Gary V. Pfeiffer

Structures and properties of linear Cn molecules

Abstract Total energies, bond lengths, charge distributions, electronic configurations, and cohesive energies are calculated for linear C n clusters, n = 2–6. The calculations are done at the single determinant Hartree-Fock level using both double-zeta and double-zeta plus polarization basis sets.

Theoretical study of linear Cn (n=6−10) and HCnH (n=2−10) molecules

The geometric and electronic structures and the total and binding (chemisorption) energies of linear Cn (n=6−10) and HCnH (n=2−10) molecules have been predicted via RHF ab initio calculations. For Cn clusters the bonding structures are predicted to be cumulene type, with n-even clusters having triplet ground states and n-odd species having singlet ground states. For HCnH molecules, the n-even species are predicted to have singlet ground states with polyyne-type bonding while the n-odd molecules are predicted to have triplet ground states with intermediate cumulene—polyyne-type bonding.

Structures of C5

Abstract Various geometric isomers of C 5 have been studied via ab initio calculations in order to determine the equilibrium structure. Hartree-Fock calculations were performed using both double-zeta and double-zeta plus polarization basis sets. Electron correlation was included via second-order Mriller-Plesset perturbation theory. It is concluded that C 5 is linear, in agreement with recent laser vaporization studies.

Theoretical Studies of the Binding Energy and Geometry of the H5+ Molecular Ion

The geometry and binding energy of the H5+ molecular ion have been examined by two different ab initio quantum mechanical variational methods. In the first a CI wavefunction was made from the 10 covalent valence‐bond structures which could be constructed from 1s orbitals at the nuclei, each 1s orbital being represented by a five‐term Gaussian expansion and having a variable scale factor. For geometries identical or similar to the D2d geometry previously predicted from analogous calculations with cruder basis sets, we found no stability with respect to H3++H2. Other geometries were examined, especially those arising naturally from the approach of H3+ and H2; however, binding was never greater than a tiny 0.6 kcal/mole. We thus concluded that the method had failed adequately to describe H5+, as it had for H3+, and that it is probably unreliable for studying ions with small binding energies. The second method used the SCF MO model with a flexible basis set to account for distortion and polarization. This gav...

Ab initio Valence-Bond Calculations of H20 I. Bond Dissociation Energies

The first and second bond dissociation energies for H2O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H2O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H2O at this geometry was −76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was −75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H2O → OH + H, D1=75.38 kcal/mole and OH → O+H, D2=54.79 kcal/mole.ZusammenfassungDie ersten und zweiten Dissoziationsenergien der Bindungen von H2O wurden mit einemab initio Verfahren nach der Valenzstrukturmethode berechnet. Die Basis bestand aus einer minimalen Anzahl von nicht-orthogonalen Atomorbitalen, die durch Gaußfunktionen ausgedrückt wurden. Die beteiligten Valenzstrukturen beschrieben in geeigneter Weise den Wechsel in der Molekülstruktur bei Abspaltung der einzelnen Wasserstoffatome. Die berechnete Gleichgewichtsgeometrie des H2O-Moleküls hat eine O-H-Bindungslänge von 1,83 Bohr und einen HOH-Winkel von 106,5°. Mit 49 Valenzstrukturen betrug die Energie des H2O bei dieser Geometrie −76,0202 Hartree. Die berechnete Bindungslänge des OH-Radikals für das Gleichgewicht betrug 1,86 Bohr und die Energie wurde mit derselben Basis zu −75,3875 Hartree berechnet. Nach Korrekturen für die Nullpunktenergien betrugen die berechneten Dissoziationsenergien der Bindungen: H2O+ → OH + H, D1=75,38 kcal/Mol und OH → O+H, D2=54,79 kcal/Mol.

Dependence of Correlation Energy upon Bond Angle: Investigation of Interactions between Nearly Degenerate Electronic Configurations

Configuration interaction of low‐lying electronic states is used to investigate the molecular correlation energy as a function of bond angle. Qualitative aspects of the problem are discussed using the molecular‐orbital diagrams of Walsh and Mulliken. Results of configuration‐interaction calculations of BeH2 and NO2+ are reported. The possibility of double minima in the potential surfaces for AB2 molecules is considered.

Theoretical study of linear LiCnLi (n=2–8) molecules

Abstract Restricted Hartree-Fock calculations have been carried out on linear LiC n Li ( n =2–8) molecules. Ground state electronic structures, total energies, binding energies, and optimized geometries are reported. The possibility of basis set superposition error is examined. Extensive bond pattern change is found by comparison with linear C n molecules.

Ab initio valence-bond calculations of H2O

A new technique for the determination of atomic valence states and their energies has been developed and applied to the water molecule and hydroxyl radical. This method is applicable to valence-bond studies involving a large number of resonance structures rather than simply a one structure perfect pairing approach. The original basis of resonance structures is transformed into a basis of approximate composite functions which are orthogonal and non-interacting for the separated atoms. The equilibrium molecular eigenfunction is analyzed in terms of the composite functions by means of structure projections. A description of the valence states and the promotional energies of each of the component atoms in H2O and OH is obtained.

Structures of C5 and C6

Recent experiments involving laser vaporization of graphite, followed by supersonic expansion, 1–4 have rekindled interest in the structures of small carbon clusters. While theoretical calculations have firmly established the structure(s) of 04, 5–8 relatively little theoretical work has thusfar been reported for C5 and C5, most such studies being small model calculations on diamond and graphite. There has been, however, a MINDO/2 geometry search on small carbon clusters which included C5 and C6 9. in that work C5 was found to be trigonal bipyramidal, and C5 was found to be a distorted octahedron of C2V symmetry. The available experimental data, from matrix isolation studies, indicate that C5 and C5 are linear.10–11