Cychc polyenes

A set of a-type atomic orbitals transform amongst themselves under point group operations like scalar quantities. Hence, to consttuct descent-in-symmetry-type LCAO cluster orbitals, we simply follow the same reasoning as for the linear and cychc polyenes, above, and define (unnormalized) cr-type cluster orbitals as... 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

It systems of cychc polyenes [22,23]. One finds here both charge-density-wave RHF solutions, where the bond indexes are alternant (one strong bond (2i, 2i+i) between two weak bonds (2i-i, 2i) and (2i+l, 2i+2) and spin-density-wave UHF solutions where the electrons are spin-alternant (one a electron on atom 2i surrounded by two pelectrons on atoms 2i l). The first one does not "dissociate" properly (when t/u tends to zero), since it remains half neutral and half ionic but it reduces the weight of the most irrelevant VB situations with respect to their importance in the symmetry-adapted solution. The charge-density-wave solution tends to localize the electrons by (a,P)pairs on the "strong bonds", each one supporting a localized MO... 

Besides the common olefinic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acychc electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cycHc polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

Restricted to 7t-electrons. These methods exist for the calculation of electronically excited states of polyenes, both cycHc and linear. These methods, such as the Pariser-Parr-Pople method (PPP), can provide good estimates of the 7t-electronic excited states, when parameterized well. Indeed, for many years, the PPP method outperformed ah initio excited state calculations [6]. 

Chemists have argued for years about the special stabihty called aromaticity that attends cychc, planar polyenes with the proper numbers of n electrons. The wonderfiil quote that begins this chapter is not so far from an accurate representation of iemarl heard at many a chemical conference However, even though it is not easy to define this shppery concept precisely, especially in marginal cases, it is not difficult to understand the simple examples, to see why the number of ti electrons is important, and to learn which molecules are fikely to exhibit this property. 

The Frost circle allows you to determine quickly the relative energies of the molecular orbitals for any planar, cychc, fully conjugated polyene. Remember. The polygon must be inscribed vertex down. 

Matrix isolated aryl carbenes have yielded routes to a number of strained polyenic systems, including strained cyclopropenes and cychc cumulenes. For example, photolysis of phenyldiazomethane (65) with visible hght (A, > 478 nm) gave phenyhnethylene (66), which was characterized by its highly structured UV absorptions and its IR spectrum (Scheme 15). Further irradiation of 66 at shorter wavelengths (A > 416 nm) prod need a new species with a broad UV absorption with 271 nm and IR absorptions... 

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