The Hückel theory, with an extended basis set consisting of 2s and 2p carbon and 1s hydrogen orbitals, with inclusion of overlap and all interactions, yields a good qualitative solution of most hydrocarbon conformational problems. Calculations have been performed within the same parametrization for nearly all simple saturated and unsaturated compounds, testing a variety of geometries for each. Barriers to internal rotation, ring conformations, and geometrical isomerism are among the topics treated. Consistent σ and π charge distributions and overlap populations are obtained for aromatics and their relative roles discussed. For alkanes and alkenes charge distributions are also presented. Failures include overemphasis on steric factors, which leads to some incorrect isomerization energies; also the failure to predict strain energies. It is stressed that the geometry of a molecule appears to be its most predictable quality.
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A. Streitwieser, Molecular Orbital Theory (John Wiley & Sons, Inc., New York, 1961). R. Daudel, R. Lefebvre, C. Moser, Quantum Chemistry (Interscience Publishers, Inc., New York, 1959). B. Pullman and A. Pullman, Les théories élecironiques de la chimie organique (Masson et Cie., Paris, 1952).
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A recent example of the consequences of this attitude may be seen in the work of
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The exceptions are some diatomics and triatomics for which this theory fails; for instance the obvious case of the ground state of the hydrogen molecule.
11.
For the simplest LCAO and VB functions give a poor binding energy but an equilibrium separation within 10% of the correct value. Scaling, i.e., varying the Slater exponent, improves the energy somewhat and predicts the distance to 1% [see the review of calculations in
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(a) W. G. Dauben and K. S. Pitzer in Steric Effects in Organic Chemistry, edited by M. S. Newman (John Wiley & Sons, Inc., New York, 1956), p. 1;
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In the minimization procedure the staggered form stabilized at a slightly shorter C‐C distance than the eclipsed.
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R. M. Pitzer (to be published).
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The acetylene wavefunction we obtain compares favorably with the SCF functions calculated by
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28.
M. P. Gouterman has suggested that a careful search for transitions arising from excitations, and thus polarized perpendicular to the aromatic ring plane, would be useful in this respect. Our energy spectrum supports that given schematically by J. C. Slater, Quantum Theory of Molecules and Solids (McGraw‐Hill Book Company, Inc., New York, 1963), Vol. 1, p. 234.
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C. A. Coulson, Conference on Quantum Mechanical Methods in Valence Theory, Shelter Island, New York, 1961, p. 42.
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41.
Note that as usual with LCAO‐MO calculations, the wave‐ function does not have the correct behavior at infinity.
42.
It is interesting in this connection to note that in a calculation in which the Mulliken approximation was used for three‐ and four‐center integrals, the binding energy was also overestimated. (L. Burnelle, Ref. 27).
43.
L. L. Lohr, Jr., has used a similar expression which differs from ours only in second order and has certain computational advantages.
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See also
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J. C. Slater, Quantum Theory of Molecules and Solids (McGraw‐Hill Book Company, Inc., New York, 1963), Vol. I, p. 108.
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