Correspondence between the reaction force minimum and the onset of abrupt variations of the kinetic and potential energies in bond dissociation and formation.

Context: We demonstrate that the minimum of the reaction force curve of a diatomic or polyatomic molecule undergoing bond dissociation is significant in several respects. As has been pointed out in the past, it is the point at which the force opposing dissociation is strongest. It marks the boundary between the primarily structural stage of a bond dissociation (stretching) and the transition region between the stretched bond and independent atoms.

Interpreting the variations in the kinetic and potential energies in the formation of a covalent bond.

We address the long-standing controversy as to the physical origin of covalent bonding, whether it involves a lowering of the potential energy or a lowering of the kinetic energy. We conclude that both of these do occur and contribute to the formation of the bond. The analysis is in terms of the virial theorem and the variations in the potential energy and the kinetic energy as the atoms approach each other.

Are HOMO-LUMO gaps reliable indicators of explosive impact sensitivity?

A high priority in designing and evaluating proposed explosives is to minimize sensitivity, i.e., vulnerability to unintended detonation due to an accidental stimulus, such as impact.

The π-hole revisited.

It follows from the Schrödinger equation that the forces operating within molecules and molecular complexes are Coulombic, which necessarily entails both electrostatics and polarization. A common and important class of molecular complexes is due to π-holes. These are molecular regions of low electronic density that are perpendicular to planar portions of the molecular frameworks.

The Neglected Nuclei.

Since the nuclei in a molecule are treated as stationary, it is perhaps natural that interpretations of molecular properties and reactivity have focused primarily upon the electronic density distribution. The role of the nuclei has generally received little explicit consideration. Our objective has been to at least partially redress this imbalance in emphasis.

Can Counter-Intuitive Halogen Bonding Be Coulombic?

We use the term "counter-intuitive" to describe an intermolecular interaction in which the electrostatic potentials of the interacting regions of the ground-state molecules have the same sign, both positive or both negative. In the present work, we consider counter-intuitive halogen bonding with nitrogen bases, in which both the halogen σ-hole and the nitrogen lone pair have negative potentials on their molecular surfaces. We show that these interactions can be treated as Coulombic despite the apparent repulsion between the ground-state molecules, provided that both electrostatics and polarization are explicitly taken into account.

Electrostatics and Polarization in σ- and π-Hole Noncovalent Interactions: An Overview.

The energetics of σ- and π-hole interactions can be described very well in terms of electrostatics and polarization, consistent with their Coulombic natures. When both of these components are taken into account, very good correlations with quantum-chemically computed interaction energies are obtained. If polarization is only minor, as when the interactions are quite weak, then electrostatics can suffice, as represented by the most positive electrostatic potential associated with the σ- or π-hole.

Explicit Inclusion of Polarizing Electric Fields in σ- and π-Hole Interactions.

The interactions between a wide variety of molecules having σ-holes or π-holes and several nitrogen bases have been analyzed computationally. The σ- and π-hole atoms span groups III-VII of the periodic table. The interaction energies range from quite weak, typical of non-covalent bonding, to unusually strong: from -4.

σ-Holes and Si···N intramolecular interactions.

We report a computational study of two series of molecules, one having the Si-O-N linkage and the other with the Si-(CH)-N linkage, where n = 1-4. The silicons have various substituents-combinations of H, CH, F, Cl and CF. Many of these compounds have been prepared and characterized experimentally.

A perspective on quantum mechanics and chemical concepts in describing noncovalent interactions.

Since quantum mechanical calculations do not typically lend themselves to chemical interpretation, analyses of bonding interactions depend largely upon models (the octet rule, resonance theory, charge transfer, etc.). This sometimes leads to a blurring of the distinction between mathematical modelling and physical reality.

An Occam's razor approach to chemical hardness: lex parsimoniae.

The term "chemical hardness" refers to the resistance to deformation of the electronic density of a system; the greater this resistance, the "harder" the system. Polarizability, a physical property, is an inverse measure of resistance to deformation and thus should be inversely related to hardness. This is indeed generally accepted.

The Hellmann-Feynman theorem: a perspective.

The Hellmann-Feynman theorem has, with a few exceptions, not been exploited to the degree that it merits. This is due, at least in part, to a widespread failure to recognize that its greatest value may be conceptual rather than numerical, i.e.

The σ-Hole Coulombic Interpretation of Trihalide Anion Formation.

It is shown that the interactions of dihalogen molecules XY with halide anions Z to form trihalide anions (XYZ) can be satisfactorily described as Coulombic, involving the σ-holes on the atoms Y, but only if polarization is taken into account. We have approximated the polarizing effect of the halide anion Z by means of a unit negative point charge. The CCSD/aug-cc-pVTZ computed interaction energies ΔE correlate well with the most positive electrostatic potentials associated with the induced σ-holes over a ΔE range of -12 to -63 kcal mol .

Electronegativity-a perspective.

Electronegativity is a very useful concept but it is not a physical observable; it cannot be determined experimentally. Most practicing chemists view it as the electron-attracting power of an atom in a molecule. Various formulations of electronegativity have been proposed on this basis, and predictions made using different formulations generally agree reasonably well with each other and with chemical experience.

The σ-hole revisited.

A covalently-bonded atom typically has a region of lower electronic density, a "σ-hole," on the side of the atom opposite to the bond, along its extension. There is frequently a positive electrostatic potential associated with this region, through which the atom can interact attractively but noncovalently with negative sites. This positive potential reflects not only the lower electronic density of the σ-hole but also contributions from other portions of the molecule.

σ-holes and π-holes: Similarities and differences.

σ-Holes and π-holes are regions of molecules with electronic densities lower than their surroundings. There are often positive electrostatic potentials associated with them. Through these potentials, the molecule can interact attractively with negative sites, such as lone pairs, π electrons, and anions.

Halogen bonding in hypervalent iodine and bromine derivatives: halonium salts.

Halogen bonds have been identified in a series of ionic compounds involving bromonium and iodonium cations and several different anions, some also containing hypervalent atoms. The hypervalent bromine and iodine atoms in the examined compounds are found to have positive σ-holes on the extensions of their covalent bonds, while the hypervalent atoms in the anions have negative σ-holes. The positive σ-holes on the halogens of the studied halonium salts determine the linearity of the short contacts between the halogen and neutral or anionic electron donors, as usual in halogen bonds.

Close contacts and noncovalent interactions in crystals.

Close contacts, defined as interatomic separations less than the sum of the respective van der Waals radii, are commonly invoked to identify attractive nonbonded interactions in crystal lattices. While this is often effective, it can also be misleading because (a) there are significant uncertainties associated with van der Waals radii, and (b) it may not be valid to attribute the interactions solely to specific pairs of atoms. The interactions within crystal lattices are Coulombic, and the strongest positive and/or negative regions do not always correspond to the positions of atoms; they are sometimes located between atoms.

Hydrogen Bonding between Metal-Ion Complexes and Noncoordinated Water: Electrostatic Potentials and Interaction Energies.

The hydrogen bonding of noncoordinated water molecules to each other and to water molecules that are coordinated to metal-ion complexes has been investigated by means of a search of the Cambridge Structural Database (CSD) and through quantum chemical calculations. Tetrahedral and octahedral complexes that were both charged and neutral were studied. A general conclusion is that hydrogen bonds between noncoordinated water and coordinated water are much stronger than those between noncoordinated waters, whereas hydrogen bonds of water molecule in tetrahedral complexes are stronger than in octahedral complexes.

Impact sensitivity and the maximum heat of detonation.

We demonstrate that a large heat of detonation is undesirable from the standpoint of the impact sensitivity of an explosive and also unnecessary from the standpoints of its detonation velocity and detonation pressure. High values of the latter properties can be achieved even with a moderate heat of detonation, and this in turn enhances the likelihood of relatively low sensitivity.

Mathematical modeling and physical reality in noncovalent interactions.

The Hellmann-Feynman theorem provides a straightforward interpretation of noncovalent bonding in terms of Coulombic interactions, which encompass polarization (and accordingly include dispersion). Exchange, Pauli repulsion, orbitals, etc., are part of the mathematics of obtaining the system's wave function and subsequently its electronic density.

Some molecular/crystalline factors that affect the sensitivities of energetic materials: molecular surface electrostatic potentials, lattice free space and maximum heat of detonation per unit volume.

We discuss three molecular/crystalline properties that we believe to be among the factors that influence the impact/shock sensitivities of energetic materials (i.e., their vulnerabilities to unintended detonation due to impact or shock).