Molecular Electrostatic Potential-Based Atoms in Molecules: Shielding Effects and Reactivity Patterns (original) (raw)
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Bonding and Reactivity Patterns from Electrostatic Landscapes of Molecules
Journal of Chemical Sciences
The topographical analysis of molecular electron density (MED) and molecular electrostatic potential (MESP) offers insights into the bonding and reactivity patterns through the critical points (CPs) of these scalar fields. The MESP is found to be particularly useful for describing sites of electrophilic attack and weak intermolecular interactions. MESP is also shown to clearly distinguish between the lone pairs and πdelocalization. The concept of atoms in molecules (AIM) which has so far been primarily based on the gradients of MED, has recently been extended via the use of MESP. The portrayal of AIM through MESP clearly reveals the electron rich atoms in the molecule and also provides the details of the preferred direction of approach of an electrophile. This perspective briefly summarizes the prominent features of MESP topography and provides a future outlook.
Electrostatic Potential Topology for Probing Molecular Structure, Bonding and Reactivity
Molecules
Following the pioneering investigations of Bader on the topology of molecular electron density, the topology analysis of its sister field viz. molecular electrostatic potential (MESP) was taken up by the authors’ groups. Through these studies, MESP topology emerged as a powerful tool for exploring molecular bonding and reactivity patterns. The MESP topology features are mapped in terms of its critical points (CPs), such as bond critical points (BCPs), while the minima identify electron-rich locations, such as lone pairs and π-bonds. The gradient paths of MESP vividly bring out the atoms-in-molecule picture of neutral molecules and anions. The MESP-based characterization of a molecule in terms of electron-rich and -deficient regions provides a robust prediction about its interaction with other molecules. This leads to a clear picture of molecular aggregation, hydrogen bonding, lone pair–π interactions, π-conjugation, aromaticity and reaction mechanisms. This review summarizes the con...
Journal of computational chemistry, 2018
This work considers the features of the electrostatic potential (ESP), and the potential acting on an electron in a molecule (PAEM) for the series of isolated dihalide molecules and for their molecular complexes. The joint analysis of these functions enriches the vision of atomic predispositions to the halogen bond formation and reveals details for their characterization. The account for the exchange-correlation interaction in PAEM retains the specific anisotropy of the ESP, which is commonly used for the halogen bonding explanation within σ-hole concept. Along the halogen bonds, the curvatures of PAEM and ESP functions are opposite. Being jointly mapped on the closed isosurfaces of the reduced density gradient, placed between bound atoms, they are significantly differed from the side facing the halogen atom and from the side looking at the electron donor atom. © 2017 Wiley Periodicals, Inc.
Reduction potentials (E 0) of six mononuclear cobalt catalysts (1−6) for hydrogen evolution reaction and electron donating/withdrawing effect of nine X-substituents on their macrocyclic ligand are reported at solvation effect-included B3P86/ 6-311+G** level of density functional theory. The electrostatic potential at the Co nucleus (V Co) is found to be a powerful descriptor of the electronic effect experienced by Co from the ligand environment. The V Co values vary substantially with respect to the nature of macrocycle, type of apical ligands, nature of substituent and oxidation state of the metal center. Most importantly, V Co values of both the oxidized and reduced states of all the six complexes show strong linear correlation with E 0. The correlation plots between V Co and E 0 provide an easy-to-interpret graphical interpretation and quantification of the effect of ligand environment on the reduction potential. Further, on the basis of a correlation between the relative V Co and relative E 0 values of a catalyst with respect to the CF 3-substituted reference system, the E 0 of any X-substituted 1−6 complexes is predicted. ■ INTRODUCTION Mononuclear cobalt complexes of tetraazamacrocyclic ligands are promising class of hydrogen evolving electro catalysts, known to work at modest over potential. 1−10 Recently Solis and Hammes-Schiffer studied the effect of substituents on tuning the reduction potentials (E 0) of cobalt diglyoxime complex referred to here as 1-X. 11 Solis and Hammes-Schiffer showed that E 0 and pK a values of 1-X correlates linearly to the Hammett substituent constant. The E 0 becomes more negative with increase in the electron donating character of the substituent. Theoretical calculation of E 0 to the experimental accuracy is very difficult to achieve as it demands very accurate estimation of thermo-dynamic parameters for both oxidized and reduced forms of the complex. This becomes even more challenging for various substituted cases as the finer substituent effects may lead to subtle variations in E 0 values. Solis and Hammes-Schiffer's work suggests that E 0 for the 1-X complex can be predicted with a knowledge of substituent effect. Among the theoretically derived properties useful for the interpretation and quantification of substituent effects in molecular systems, the topographical and surface features of molecular electrostatic potential (MESP) have been widely used. The MESP can be experimentally determined from electron density data derived from X-ray diffraction studies on crystals whereas being a one electron property, its accurate calculation is rather easy with theoretical methods implemented in many of the standard ab initio/DFT program packages. The use of the theoretically derived MESP to understand molecular reactivity has been pioneered by the works of Tomasi, 12 Pullman, 13 Politzer, 14−19 and Gadre. 20−23 Recently the works of Wheeler and Houk 24−28 have contributed to the growth of this area. In many of the studies from our group, we have shown that MESP based analysis is useful to interpret and quantify resonance effect, 29 inductive effect, 30 substituent effects, 31,32 trans influence, 33 cation-π interactions, 34−36 lone pair-π interactions , 37 noncovalent interactions including a large variety of hydrogen bonds, 38 aromatic character of benzenoid hydrocarbons , 39,40 stereoelectronic features of ligands in organo-metallic/inorganic chemistry 41−46 etc. Very recently we have
Intricate behavior of one-electron potentials from the Euler equation for electron density and corresponding gradient force fields in crystals was studied. Bosonic and fermionic quantum potentials were utilized in bonding analysis as descriptors of the localization of electrons and electron pairs. Channels of locally enhanced kinetic potential and the corresponding saddle Lagrange points were found between chemically bonded atoms linked by the bond paths. Superposition of electrostatic φ_es (r) and kinetic φ_k (r) potentials and electron density ρ(r) allowed partitioning any molecules and crystals into atomic ρ- and potential-based φ-basins; the φ_k-basins explicitly account for electron exchange effect, which is missed for φ_es-ones. Phenomena of interatomic charge transfer and related electron exchange were explained in terms of space gaps between ρ- and φ-zero-flux surfaces. The gap between φ_es- and ρ-basins represents the charge transfer, while the gap between φ_k- and ρ-basins...
Influence of External Electric Fields on Atomic and Bond Properties of Diatomic Molecule
arXiv: Chemical Physics, 2020
Atomic and molecular (bond) properties of a set of homo- (H$_2$, N$_2$, O$_2$, F$_2$, and Cl$_2$) and hetero-diatomic (HF, HCl, CO, and NO) molecules under intense external electric fields are studied in the context of quantum theory of atoms in molecules (QTAIM). Field-effects on basic QTAIM properties are elucidated. Atomic charges and energies are found to be linearly correlated with the field strengths within the studied range of field intensities, but some molecular properties that depend on polarizability exhibit non-linear responses to the imposed fields. The electron density distribution responds to the imposed fields altering the shape and location of the zero-flux surfaces, atomic volumes, atomic electron population, and localization/delocalization indices. The external fields also perturb the covalent-polar-ionic characteristic of the studied chemical bonds. The topography and topology of the molecular electrostatic potential is dramatically changed under the influence of...
Journal of Computational Chemistry, 1995
A general methodology for deriving geometry-dependent atomic charges is presented. The main ingredient of the method is a model that describes the molecular dipole moment in terms of geometry-dependent point charges. The parameters of the model are determined from ab initio calculations of molecular dipole moments and their Cartesian derivatives at various molecular geometries. Transferability of the parameters is built into the model by fitting ab initio calculations for various molecules simultaneously. The results show that charge flux along the bonds is a major contributing factor to the geometry dependence of the atomic charges, with additional contributions from fluxes along valence angles and adjacent bonds. Torsion flux is found to be smaller in magnitude than the bond and valence angle fluxes but is not always unimportant. A set of electrostatic parameters is presented for alkanes, aldehydes, ketones, and amides. Transferability of these parameters for a host of molecules is established to within 3-5% error in the predicted dipole moments. A possible extension of the method to include atomic dipoles is outlined. With the inclusion of such atomic dipoles and with the set of transferable point charges and charge flux parameters, it is demonstrated that molecular electrostatic potentials as well as electrostatic forces on nuclei can be reproduced much better than is possible with other models (such as potential derived charges).
A Molecular Electrostatic Potential Analysis of Hydrogen, Halogen, and Dihydrogen Bonds
Hydrogen, halogen, and dihydrogen bonds in weak, medium and strong regimes (<1 to ∼60 kcal/mol) have been investigated for several intermolecular donor−acceptor (D-A) complexes at ab initio MP4//MP2 method coupled with atoms-in-molecules and molecular electrostatic potential (MESP) approaches. Electron density ρ at bond critical point correlates well with interaction energy (E nb ) for each homogeneous sample of complexes, but its applicability to the entire set of complexes is not satisfactory. Analysis of MESP minimum (V min ) and MESP at the nuclei (V n ) shows that in all D-A complexes, MESP of A becomes more negative and that of D becomes less negative suggesting donation of electrons from D to A leading to electron donor−acceptor (eDA) interaction between A and D. MESP based parameter ΔΔV n measures donor−acceptor strength of the eDA interactions as it shows a good linear correlation with E nb for all D-A complexes (R 2 = 0.976) except the strongly bound bridged structures. The bridged structures are classified as donor−acceptor−donor complexes. MESP provides a clear evidence for hydrogen, halogen, and dihydrogen bond formation and defines them as eDA interactions in which hydrogen acts as electron acceptor in hydrogen and dihydrogen bonds while halogen acts as electron acceptor in halogen bonds. − (electron donors) with different electron acceptors. Throughout this paper, E nb represents the interaction energy calculated at MP4//MP2 method and the standard notations ρ and ∇ 2 ρ are used to indicate the electron density at the bond critical point (bcp) of the electron donor−acceptor bond and the Laplacian of the electron density at the bcp. Figure 7. Change in V min upon bond formation in electron donor−acceptor−donor complexes (a) F − ...IF and (b) F − ...IBr. The black dots represent the location of the most negative MESP-valued point and the corresponding V min values in kcal/mol are also depicted.
Journal of Molecular Structure: THEOCHEM, 2010
A simple coarse grained description of the electron density changes in molecular systems due to change in external potential, which may include the effect of external electric fields in addition to the potential due to the nuclei, has been proposed in terms of the induced atom-atom charges and atomic dipoles. The density functional perturbation theory has been used for deriving the expressions for the interaction energy and the effective chemical potentials in terms of these coarse grained variables. A route to the calculation of these quantities and hence the dipole polarizability of the molecular system is provided. The proposed approach would also be useful for obtaining polarizable charge based force field for intermolecular interaction in computer simulation.
Valence-State Atoms in Molecules. 6. Universal Ionic−Covalent Potential Energy Curves
Journal of Physical Chemistry A, 2001
A semiempirical approach for constructing a universal ionic-covalent (UIC) potential energy curve is presented, and two related UIC functions are discussed. In the vicinity of the equilibrium bond length, the attraction between the atoms in the molecule (AIM) is modeled as purely Coulombic, -C/R, as implied by the asymptotic reference to the promoted valence-state energy of partially charged atoms Szentpály, L. v. J. Phys. Chem. A 1999, 103, 9313]. The partial charge is calculated by electronegativity equalization. Along the dissociation coordinate R, we model the decreasing contribution of "ionic structures" as a "soft" Coulson-Fischer transition: the composite UIC function is generated by continuously reducing the weight of the valencestate potential energy function by the admixture of a modified Morse function. Average unsigned errors of 1.42% and 1.16% of D e are obtained by comparing our five-parameter UIC and UIC R curves with the full Rydberg-Klein-Rees, or ab initio, curves of 42 covalent or polar diatomic molecules (from H 2 to NaCl). The evaluation of the rotation-vibration coupling constant, R e , requires only three parameters and yields an average unsigned error of 6.37% for 50 molecules.