Hiickel theory double bond

The first calculations of the frontier orbitals for acrolein gave the HOMO coefficients on the C=C double bond of acrolein, with the a carbon having the larger coefficient. This failed to explain the regiochemistry, but only because the simple Hiickel theory that was used is notoriously weak in dealing with electron distribution in heteroatom-containing systems. Later calculations gave a better set of coefficients, as shown in Fig. 6.29. 

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

The first application of quantitative qnantnm theory to chemical species significantly more complex than the hydrogen atom was the work of HiickeP on unsaturated organic componnds, in 1930-1937 [19], This approach, in its simplest form, focuses on the p electrons of double bonds, aromatic rings and heteroatoms. Althongh Hiickel did not initially explicitly consider orbital hybridization (the concept is nsnally credited to Panling, 1931 [20]), the method as it became widely applied [21] confines itself to planar arrays of -hybridized atoms, nsnally carbon atoms, and evaluates the consequences of the interactions among the p electrons (Fig. 4.4). Actually, the simple Hiickel method has been occasionally applied to nonplanar systems [22]. Because of the importance of the concept of hybridization in the simple Hiickel method a brief discussion of this concept is warranted. 

All carbon-carbon bonds in the skeleton have 50% double bond character. This fact was later confirmed by X-ray diffraction studies. A simple free-electron model calculation shows that there is no energy gap between the valence and conduction bands and that the limit of the first UV-visible transition for an infinite chain is zero. Thus a simple free-electron model correctly reproduces the first UV transition with a metallic extrapolation for the infinite system. Conversely, in the polyene series, CH2=CH-(CH=CH) -CH=CH2, he had to disturb the constant potential using a sinusoidal potential in order to cover the experimental trends. The role of the sinusoidal potential is to take into account the structural bond alternation between bond lengths of single- and double-bond character. When applied to the infinite system, in this type of disturbed free-electron model or Hiickel-type theory, a non-zero energy gap is obtained (about 1.90 eV in Kuhn s calculation), as illustrated in Fig. 36.9. 

A theoretical study of the interaction of sulfur atoms with ethylene within the framework of the Extended Hiickel MO theory has been reported by HoflFmann and co-workers (19). Potential surface calculations revealed two minima for the 8( 02) + C2H4 system. The higher corresponds to vinyl mercaptan formation via C-H bond insertion, and the lower, lying about 20 kcal below the former, to the least-motion, symmetry-allowed addition of sulfur across the double bond. The two are viewed as competing concerted processes. Similar calculations for the... 

The bent bond picture, later restated in terms of localized molecular orbitals, extends the model presented by Pauling and Slater for ethylene to a stem with three centers. Walsh s model parallels that of Mulliken for the a, w model of the double bond widely used in Hiickel theory. Since the canonical orbitals are good models for the interpretation of photoelectron spectra (see Introduction) we will discuss the Walsh model briefly. 

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