Sándor Fliszár

Charge Distributions and Chemical Effects

Abstract SCF—xα—SW calculations have been performed for the electronic structure of methane, ethane, propane, isobutane and neopentane. The touching and overlapping sphere models with different degrees of overlap were used. The charge distributions were partitioned into atomic charges by means of a modified overlapping sphere model. These charges which indicate a C + H − polarity reflect closely the customary inductive effects and are unique in that they represent the only set of theoretically determined charges which can be related directly to experiment.

Atoms, chemical bonds and bond dissociation energies

Core and Valence Regions of Atoms.- The Valence Region of Molecules.- The Chemical Bond.- Bond Dissociation Energies.- Electronic Charge Distributions.- Applications.- Assessment.

On the dissociative nature of the first excited states of nitromethane

Abstract SCF + Cl ab initio calculations on nitromethane show the multiconfigurational nature of the ground and first excited singlet states. The potential surfaces relatives to the CN bond lengths are investigated; each of the excited states studied is strongly predissociative. A re-evaluation of recent picosecond UV photolysis experiments in terms of the calculated potential surfaces and the primary photoreaction leading to electronically excited nitrogen dioxide and methyl radicals is presented.

Charge distributions and chemical effects. XVI. Valence electron energies in atoms and ions

Valence region energies of first‐row atoms and ions of nuclear charge Z, with N1 core and N valence electrons are calculated from the formula EN=−(3/7) (Z−N1) N〈r−12〉, where 〈r−12〉 is the average inverse distance of the N electrons from the nucleus. The loss of an electron is described in terms of a shrinking of the valence region, 〈r−12〉N −〈r−12〉N−1= (∂〈r−12〉/∂N)0 [N/(N−1)], where the derivative β = (∂〈r−12〉/∂N)0 can be treated as a constant for the removal of any p electron. In this approximation it is 〈r−12〉N=〈r−12〉N=2 +β[N/(N−1)+(N−1)/(N−2)+⋅⋅⋅] where 〈r−12〉N=2 is the value for systems with two valence region electrons. Finally, since the ratio α=〈r−12〉N=2/(Z−N1) is remarkably constant, we have all the elements for evaluating 〈r−12〉N and, consequently, the valence region energy EN of the N valence electron ion or atom.

Charge distributions and chemical effects. XVII. Valence region energies and electronegativity of atoms and charged species

The derivative (∂EN/∂N)z of the N valence‐electron energry in atoms and charged species can be expressed in a simple manner using the Politzer–Parr partitioning into core and valence regions. A finite difference approximation is presented for (∂EN/∂N)z, thus avoiding the strategy of expressing EN in terms of an approximation leading to (∂EN/∂N)z as a function of this approximation.

Influence of the physical state of an explosive on its sensitivity. Is nitromethane sensitive or insensitive

Abstract Nitromethane is selected as a typical example illustrating the complexity of detonation phenomena, which depend on both the physical state of an explosive and the dynamic or static nature of the stimulus. Self-consistent field calculations indicate that the crystalline environment significantly reinforces the C-N bond in nitromethane, by about 4.7 kcal mol −1 relative to a gas phase molecule, and offer arguments in the discussion of pertinent static high presure (diamond anvil) experiments on nitromethane. Although changes in bond dissociation energies dependent upon the physical state of a substance are certainly relevant, a critical review also illustrates that an oversimplified one-toone correspondence between sensitivity and molecular structure should be warned against in the context of safety rules.

Xα local spin density calculations. First-row diatomic molecules

Abstract The Xα local spin density approximation is examined, with special reference to the selection of the appropriate α parameter. A simple rule is developed for molecules, on the basis of α terms reproducing the Hartree-Fock energies of the isolated atoms, giving sufficiently accurate α values for use in the calculation of intemuclear distances and vibrational frequencies. An additional refinement is required, however, for the study of dissociation energies. Detailed examples, worked out for the first-row diatomics, illustrate both the merits and the limitations of X α theory. Calculated intemuclear distances, vibrational frequencies and dissociation energies agree within about 0.030–0.037 bohr, about 100 cm −1 and about 0.17 eV, respectively, with their experimental coun-terparts. It is also shown under what precise conditions X α theory can prove a valuable alternative for the study of atomization energies, at a level approaching experimental accuracy, thus explaining earlier successes in applications to organic chemistry.

Charge distributions and chemical effects. XVIII. On the relationship between total valence‐electron energies and nuclear–electronic interaction energies in atoms and ions

Valence shell electronic energies EN of atoms and ions, interpreted as multiplet averages, can be calculated accurately within the framework of a theory developed in terms of nuclear–electronic interaction energies Vne. The results point to a constant EN/Vne ratio, both for neutral atoms and charged species.

Applications of density functional theory approaching chemical accuracy to the study of typical carbon-carbon and carbon-hydrogen bonds

Abstract This article examines the merits of cost-efficient density functional variants in the computation of prototype hydrocarbons: CH4, C2H6, C2H4, C2H2, benzene and cyclopropane. Inclusion of some Hartree-Fock exchange in a fully self-consistent density functional approach with gradient corrections leads to atomization energies close to experimental accuracy. Validation of Politzers energy formula, E molecule  ∑ k V k mol γ k mol , where Vkmol is the potential energy involving the nucleus of atom k in the molecule and γkmol is a parameter ( ∼ 7 3 for atoms other than H), with better than Hartree-Fock wave functions provides a useful link with classical chemical concepts, namely bond energies.

Charge distributions and chemical effects. XXII. On the partitioning of molecular energies and the relationships between energy components

The exact quantum mechanical formulation of atomic and molecular energies and the postulate that ’’chemical bonds’’ exist combine to show that the nuclear–electronic and nuclear–nuclear potential energy V (k,mol) involving the kth atom in a molecule is for its largest part determined by ’’local effects’’ related to the number and type of bonds formed by k. These local effects are measured by the derivatives ∂ekj/∂Zk of the bond energies ekj involving k, with respect to its nuclear charge Zk. To a good approximation, neglecting the small nonbonded contributions, V (k,mol) =Vne (free atom k)−ZkjJ∂ekj/∂Zk. Applications to saturated hydrocarbons at the level of experimental accuracy indicate that the Emol/(Vne+2Vnn) ratios derived in this manner are close to their ab initio counterparts. These ratios are averages KmolAv=JKmolkV (k,mol)/JV (k,mol) of the ratios Kmolk=Ek(mol)/V (k,mol) defining the individual total energies Ek(mol) of atoms being part of a molecule, whereby Emol=JEk(mol). The Kmolk’s, in turn, ...