The molecular electrostatic potential (MESP) V(r) data derived from a reliable quantum chemical method has been widely used for the interpretation and prediction of various aspects of chemical reactivity. A rigorous mapping of the MESP topology is achieved by computing both rV(r) data and the elements of the Hessian matrix at the critical points where rV(r) = 0. In the MESP topology, intra-and inter-molecular bonded regions show the characteristic (3, À1) bond critical points (BCPs) while the electron-rich regions such as lone pair and π-bonds show (3, +3) minimum (V min ) CPs. The V min analysis provides a simple and powerful technique to characterize the electron-rich region in a molecular system as it corresponds to the condensed information of the wave function at this point due to the nuclei and electronic distribution through the Coulomb's law. The V min analysis has been successfully applied to explain the phenomena related to chemical reactivity such as π-conjugation, aromaticity, substituent effect, ligand electronic effects, trans-influence, redox potential, activation energy, cooperativity, noncovalent interactions, and so on. The MESP parameters ΔV min and ΔV n , derived for arene systems have been used as powerful measures of substituent effects while V min at the lone pair region of ligands has been used as a reliable electronic parameter to assess their σ-donating ability to metal centers. Furthermore, strong predictions on the intermolecular interactive behavior of molecular systems can be made from MESP topology studies. This review summarizes the chemical reactivity applications offered by MESP topology analysis for a large variety of organic, organometallic, and inorganic molecular systems.

Substituent effects in organic chemistry are generally described in terms of experimentally derived Hammett parameters whereas a convenient theoretical tool to study these effects in π-conjugated molecular systems is molecular electrostatic potential (MESP) analysis. The present study shows that the difference between MESP at the nucleus of the para carbon of substituted benzene and a carbon atom in benzene, designated as ΔVC, is very useful to quantify and classify substituent effects. On the basis of positive and negative ΔVC values, a broad classification of around 381 substituents into electron withdrawing and donating categories is made. Each category is again sorted based on the magnitude of ΔVC into subcategories such as very strong, strong, medium, and weak electron donating/withdrawing. Furthermore, the data are used to show the transferability and additivity of substituent effects in π-conjugated organic molecules such as condensed aromatic, olefinic, acetylenic, and heterocyclic systems. The transferability properties hold good for ΔVC in all these molecular systems. The additive properties of substituent effects are strongly reflected on ΔVC and the predictive power of the data to assign the total substituent effects of multi-substituted systems is verified. The ΔVC data and the present classification of substituents are very useful to design π-conjugated organic molecular systems with desired electron rich/poor character.

Modifications on the ligand environment of Milstein ruthenium(II) pincer hydride catalysts have been proposed to fine-tune the activation free energy, ΔG(⧧) for the key step of H2 elimination in the water splitting reaction. This study conducted at the B3LYP level of density functional theory including the solvation effect reveals that changing the bulky t-butyl group at the P-arm of the pincer ligand by methyl or ethyl group can reduce the ΔG(⧧) by a substantial margin, ∼ 10 kcal/mol. The reduction in the steric effect of the pincer ligand causes exothermic association of the water molecule to the metal center and leads to significant stabilization of all the subsequent reaction intermediates and the transition state compared to those of the original Milstein catalyst that promotes endothermic association of the water molecule. Though electron donating groups on the pyridyl unit of the pincer ligand are advantageous for reducing the activation barrier in the gas phase, the effect is only 1-1.4 kcal/mol compared to that of an electron withdrawing group. The absolute minimum of the electrostatic potential at the hydride ligand and carbonyl stretching frequency of the catalyst are useful parameters to gauge the effect of ligand environment on the H2 elimination step of the water splitting reaction.

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