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Politzer P.,CleveTheoComp | Riley K.E.,Czech Institute of Organic Chemistry And Biochemistry | Bulat F.A.,Fable Theory and Computation | Murray J.S.,CleveTheoComp
Computational and Theoretical Chemistry | Year: 2012

Many covalently-bonded atoms of Groups IV-VII have regions of positive electrostatic potential (σ-holes) opposite to the bonds, along their extensions. Through these positive regions, the atoms can interact highly directionally with negative sites. (Halogen bonding, in which the σ-hole is on a Group VII atom, is an example of this, and we suggest that hydrogen bonding is as well.) The formation and observed properties of the resulting noncovalent complexes can be fully explained in terms of electrostatics/polarization plus dispersion as the driving forces; this straightforward interpretation is based largely upon physical observables - electrostatic potentials, geometries, interaction energies and electric fields. More elaborate interpretations, involving less physically-based methods and models, have also been advanced. In this paper, we try to reconcile some of these seemingly different approaches. © 2012 Elsevier B.V.

Politzer P.,CleveTheoComp | Murray J.S.,CleveTheoComp | Bulat F.A.,Global Strategies Group
Journal of Molecular Modeling | Year: 2010

The average local ionization energy ̄I(r) is the energy necessary to remove an electron from the point r in the space of a system. Its lowest values reveal the locations of the least tightly-held electrons, and thus the favored sites for reaction with electrophiles or radicals. In this paper, we review the definition of ̄I(r) and some of its key properties. Apart from its relevance to reactive behavior, ̄I(r) has an important role in several fundamental areas, including atomic shell structure, electronegativity and local polarizability and hardness. All of these aspects of ̄I(r) are discussed. [Figure not available: see fulltext.] © 2010 Springer-Verlag.

Politzer P.,CleveTheoComp | Murray J.S.,CleveTheoComp | Clark T.,The Interdisciplinary Center | Clark T.,University of Portsmouth
Physical Chemistry Chemical Physics | Year: 2010

A halogen bond is a highly directional, electrostatically-driven noncovalent interaction between a region of positive electrostatic potential on the outer side of the halogen X in a molecule R-X and a negative site B, such as a lone pair of a Lewis base or the π-electrons of an unsaturated system. The positive region on X corresponds to the electronically-depleted outer lobe of the half-filled p-type orbital of X that is involved in forming the covalent bond to R. This depletion is labeled a σ-hole. The resulting positive electrostatic potential is along the extension of the R-X bond, which accounts for the directionality of halogen bonding. Positive σ-holes can also be found on covalently-bonded Group IV-VI atoms, which can similarly interact electrostatically with negative sites. Since positive σ-holes often exist in conjunction with negative potentials on other portions of the atom's surface, such atoms can interact electrostatically with both nucleophiles and electrophiles, as has been observed in surveys of crystallographic structures. Experimental as well as computational studies indicate that halogen and other σ-hole interactions can be competitive with hydrogen bonding, which itself can be viewed as a subset of σ-hole bonding. © the Owner Societies 2010.

Murray J.S.,CleveTheoComp | Lane P.,University of New Orleans | Clark T.,The Interdisciplinary Center | Clark T.,University of Portsmouth | And 2 more authors.
Journal of Molecular Modeling | Year: 2012

A positive π-hole is a region of positive electrostatic potential that is perpendicular to a portion of a molecular framework. It is the counterpart of a σ-hole, which is along the extension of a covalent bond to an atom. Both σ-holes and π-holes become more positive (a) in going from the lighter to the heavier atoms in a given Group of the periodic table, and (b) as the remainder of the molecule is more electronwithdrawing. Positive σ- and π-holes can interact in a highly directional manner with negative sites, e.g., the lone pairs of Lewis bases. In this work, the complexes of 13 π-holecontaining molecules with the nitrogen lone pairs of HCN and NH 3 have been characterized computationally using the MP2, M06-2X and B3PW91 procedures. While the electrostatic interaction is a major driving force in π-hole bonding, a gradation is found from weakly noncovalent to considerably stronger with possible indications of some degree of coordinate covalency. © Springer-Verlag 2011.

Politzer P.,University of New Orleans | Murray J.S.,CleveTheoComp
ChemPhysChem | Year: 2013

Halogen bonding is a noncovalent interaction that is receiving rapidly increasing attention because of its significance in biological systems and its importance in the design of new materials in a variety of areas, for example, electronics, nonlinear optical activity, and pharmaceuticals. The interactions can be understood in terms of electrostatics/polarization and dispersion; they involve a region of positive electrostatic potential on a covalently bonded halogen and a negative site, such as the lone pair of a Lewis base. The positive potential, labeled a σ hole, is on the extension of the covalent bond to the halogen, which accounts for the characteristic near-linearity of halogen bonding. In many instances, the lateral sides of the halogen have negative electrostatic potentials, allowing it to also interact favorably with positive sites. In this discussion, after looking at some of the experimental observations of halogen bonding, we address the origins of σ holes, the factors that govern the magnitudes of their electrostatic potentials, and the properties of the resulting complexes with negative sites. The relationship of halogen and hydrogen bonding is examined. We also point out that σ-hole interactions are not limited to halogens, but can also involve covalently bonded atoms of Groups IV-VI. Examples of applications in biological/medicinal chemistry and in crystal engineering are mentioned, taking note that halogen bonding can be "tuned" to fit various requirements, that is, strength of interaction, steric factors, and so forth. To bond or not to bond: The factors governing halogen-bonding interactions, which involve a region of positive electrostatic potential on a covalently bonded halogen and a negative site (see picture), such as the lone pair of a Lewis base, are described. Particularly important is the positive potential, labeled a σ hole, which is an extension of the covalent bond to the halogen. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Murray J.S.,University of New Orleans | Politzer P.,CleveTheoComp
Wiley Interdisciplinary Reviews: Computational Molecular Science | Year: 2011

The electrostatic potential V(r) that is created by a system of nuclei and electrons is formulated directly from Coulomb's law and is a physical observable, which can be determined both experimentally and computationally. When V(r) is evaluated in the outer regions of a molecule, it shows how the latter is 'seen' by an approaching reactant, and thus is a useful guide to the molecule's reactive behavior, especially in noncovalent interactions. However, V(r) is a fundamental property of a system, the significance of which goes beyond its role in reactivity. For example, the energy of an atom or molecule can be expressed rigorously in terms of the electrostatic potentials at its nuclei. These and other features of V(r) are discussed in this overview. © 2011 John Wiley & Sons, Ltd.

Saha S.,Jackson State University | Dinadayalane T.C.,Jackson State University | Murray J.S.,CleveTheoComp | Leszczynska D.,Jackson State University | Leszczynski J.,Jackson State University
Journal of Physical Chemistry C | Year: 2012

In the present work, we first attached one chlorine radical and then examined the site selectivity for the addition of a second chlorine radical on the external surface of (5,5) armchair single-walled carbon nanotubes (SWCNTs) of 9 and 15 carbon layers. Optimized geometries and energetics were determined using density functional theory (DFT), at the M06-2X/6-31G(d,p) level. Computed reaction energy values for second chlorination have been used to corroborate the prediction of the reactivities of different sites based on the electron density distribution in the singly occupied molecular orbital (SOMO), total spin density, the DFT based local reactivity descriptors (viz., Fukui function, local softness), and the average local ionization energy. Natural bond orbital (NBO) analysis was performed to confirm the covalent bonding between the chlorine and carbon of the SWCNT. The present study reveals that the first chlorine attached to the SWCNT behaves like an ortho- and para-director. The addition of a second chlorine on an already chlorinated SWCNT exhibits positional preference. © 2012 American Chemical Society.

Politzer P.,CleveTheoComp | Murray J.S.,CleveTheoComp
Central European Journal of Energetic Materials | Year: 2011

We have explored various aspects of the Kamlet-Jacobs equations for estimating detonation velocities and pressures. While the loading density of the explosive compound is certainly an important determinant of these properties, its effect can sometimes be overridden by other factors, such as the detonation heat release and/or the number of moles of gaseous products. Using a gas phase rather than solid phase enthalpy of formation in obtaining a compound's heat release can produce a significant error in the calculated detonation velocity. However a negative enthalpy of formation is not necessarily incompatible with excellent detonation properties. Additional evidence is presented to support Kamlet and Jacobs' conclusion that, for C, H, N, O explosives, assuming the detonation product composition to be N 2(g)/H 2O(g)/CO 2(g)/C(s) gives overall quite satisfactory results.

Bundhun A.,University of Mauritius | Ramasami P.,University of Mauritius | Murray J.S.,CleveTheoComp | Politzer P.,CleveTheoComp
Journal of Molecular Modeling | Year: 2013

It is well-established that many covalently-bonded atoms of Groups IV-VII have directionally-specific regions of positive electrostatic potential (σ-holes) through which they can interact with negative sites. In the case of Group VII, this is called "halogen bonding." We have studied two series of molecules: the F3MX and, for comparison, the H 3MX (M = C, Si and Ge; X = F, Cl, Br and I). Our objective was to determine how the interplay between M and X in each molecule affects the σ-holes of both, and consequently their interactions with the nitrogen lone pair of HCN. We find that the relative electronegativities of M and X are not sufficient to explain their effects upon each other's σ-holes; consideration of charge capacity/polarizability (and perhaps other factors) also appears to be necessary. However the results do support the description of normal σ-hole interactions as being largely electrostatically-driven. © 2012 Springer-Verlag.

Politzer P.,CleveTheoComp | Murray J.S.,CleveTheoComp
Theoretical Chemistry Accounts | Year: 2012

It was recently shown that complexes of the type F-Cl···CN-R, ostensibly halogen bonded, sometimes have properties (relatively high binding energies, short Cl-C separations and considerably lengthened F-Cl distances) that are inconsistent with typical halogen bonds (Del Bene et al. in J Phys Chem A 114:12958-12962, 2010). We attribute these anomalous features, as well as analogous observations for F-Cl···SiN-R systems, to the strong polarization of the CN-R carbons and SiN-R silicons by the electric field of the positive σ-hole of the F-Cl chlorine. This polarization may evolve into some degree of dative sharing of electrons by the carbons and silicons. This interpretation is supported by the fact that complexes of CN-R and SiN-R with Cl-Cl, which has a much weaker positive σ-hole than F-Cl, are considerably less likely to show the unusual features. It is demonstrated that the full ranges of binding energies of the CN-R and SiN-R complexes with either F-Cl or Cl-Cl can be represented well (R 2 > 0. 96) in terms of the most negative electrostatic potentials and the lowest local ionization energies on the carbon and silicon surfaces. These properties reflect the electrostatic components of the interactions and the polarizabilities/dative reactivities of the carbons and silicons. © 2012 Springer-Verlag.

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