Abstract
Approximately two-thirds of the chapters in these two companion volumes are devoted to methods of obtaining high-accuracy electronic wave functions for molecules and solids. The remaining third are concerned with particular chemical species or properties, and our chapter fits the latter category. Within this category the extensive literature on barriers offers two special opportunities of general interest to chemical theorists. First, it is possible to make rather definitive statements on the quality of wave functions required to yield quantitative predictions. Second, methods for analyzing ab initio wave functions to ascertain the physical origin of the barrier and provide a quantum mechanically well-defined, but simple picture of the mechanism have been more extensively developed for this topic than any other.
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References
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Note Added in Proof
The contrasting roles of orbital orthogonality and electron exchange are further clarified in a recent paper by Levy.(277) Levy has found that application of Edmiston-Ruedenberg exchange localization without orbital orthogonality constraints generates localized orbitals that are nearly orthogonal. This result helps rationalize the apparent dependence of some barrier models on electron exchange energy. Exchange energy minimization has little intrinsic importance for rotational barrier mechanisms; but exchange energy minimization tends to orthogonalize orbitals, and orthogonality is important for the barrier mechanism.
Brunck and Weinhold(278) attribute rotational barriers in ethane, methylamine, and methanol to vicinal mixing between bonds and antibonds. Their key step is expansion of the INDO Hamiltonian matrix in a basis set of local bonding and antibonding orbitals. Since the rotational barriers disappear if the pseudomolecular orbitals are constructed as a linear combination of local bonding orbitals, they claim that vicinal mixing between bonds and antibonds is at the heart of barriers. This model has a strong intuitive appeal. In order to establish its credibility, further work is needed on the following problems: First, the definitions of local bonding and antibonding orbitals are arbitrary. It is not clear that the model would hold up under small adjustments in the bond orbitals. Second, the balance between INDO matrix elements is often quite different from that in an ab initio theory. Third, a barrier model should not be sensitive to geometry optimization if total energy is insensitive. Because INDO barrier heights are poor when geometries are optimized, any barrier model derived from INDO wave functions is tentatively best.
Another interesting paper that is formulated within the framework of a one-electron orbital theory and addresses a long-standing problem is that of Salem, Hoffmann, and Otto on barriers in substituted ethanes.(279)
M. Levy, Unconstrained exchange localization and distant orbital tails, J. Chem. Phys. 65, 2473–2475 (1976).
T. K. Brunck and F. Weinhold, Quantum-mechanical origin of barriers to internal rotation about single bonds, J. Am. Chem. Soc. (1977), in press.
L. Salem, R. Hoffmann, and P. Otto, The energy of substituted ethanes: Asymmetry orbitals, Proc. Nat. Acad. Sci. (U.S.A.), 70, 531–532 (1970).
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Payne, P.W., Allen, L.C. (1977). Barriers to Rotation and Inversion. In: Schaefer, H.F. (eds) Applications of Electronic Structure Theory. Modern Theoretical Chemistry, vol 4. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-8541-7_2
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