James R. Hoyland

Internal Rotation in Propane: Reanalysis of the Microwave Spectrum and Quantum‐Mechanical Calculations

Quantum‐mechanical calculations are carried out on various conformations of the propane molecule using a Gaussian basis set and the Hartree–Fock–Roothaan formalism. These calculations indicate that internal rotation in propane can be adequately described by a simple equation of the form E(α1, α2) = 12V0 (2 − cos3α1 − cos3α2)−12V1[1 − cos3(α1 + α2)] where α1 and β2 are the two angles describing the rotation of the two methyl groups. The microwave spectrum of propane in the first excited torsional states reported by Hirota, Matsumura, and Morino was analyzed assuming this potential, leading to values for V0 and V1 of 3575 ± 100 cal/mole and 310 ± 40 cal/mole, respectively.

Ab Initio Bond‐Orbital Calculations. II. An Improved Procedure for Saturated Hydrocarbons

The bond‐orbital approach to calculations on saturated hydrocarbons given previously is improved by use of a more optimum basis set and through a more judicious choice of the bond orbitals. Calculations are carried out on methane, ethane, propane, and butane. Energy values, population analysis, and various computed properties are given and compared with the results of SCF calculations utilizing the same basis set.

Internal Rotation in Butane

Quantum‐mechanical calculations within the Hartree–Fock framework and using two Gaussian basis sets are carried out on several conformations of the butane molecule. The smaller basis set predicts a transgauche barrier of 3.536 kcal/mole, an energy difference of 0.822 kcal/mole between the gauche and trans forms, and a barrier of 6.821 kcal/mole for interconversion of the two gauche conformations. The values of these quantities as given by the larger basis set are 3.619, 0.761, and 6.834 kcal/mole. Taking the angle of rotation about the central bond to be 0 for the trans conformation, the gauche form is predicted to occur at 110.9° for the smaller and at 111.3° for the larger basis set. The barrier to internal rotation of a methyl group is predicted to be 2.92 kcal/mole and 2.94 kcal/mole for the smaller and larger basis sets, respectively.

MINDO/2 calculations on the reaction of methyl radicals with ethylene and butadiene

Semiempirical calculations using the MINDO/2 procedure have been carried out on the potential surface for the reaction of a methyl radical with ethylene and trans-butadiene. The transition state is predicted to be reactant-like in character and no evidence of resonance stabilization of the activated complex is found for butadiene. It is conjectured that the experimentally observed lowering of the activation energy for butadiene relative to ethylene may be attributed to differential correlation effects.

Improved Single‐Center LCAO SCF Calculations on HF and CH4

Calculations within the LCAO SCF framework utilizing large basis sets of Slater orbitals centered at the heavy nucleus are carried out on HF and CH4 in order to assess the feasibility of computing near‐Hartree—Fock wavefunctions and energies for simple AHn‐type molecules. Considerable improvement is obtained over previous results found by the single‐center method, but it is concluded that it is not economically possible to extend the calculations to the accuracy given by poly‐centered methods.

One‐ and Two‐Center Calculations on the Lowest‐Lying Π States of HeH+

Accurate two‐center calculations using a basis set of elliptical orbitals are made on the lowest‐lying singlet and triplet‐Π states of HeH+. The calculated dissociation energies of these states are 0.234 eV for the singlet and 0.147 eV for the triplet. Single‐center wavefunctions for these two states are also computed at the equilibrium internuclear separation predicted by the two‐center results (8.05 bohrs for the singlet and 7.67 bohrs for the triplet). The differences in the energies of the one‐ and two‐center calculations are 144 cal and 94 cal for the singlet and triplet, respectively. Expectation values of several operators are given along with calculated values of the spectroscopic constants.

An improved iterative Extended Hückel method for conjugated molecules

The deficiencies of iterative extended Hückel theory as applied to conjugated molecules are discussed. An improved treatment is suggested which sacrifices complete rotational invariance but which is capable of a better representation of the πt-electron energy levels and molecular ionization potentials.

Semiempirical MO Theories: A Critique and a Review of Progress

The basic principles and methods of quantum mechanics have gradually become familiar to an increasingly large number of scientists working in many diverse areas of research. At the present time, application of quantum mechanical methods to such problems as protein structure, drug design, hydrogen bonding, and many other phenomena important in biological studies is becoming common. Nearly all such applications to phenomena of biological interest are made by means of some type of semiempirical formulation. Therefore it seems of interest to review these semiempirical methods and to discuss in detail their advantages and shortcomings. This chapter is an attempt at such a review and will be restricted to a discussion of empirical and semiempirical molecular orbital (MO) approaches. Emphasis will be placed on recent all-valence electron approaches and no discussion of the older methods will be given since these latter formulations have already been exhaus- tively reviewed elsewhere.

Use of simple thermodynamic and structural parameters to predict self-reactivity hazard ratings of chemicals☆

Abstract A major problem in the transportation, transfer and storage of bulk chemicals is the problem of catastrophic instability under unforeseen situations. N