Atomic orbitals rigid

In the Hiickel treatment, atomic orbitals on nonadjacent atoms are assumed to have no interacticai. They are neither bonding nor antibonding. The concept of homoconjugation suggests that such orbitals may interact, especially in rigid structures which direct orbitals toward one another. Consider, for example, bicyclo[2.2.1]hepta-2,S-diene ... 

A similar qualitative approach, although with a less rigid separation between the starting atomic orbitals, has been applied by Dahl to Fe5(CO)i3C and [M2Ni3(CO)i6]2- (M = Cr, Mo, W) (26,111). 

E is calculated by summing and integrating the forces on the nuclei as the atoms approach each other due to the superposition of rigid atom density functions using Slater atomic orbitals. Eq is approximated by the change in one-electron molecular orbital energy, A mo> calculated using... 

There is no quantum-mechanical evidence for spatially directed bonds between the atoms in a molecule. Directed valency is an assumption, made in analogy with the classical definition of molecular frameworks, stabilized by rigid links between atoms. Attempts to rationalize the occurrence of these presumed covalent bonds resulted in the notion of orbital hybridization, probably the single most misleading concept of theoretical chemistry. As chemistry is traditionally introduced at the elementary level by medium of atomic orbitals, chemists are conditioned to equate molecular shape with orbital hybridization, and reluctant to consider alternative models. Here is another attempt to reconsider the issue in balanced perspective. 

Spin properties are notoriously difficult to calculate accurately57. Here, we are actually calculating spin populations, with their intrinsic uncertainties, and not the directly observed hyperfine interactions. On the other hand, analyses of the hyperfine interactions in the ESR spectra to give experimental atomic orbital occupancies for the radical electron are based on a simplistic, rigid linear combination of atomic orbitals (LCAO)-MO model with the reference electron-nuclear coupling parameters taken from the free atom. No allowance is made for radial or angular polarization of the atomic orbitals in the molecular environment. Thus agreement at these levels between calculated and experimental values can only be qualitative, at best. 

The interaction between the S3p atomic orbital and the highest occupied benzene tt orbitals in the case of thioanisole is indicated in Fig. 25. The resulting three levels correspond to the first three main PE bands " However, an additional complication is the non-rigidity of the molecule. Dewar et al. suggested the existence of a conformational equilibrium with two predominant conformers, one essentially planar with maximum conju tion and the other with the S3p orbital orthogonal to the tt system. The shoulder at 8.6 eV in the PE spectrum of thioanisole (Fig. 25) was attributed to the presence of ca. 10% of the latter conformer... 

There are molecular orbitals constructed from atomic orbitals, hybrid in general, belonging to certain atoms of a tridimensional rigid framework, and from orbitals of one or many metallic atoms. 

The bonding and geometry in a molecule can be described in terms of valence bond theory. In this theory, a bond is formed by the overlap of orbitals from two atoms. Hybrid orbitals, a set of equivalent orbitals formed by combining atomic orbitals, are often needed to describe this bond. Multiple bonds occur via the overlap of atomic orbitals to give cr bonds and tr bonds. Cis-trans isomers result from the molecular rigidity imposed by a tt bond. 

A 10.8 The Chemical Bond Formation Energy Based on Rigid Atomic Orbitals... 

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