Showing papers on "Ab initio quantum chemistry methods published in 1969"

Ab initio Calculations on cytosine, thymine and adenine

Abstract: An ab initio LCAO SCF calculation has been performed in a small contracted GTO basis set on three bases of ADN. A check of the representativity of the basis set is reported for formamide and pyrrole.

Ab Initio Calculations on the H2+D2=2HD Four‐Center Exchange Reaction. I. Elements of the Reaction Surface

Abstract: We present the results of ab initio calculations on some interesting regions of the reaction surface for the four‐center exchange reaction H_(2)+D_(2)=2HD. These calculations, which use a minimum basis set of Slater orbitals, indicate that for all geometries appropriate to the transition state of the reaction, a barrier height of at least 148 kcal/mole is present. This is far greater than the energy required to produce free radicals and more than three times the experimental energy of activation, 42 kcal/mole. Considering the sources and magnitudes for errors due to correlation and basis set restrictions, we estimate the barrier height for this exchange reaction to be 132 ± 20 kcal/mole exclusive of zero‐point energies. In this paper we discuss the surface as determined by configuration interaction techniques. We find that the most favorable geometries for the exchange reactions are the square, rhombus, and kite configurations. However, all of these states are unstable with respect to H_(2) + 2H. In addition we find no evidence of collision complexes for any of the likely transition state geometries. In the following paper we will examine the G1 wavefunctions for this system in order to obtain an understanding of the factors responsible for the shape of the surface.

Ab Initio Predictions for Very Small Ions

Abstract: Approximate ab initio calculations are made on all polyatomic unipositive ions with fewer than six electrons. Predictions are made regarding the stability of such ions. For those which are stable, estimates are made of binding energies, geometries, vibration frequencies, and center of charge. The wavefunctions are valence‐bond configuration‐interaction functions built form Gaussian orbitals. All possible valence‐bond configurations within our chosen basis are used in order to eliminate a serious prejudice regarding geometry. BeH2+, LiHeH+, He3+, LiH3+, LiH2+, He2H+, HeH3+, H5+, HeH2+, H4+, and H3+ are found to be stable. (More accurate calculations on H4+ show that this ion is not stable relative to H3+ and H.) He2H2+, HeH4+, and H6+ are found to be unstable. Two stable isomers of BeH2+ are found: a stable triangular form and a metastable linear form. A semiclassical model is used to interpret and extrapolate our results.

Five- and six-coordinated complexes of cobalt(II) with 2,2',2'-terpyridyl: Unusual structure and magnetism

Abstract: Cobalt(II) complexes of the type [Co(terpy)X2] (I), [Co(terpy)2]X2,nH2O (II), and [Co(terpy)2]Y2,nH2O (III), where X = Cl-, Br-, I-, NCS-, and Y = ClO4-, [BPh4]-, have been prepared and characterized by magnetic, spectroscopic, and conductance measurements. The ?mono? complexes (I) are 5-coordinated and monomeric, non-ionic in nitrobenzene, and have high-spin magnetic moments in the range 4.7-5.0 B.M. They are readily and reversibly converted into the ?bis? complexes (II) with the addition or loss of a molecule of terpyridyl: [Co(terpy)X2]+ terpy ↔ [Co(terpy)2]X2 The bis complexes (II) and (III) exhibit the characteristic features of the electronic cross-over phenomenon, their magnetic moments being strongly dependent on temperature, the anion X or Y, and water of crystallization. A model based on the 4T1-2E cross-over is set up which gives a good representation of the magnetic properties. Values of the difference between the zero-point energies of the 4T1 and 2E states are given for the various compounds. Attempts to isolate halogeno complexes of the type [Co(terpy)X]+ were unsuccessful.

Molecular Schrödinger Equation. IX. Square and Rectangular States of H4 and the Molecular Ions H43+ and H42+

Abstract: Our method for the solution of the molecular Schrodinger equation has been further developed and extended to the case of the four‐electron system Ab initio calculations are presented which lead to the electronic wavefunctions and energies of square and rectangular forms of H4 As a preliminary to the four‐electron studies, the energies and wavefunctions for a number of states of the ions H43+ and H42+ are calculated Complete contour maps for the potential energy surfaces of these systems are reported and the H4 surface is discussed in connection with the bimolecular isotope exchange reaction H2+D2→2HD Contour maps for the electron density distribution in square and rectangular H4 are presented and discussed

The preparation and magnetic properties of oxovanadium(IV) acetate

Abstract: The variation of the magnetic susceptibility of oxovanadium(IV) acetate with temperature may be fitted to the Ising model of an infinite linear antiferromagnetic chain with the exchange energy of -166 cm-1. On the basis of this and infrared evidence a structure for the compound is proposed.

Approximate ab initio calculations on polyatomic molecules. II

Abstract: A new method of molecular wave-function calculation, which combines the advantages of Slater and gaussian orbitals, is presented. The method is applied to a selection of small molecules displaying σ, π and ‘three-centre’ bonds.

Elastic Scattering, Diffusion, and Excitation Transfer of Metastable Helium in Helium

Abstract: The dynamics of the resonance collision of metastable and normal helium atoms has been examined and calculations have been performed for elastic scattering, diffusion, and excitation transfer. To facilitate this analysis, accurate ab initio calculations of the pertinent interaction potentials have been carried out. The use of these more accurate potential curves in the framework of the original dynamical formulation of the collision process by Buckingham and Dalgarno leads to serious discrepancies between calculations and experiment for the elastic scattering cross section and the coefficient of diffusion. The rate constant for the excitation transfer reaction, however, is in reasonable agreement with experiment, over a wide range of temperatures.

The hydroxyl stretching frequencies and conformations of flavan-4-ols and thiaflavan-4-ols,

Abstract: : The hydroxyl stretching frequencies of some 2,4-cis and 2,4-trans flavan-4-ols, thiaflavan-4-ols, and related compounds have been measured in dilute carbon tetrachloride solution. Characteristic frequencies are observed at 3626/cm (free hydroxyl), 3616, and 3597/cm. The 3616 and 3597/cm bands are assigned to pseudo-axial and pseudo-equatorial hydroxyl groups respectively. Evidence is presented to show that the frequency shifts arise from differences in the interaction of the hydroxyl group with the pi-elections of the fused benzene ring, and that these flavans and thiaflavans exist in half-chair, rather than sofa conformations. (Author)

Electronic structure of aminoborane: a comparison between non-empirical and semiempirical theoretical methods

Abstract: The electronic structure of aminoborane, BH2·NH2, has been calculated ab initio by use of extended basis sets of Gaussian-type orbitals. The binding energy (corrected for correlation) is 550 kcal. mole–1 and the energy of rotation about the B–N bond is 40·7 kcal. mole–1. The B–N bond is polar in the sense [graphic omitted] and the calculated dipole moment of the molecule is 2·071 D. The highest bonding orbital is of σ type with energy –12·0 eV. Extended Huckel calculations on the same compound give orbital energies which agree fairly well with those above whilst the charge distribution can be reproduced by the CNDO method. However neither of the semiempirical techniques yield good overall agreement with the ab initio calculations.

Ab initio calculations, including d-orbitals, of the electronic structure of the sulphate ion

Abstract: Ab initio molecular orbital calculations using a Gaussian basis set, of the electronic structure of the sulphate ion reveal the importance of 3d-orbitals in describing the bonding in this ion.