assessing the effect of a substituent on the aromatic π-electron distribution based on the molecular electrostatic potential (MESP) topography of monosub-stituted benzenes. MESP is a well-established tool for exploring molecular reactivities, intermolecular interactions, and a variety of other chemical phenomena. 5 It has been extensively used by Politzer et al. for understanding the general electro-philic substitution reactions, in particular for substituted benzenes and many other chemical applications. 6-10 Average local ionization energy has also been employed 11 for this purpose. Recently, Gadre et al. have proposed that a detailed investigation of the topography of the MESP is capable of revealing subtle changes observed in the spatial electronic distribution due to changes in the molecular framework by locating and characterizing the critical points (CPs) of the MESP. 12-13 The MESP, V(r), at a point r due to a molecular system with nuclear charges {Z A } located at {R A } and electron density F(r) is given by Here N is the total number of nuclei in the molecule. The delicate balance between the two opposing terms in the above equation brings out the electron-rich regions or electron-deficient regions surrounding a molecule in terms of the corresponding MESP critical points (minima and saddle points). 12-15 In this work, we report the results of a study of 13 typical neutral monosubstituted benzenes. These sub-stituents cover a wide range of Hammett parameters and include halogens (F, Cl), electron-donating groups with unshared pair of electrons (NHCH 3 , NHOH, NH 2 , OH, OCH 3), alkyl groups (CH 3 , CH 2 CH 3), and electron-withdrawing groups (NO 2 , CN, CHO, COOH). The molecular geometries were optimized at HF/6-31G** level using Gaussian 94 package, 16 and the minima were confirmed by the frequency calculations. The MESP topography of all these systems was obtained by employing the package INDPROP. 17 The distribution of the MESP critical points over the benzene ring in four substituted bezenes (one each from the four classes of substituent described here) and benzene projected on to the carbon framework is shown in Figure 1. Out of all these CPs, we have considered the CPs which could be (3, +3) or (3, +1) in nature, labeled as "p" (close to the para-carbon) and "m" (close to the meta-carbon) for the correlation of σ p and σ m substituent constants, 18 respectively. In all the other systems, similar CPs are located and characterized. Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Allaham, M. A.; Zatrezewski, V. G.; Ortiz, J. V.; Foresman, J. B. ; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian Inc., Pittsburgh, PA, 1995. (17) The package UNIPROP developed by S. R. Gadre and co-workers, (18) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165. V(r)) ∑ A N Z A /|r-R A |-∫ F(r′)d 3 r′/|r-r′| 2625