John D. Goddard

A note on a non-empirical molecular orbital study of some cytosine and thymine tautomers

The relative stabilities of a series of cytosine and thymine tautomers have been investigated by means of non-empirical (ab initio) LCAO-MO-SCF Hartree-Fock-Roothaan calculations employing a small contracted Gaussian basis set (STO-3G) in all cases. The relative stabilities of the various tautomeric forms agree in general with the results of earlier empirical and semi-empirical calculations on these molecules. In particular, the “enol forms” of cytosine and thymine, within the isolated molecule approximation inherent in these MO calculations, are predicted to possess greater stability than the forms commonly assigned to these molecules in aqueous solution and found in the solid state by X-ray crystallographic studies.

AB Initio molecular orbital calculations on the si2h4molecule

Abstract Ab initio molecular orbital calculations using an extended Gaussian basis set have been performed on C 2 H 4 , CH 2 SiH 2 and Si 2 H 4 . The species CH 2 and SiH 2 have also been examined. Geometries were partially optimized and the energy difference between the planar singlet and orthogonal or twist triplet geometries of Si 2 H 4 was studied in order to provide a measure of the strength of the Si-Si bond in this molecule. Mulliken population analyses were carried out on CH 2 CH 2 and SiH 2 SiH 2 , to further study the nature of the Si-Si double bond in comparison with the C—C double bond.

The effects of optimization and scaling of AO exponents on molecular properties

The combined effects of direct optimization and various scaling techniques of Gaussian AO basis sets on the calculated molecular wavefunctions were analyzed for two examples, the hydrogen fluoride and ammonia molecules. An 8s4p Gaussian basis set for the fluorine atom which had been rigorously optimized for the total atomic energy by the conjugate gradient method was employed. For nitrogen, a similar atom optimized 9s5p basis set was employed. Three basis sets obtained in the course of the exponent optimization, the final optimized basis set, and basis sets derived from the optimum AO basis sets by various scalings of the exponents were employed in molecular calculations. The total energies and numerous one‐electron properties were analyzed. The convergence of molecular one‐electron properties to their limiting values is much slower than the energy convergence, and basis sets optimized for energy may be improved in terms of their property predictions by rigorous optimization. The simple scaling procedure ...

An analysis of Gaussian basis sets in terms of molecular one‐electron properties

A large number of nitrogen sp and hydrogen s Gaussian basis sets were obtained from the literature and employed in LCAO–MO–SCF calculations on ammonia at a fixed geometry. The resultant wavefunctions were analyzed in terms of a large number of one‐electron properties. Minimal, split valence, double zeta, and extended (sp/s) basis sets were considered. An assessment is made of the size and type of basis set necessary to yield values for various one‐electron properties that approach the best calculated or experimental results.

Ab initio molecular orbital calculations on thirene: The thermodynamic stability of five C2H2S isomers

Abstract A non-empirical SCF molecular orbital study using an STO-4G minimal basis set has been carried out on the thiirene molecule and on four of its C2H2S isomeric structures. The geometries of all five structures have been optimized in both their lowest singlet and triplet states. The computed thermodynamic stabilities of the various structures agree with experimental observations and, in general, with the predicted trends in the C2H2O analogs.

Theory of lone pairs. A relationship between orbital energy contributions and the second moments of localized orbitals in ten-electron hydrides

Abstract A relationship between e i the energy contribution of the i th localized molecular orbital, and 〈 R 3 〉 i = (〈 x ′ 2 〉〈 y ′ 2 〉〈 z ′ 2 〉) 1 2 , the “statistical effective volume” of the i th LMO, is discussed for a series of ten-electron hydrides. The equation 1/ e i = m (〈 R 3 〉 i ) 2 3 where m is a constant holds accurately for core, bond, and lone pair LMO densities.

Ab initio configuration interaction calculations on the hydrogen molecular excimer, H*4

Abstract Results of ab initio configuration interaction calculations are presented for a simple model of molecular excimers, (H 4 ) * . Preliminary SCF calculations as well as several levels of CI are discussed for the lowest three singlet and triplet states of H 4 . Square, rectangular, and regular trapezoid geometries of H 4 are examined. The results of this study are significantly better in a quantitative sense than earlier work on the excited states of H 4 .

A note on scf mo ci calculations on the ground and low-lying excited states of rectangular H4: an excimer model system

Abstract Ab initio self-consistent field molecular orbital calculations in an extended gaussian basis set plus configuration interaction calculations have been performed on the rectangular approach of two hydrogen molecules with fixed bond lengths to form square H 4 . Results for the ground and two low-lying excited electronic states are reported. One of these excited electronic state of H 4 correlates with a ground state (X 1 Σ + g ) hydrogen molecule and a hydrogen molecule in its lowest singlet excited state (B 1 Σ + u ) at infinite intermolecular distance and, therefore, may be classified as an excimer state.

Ab initio molecular orbital calculations on some selected boron(I) hydrides

Abstract Ab initio LCAO-MO-SCF Hartree-Fock-Roothaan calculations have been carried out for a series of closed-shell boron hydrides (B 3+ , B + BH, BH 2 − ,BH 2 + , BH 3 and BH 4 − ). Koopmans theorem vertical ionization potentials for the core and highest occupied molecular orbitals are presented. Proton and hydride affinities of some of these molecules have been calculated along with the energies of reaction between various pairs of these boron hydrides.

Changes in molecular properties with changes in molecular geometry: A partitioning into electronic and nuclear components

Most one-electron properties vary with changes in molecular conformation. Although the nuclear component remains constant for some of the one-electron property changes and thus the overall change depends only on the electronic change this result is not general. Often the change in the nuclear component dominates the overall change in a molecular property. An analysis of the changes in a number of one-electron properties with changes in molecular geometry in terms of the changes in the nuclear and the electronic components is presented. The inversion of ammonia and the torsion of ethane were chosen as important examples of conformational changes and the changes in molecular one-electron properties studied.