Pat Lane

Expansion of the σ-hole concept

AbstractThe term “σ-hole” originally referred to the electron-deficient outer lobe of a half-filled p (or nearly p) orbital involved in forming a covalent bond. If the electron deficiency is sufficient, there can result a region of positive electrostatic potential which can interact attractively (noncovalently) with negative sites on other molecules (σ-hole bonding). The interaction is highly directional, along the extension of the covalent bond giving rise to the σ-hole. σ-Hole bonding has been observed, experimentally and computationally, for many covalently-bonded atoms of Groups V–VII. The positive character of the σ-hole increases in going from the lighter to the heavier (more polarizable) atoms within a Group, and as the remainder of the molecule becomes more electron-withdrawing. In this paper, we show computationally that significantly positive σ-holes, and subsequent noncovalent interactions, can also occur for atoms of Group IV. This observation, together with analogous ones for the molecules (H3C)2SO, (H3C)2SO2 and Cl3PO, demonstrates a need to expand the interpretation of the origins of σ-holes: (1) While the bonding orbital does require considerable p character, in view of the well-established highly directional nature of σ-hole bonding, a sizeable s contribution is not precluded. (2) It is possible for the bonding orbital to be doubly-occupied and forming a coordinate covalent bond. FigureTwo views of the calculated electrostatic potential on the 0.001 au molecular surface of SiCl4. Color ranges, in kcal/mole, are: purple, negative; blue, between 0 and 8; green, between 8 and 11; yellow, between 11 and 18; red, more positive than 18. The top view shows three of the four chlorines. In the center is the σ-hole due to the fourth Cl−Si bond, its most positive portion (red) being on the extension of that bond. In the bottom view are visible two of the σ-holes on the silicon. In both views can be seen the σ-holes on the chlorines, on the extensions of the Si−Cl bonds; their most positive portions are green

σ-Holes, π-holes and electrostatically-driven interactions

AbstractA 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 electron-withdrawing. 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-hole-containing molecules with the nitrogen lone pairs of HCN and NH3 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. FigureComputed molecular surface electrostatic potential of SeO2 showing the π-hole above the selenium atom (middle). The position of the most positive electrostatic potential associated with the π-hole is indicated by a black hemisphere. Color ranges, in kcal mol-1, are: red, greater than 33; yellow, from 33 to 20; green, from 20 to 0; blue, less than 0 (negative).

Blue shifts vs red shifts in σ-hole bonding

Abstractσ-Hole bonding is a noncovalent interaction between a region of positive electrostatic potential on the outer surface of a Group V, VI, or VII covalently-bonded atom (a σ-hole) and a region of negative potential on another molecule, e.g., a lone pair of a Lewis base. We have investigated computationally the occurrence of increased vibration frequencies (blue shifts) and bond shortening vs decreased frequencies (red shifts) and bond lengthening for the covalent bonds to the atoms having the σ-holes (the σ-hole donors). Both are possible, depending upon the properties of the donor and the acceptor. Our results are consistent with models that were developed earlier by Hermansson and by Qian and Krimm in relation to blue vs red shifting in hydrogen bond formation. These models invoke the derivatives of the permanent and the induced dipole moments of the donor molecule. FigureComputed electrostatic potential on the molecular surface of Cl-NO2. Color ranges, in kcal mol−1, are: red, greater than 25; yellow, between 10 and 25; green, between 0 and 10; blue, between −4 and 0; purple, more negative than −4. The chlorine is facing the viewer, to the right. Note the yellow region of positive potential on the outer side of the chlorine, along the extension of the N–Cl bond. The blue region shows the sides of the chlorine to have negative potentials. The calculations were at the B3PW91/6–31G(d,p) level.

Why are dimethyl sulfoxide and dimethyl sulfone such good solvents

AbstractWe have carried out B3PW91 and MP2-FC computational studies of dimethyl sulfoxide, (CH3)2SO, and dimethyl sulfone, (CH3)2SO2. The objective was to establish quantitatively the basis for their high polarities and boiling points, and their strong solvent powers for a variety of solutes. Natural bond order analyses show that the sulfur–oxygen linkages are not double bonds, as widely believed, but rather are coordinate covalent single S+→O− bonds. The calculated electrostatic potentials on the molecular surfaces reveal several strongly positive and negative sites (the former including σ-holes on the sulfurs) through which a variety of simultaneous intermolecular electrostatic interactions can occur. A series of examples is given. In terms of these features the striking properties of dimethyl sulfoxide and dimethyl sulfone, their large dipole moments and dielectric constants, their high boiling points and why they are such good solvents, can readily be understood. FigureDimers of dimethyl sulfoxide (DMSO; left) and dimethyl sulfone (DMSO2; right) showing O S—O -hole bonding and C H—O hydrogen bonding. Sulfur atoms are yellow, oxygens are red, carbons are gray and hydrogens are white

Relationships between impact sensitivities and molecular surface electrostatic potentials of nitroaromatic and nitroheterocyclic molecules

For two classes of molecules that are stabilized by the delocalization of electronic charge, nitroaromatics and nitroheterocycles, we have shown that their measured impact sensitivities can be related quantitatively to the degrees of internal charge separation and the presence of strongly positive electrostatic potential maxima on their molecular surfaces. These latter properties have been obtained through ab initio HF/STO-5G* and HF/6-31G* calculations. We suggest that a key factor in determining the impact sensitivities of these compounds may be the extent to which the stabilizing effect of charge delocalization has been counteracted.

Statistically-based interaction indices derived from molecular surface electrostatic potentials: a general interaction properties function (GIPF)

Abstract A number of physical properties determined primarily by non-covalent interactions can be expressed quantitatively in terms of molecular surface area plus three statistically-based quantities obtained from the surface electrostatic potential: Π, a measure of local polarity; σ 2 tot , which indicates the variability of the potential on the surface; v , a measure of the balance between positive and negative regions. In the applications discussed, these quantities and the area are obtained through ab initio computations. The various specific relationships can be summarized through a general interaction properties function (GIPF), property = f (area, Π, σ 2 tot , v ) the functional form of which depends upon the property of interest.

Computational investigation of the structures and relative stabilities of amino/nitro derivatives of ethylene

Eight amino and/or nitro derivatives of ethylene have been investigated computationally at the density functional B3P86/631 + G** level. The molecular geometries and relative stabilities reflect the varying roles of ‘push‐pull’ electronic delocalization and intramolecular hydrogen bonding. The same two factors affect, to varying extents, the computed C‐NO2 and C‐NH2 bond dissociation energies, which are also presented, as are the heats of formation, vaporization and sublimation of the three diaminodinitroethylenes. The potential of the latter as energetic compounds is briefly discussed. q 1998 Elsevier Science B.V. All rights reserved

A relationship between impact sensitivity and the electrostatic potentials at the midpoints of CNO2 bonds in nitroaromatics

Abstract We have modified and extended an earlier relationship between nitroaromatic impact sensitivity and electrostatic potentials at the midpoints of CNO 2 bonds. Because of the anomalous effect of the hydroxyl group on nitroaromatic sensitivities, possibly due to the formation of unstable nitronic acid tautomers, hydroxynitroaromatics have been excluded from the plotted data set. Our results suggest that the instability of an individual CNO 2 linkage may be a key factor in initiating decomposition induced by impact.

Shock-sensitivity relationships for nitramines and nitroaliphatics

Abstract For groups of nitramines and nitroaliphatics, taken separately it is shown that shock, sensitivity is related to the strengths of all of NNO2 and/or CNO2 bonds in the molecule, taken in conjunction with its overall size. The reciprocals of the bond lengths are used measures of bond strengths, while molecular weight is taken as the indicator of molecular size. Linear correlation coefficients of 0.94 and 0.98 are obtained, respectively, for nitramines and nitroaliphatics of a variety of structural types. These results are indicative of the importance of the NNO2 and CNO2 bonds in the shock-induced decomposition of nitramines and nitroaliphatics.

Computational characterization of energetic materials

Abstract We present an overview, focusing primarily upon the past ten years, of our work relating to the design, characterization and evaluation of new and proposed energetic compounds. Our approach has been entirely computational, at ab initio and density functional levels. Several areas are discussed in some detail: (a) assessment of thermodynamic and kinetic stabilities; (b) investigation of factors affecting impact/shock sensitivities; (c) calculation of gas, liquid and solid phase heats of formation; (d) determination of thermodynamic/kinetic data and reaction mechanisms for decomposition and combustion processes; and (e) evaluation of possible synthetic pathways.