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. We now show that the reaction force minimum is also where the kinetic and potential energy curves tend to change direction abruptly. At this point, the total energy E(R) has increased by about 27% of the dissociation energy, for both diatomic and polyatomic molecules. METHODS:Dissociation curves are analyzed at the UHF/daug-cc-pV5Z level of theory using Gaussian 16.

There continues to be confusion concerning the concept of electronegativity. Pauling's original approach, focusing upon an atom in a molecule, continues to be widely invoked. There has also been a more recent tendency to view electronegativity as the negative of the chemical potential and to extend it to molecules. However, this leads to results that are incompatible with chemical experience. A more effective approach, which gives results in overall agreement with Pauling's values, is to relate electronegativity to the average valence electron ionization energies of atoms.

Some cases of close contacts involving rhenium of perrhenate esters are presented and investigated computationally. The structures and calculated interaction energies confirm the attractiveness of these contacts, which display geometries typical of sigma-hole interactions. The short Re center dot center dot center dot O contacts observed in three crystalline perrhenate salts confirm the general ability of rhenium compounds, both neutral and ionic, to form such interactions.

We have shown in the past that the electrostatic potential at the nucleus of a given atom A is relatively insensitive to that atom's molecular environment. However, the separate nuclear and electronic contributions to it can be quite different. For a free atom, the electrostatic potential at the nucleus is due entirely to its electrons. If this were to be subtracted from the total electronic potential at the nucleus of a molecule or polyatomic ion, this would yield the potential due to the electrons in the remainder of the molecule or ion, not including those of the atom in question itself. In this paper, we show results for ten atoms in a variety of molecules and polyatomic ions. We find that the potentials created by other electrons are almost identical in magnitude with the potentials due to the other nuclei. This is a significant finding because it demonstrates that an atom in a molecule or polyatomic ion is only very slightly affected by the nuclei and electrons of the other atoms. It supports the concept that individual atoms retain their characteristic identities in molecules and ions. The electrostatic potentials created by other electrons are almost identical with the potentials due to the other nuclei. This is a most significant finding because it demonstrates that an atom in a molecule or polyatomic ion is only very slightly affected by the nuclei and electrons of the other atoms. Their effects can truly be considered as no more than a minor perturbation.

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. At large separations, the change in kinetic energy relative to the separated atoms is negative and stabilizing, while the corresponding potential energy change is positive and destabilizing. However, as the atoms approach their equilibrium separation, these rapidly reverse; the kinetic energy increases and the potential energy decreases, so that at equilibrium the net kinetic energy is positive and the net potential energy negative. At equilibrium, the bonding is due solely to the potential energy and is electrostatic.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The power of the Hellmann–Feynman theorem is primarily conceptual. It provides insight and understanding of molecular properties and behavior. In this overview, we discuss several examples of concepts coming out of the theorem. (1) It shows that the forces exerted upon the nuclei in a molecule, which hold the molecule together, are purely Coulombic in nature. (2) It indicates whether the role of the electronic charge in different portions of a molecule’s space is bond-strengthening or bond-weakening. (3) It demonstrates the importance of the electrostatic potentials at the nuclei of a molecule, and that the total energies of atoms and molecules can be expressed rigorously in terms of just these potentials, with no explicit reference to electron–electron interactions. (4) It shows that dispersion forces arise from the interactions of nuclei with their own polarized electronic densities. Our discussion focuses particularly upon the contributions of Richard Bader in these areas.

Fourteen oxatriazoles have been investigated computationally as potential energetic compounds. They include the two isomeric parent compounds, their amino and nitro derivatives, four N-oxides and four amino-N-oxides. Densities and solid state heats of formation were calculated for all of these compounds, and used to determine their detonation velocities and detonation pressures by means of the Kamlet-Jacobs equations. Four of the compounds, all N-oxides, surpass or essentially equal Klapotke's criteria for detonation velocity and detonation pressure. Three other N-oxides meet just the detonation velocity criterion. Impact sensitivity was addressed in terms of three factors that are known to affect it: the free space per molecule in the crystal lattice, the electrostatic potential on the molecular surface, and the detonation heat release of the compound. Three of the N-oxides with the highest detonation velocities and detonation pressures have undesirably large heats of detonation, a warning of possible impact sensitivity. However 5-amino-1,2,3,4-oxatriazole-3-oxide combines good detonation properties with a moderate heat release; its free space per molecule in the crystal lattice and molecular surface electrostatic potential are also consistent with low sensitivity. Overall, we believe that the results of this work should encourage further investigation of oxatriazole derivatives as energetic compounds.

The most extensive use of computed molecular electrostatic potentials has been in relation to reactive behavior. Nucleophilic portions of each molecule would interact favorably with the regions of most positive electrostatic potential on the other, especially the surface local maxima. Molecular electrostatic potentials have been extensively used in biochemistry and pharmacology to identify characteristic patterns of positive and negative potentials that either promote or inhibit particular types of biological activities. Comparisons of the computed electrostatic potentials of a series of variously halogenated dibenzo-p-dioxins plus some related molecules revealed a pattern. The use of the electrostatic potential computed over a molecular surface to interpret and predict the molecule's reactive behavior is well known. The fundamental nature of atomic and molecular electrostatic potentials follows directly from the Schrodinger equation, which describes nuclear-nuclear, nuclear-electronic, and electronic-electronic interactions as purely Coulombic.

The electrostatic potential at the nucleus of an atom, whether in the free state or in a molecule, is qualitatively a characteristic property of the atom. It changes remarkably little from one molecular environment to another. The energies of atoms and molecules can be expressed both rigorously and approximately in terms of the electrostatic potentials at their nuclei. Molecular energies can be written entirely as summations over atomic contributions, with no explicit interatomic terms. This provides a basis for estimating the energy of an atom in a molecule. Overall, the present study supports the validity of the atoms-in-molecules concept.

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. In order to establish a capability for predicting impact sensitivity, there have been numerous attempts to correlate it with some molecular or crystal property or properties. One common approach has been to relate impact sensitivity to the difference between the energies of the highest-occupied and lowest-unoccupied molecular orbitals of the explosive molecule, the “HOMO–LUMO gap.” In the present study, we tested this approach for a series of twelve explosive nitroaromatics, using four different computational methods. We found that the HOMO–LUMO gap does not appear to be a reliable indicator of relative impact sensitivity. Since detonation initiation involves a series of steps, all of which influence sensitivity; it seems more realistic to try to identify fundamental factors and general trends related to sensitivity ‒ an approach that has already had some success ‒ rather than to seek correlations with one or two specific properties.

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. We demonstrate first that the energies of 20 counter-intuitive interactions with four nitrogen bases can be expressed very well in terms of just two molecular properties: the electrostatic potential of the halogen σ-hole and the average polarizability of the nitrogen base. Then we show that the same two properties can also represent the energies of an expanded data base that includes the 20 counter-intuitive plus an additional 20 weak and moderately-strong intuitive halogen bonding interactions (in which the σ-hole potentials are now positive).

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. π-Holes often have positive electrostatic potentials associated with them, which result in mutually polarizing attractive forces with negative sites such as lone pairs, π electrons or anions. In many molecules, π-holes correspond to a flattening of the electronic density surface but in benzene derivatives and in polyazines the π-holes are craters above and below the rings. The interaction energies of π-hole complexes can be expressed quite well in terms of regression relationships that account for both the electrostatics and the polarization. There is a marked gradation in the interaction energies, from quite weak (about -2 kcal mol-1) to relatively strong (about -40 kcal mol-1). Gradations are also evident in the ratios of the intermolecular separations to the sums of the respective van der Waals radii and in the gradual transition of the π-hole atoms from trigonal to quasi-tetrahedral configurations. These trends are consistent with the concept that chemical interactions form a continuum, from very weak to very strong.

The site of the greatest electronic density of the highest occupied molecular orbital (HOMO) is often taken to be the location of the most energetic electrons in a molecule, and thus the most likely site for reaction with an electrophile. However we show, by reference to the average local ionization energy on a molecular surface, that the HOMO alone often does not locate the most energetic electrons. This is because the HOMO, unlike the average local ionization energy, does not take explicit account of the fact that any molecular site has a significant probability of being occupied by electrons in lower-lying, less energetic molecular orbitals.

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. We discuss a number of examples in which the nuclei play the determining role with respect to molecular properties and reactive behavior. It follows that conventional interpretations based solely upon electronic densities and donating or withdrawing tendencies should be made with caution.

Experimental and theoretical studies of fluoro-, chloro-, and bromo-substituted derivatives of barbituric acid and indandione show that imide protons form short hydrogen bonds and bromine or, to a lesser extent, chlorine atoms form halogen bonds. The imide nitrogen atoms act as effective pnictogen bond donors, while C(sp(2)) and C(sp(3)) atoms act as tetrel bond donors; the resulting N center dot center dot center dot O and C center dot center dot center dot O close interactions are a distinctive feature of crystal lattices in all compounds. Importantly, halogen atoms promote the electrophilicity of C(sp(3)) sites and favor the formation of C(sp(3))center dot center dot center dot O close contacts. Oxygen atoms of carbonyl groups of barbituric and indandione units or of water molecules function as the interaction acceptor sites: namely, they donate electron density to hydrogen, halogen, nitrogen, and carbon atoms. Modeling of various barbituric acid derivatives indicates that the positive electrostatic potentials of pi-holes orthogonal to the C(sp(2)) carbons and sigma-holes on the elongation of quasi-axial F/Cl/Br-C(sp(3)) bonds merge to produce a single well-defined point of the most positive electrostatic potential on one face of the barbituric acids. This single local maximum of the potential on the molecular face is close to the site occupied by the oxygen forming the C(sp(3))center dot center dot center dot O, and C(sp(2))center dot center dot center dot O, short contacts observed in crystals.

Following a brief survey of some anomalous properties of fluorine, both as an atom and as a constituent of molecules, we computed electrostatic potentials on molecular surfaces to examine some noncovalent interactions of fluorinated compounds. We demonstrate that the relative rarity and weakness of organic fluorine acting as a hydrogen bond acceptor can be attributed to the low charge capacity of atomic fluorine, which is associated with its low polarizability. This prevents covalently-bonded fluorine from acquiring as much negative character as would be anticipated from its high intrinsic electronegativity. Then we show that the differing interactive behaviors of hydrocarbons and perfluorocarbons can also be interpreted on the basis of their molecular surface electrostatic potentials. The aqueous solubilities of linear alkanes and linear perfluoroalkanes can be expressed quite well as functions of their intrinsic polarities, defined in terms of their electrostatic potentials, and their molecular volumes. The molecular electrostatic potentials of the linear perfluoroalkanes explain their inertness, and their ability to repel water, i.e. the "Teflon effect". (C) 2020 Elsevier Ltd.

A widely used criterion for the existence of a noncovalent interaction is that the interatomic separation be less than the sum of the van der Waals radii of the respective atoms. However, this criterion should not be applied rigidly, but rather with considerable caution and flexibility, for several reasons. First, there is considerable uncertainty in the values assigned as van der Waals radii. Second, because of the manner in which they are assigned, their sums will necessarily miss a significant number of noncovalent interactions. Finally, not all interactions are between atoms; sometimes, instead of an atom, it is a region of positive or negative electrostatic potential that is involved.