Nonaltemant hydrocarbons

Polycyclic Hydrocarbons, Nonaltemant Compounds with Five Fused Rings Polycyclic Hydrocarbons, Nonalternant Compounds with Fused Rings Polycyclic Hydrocarbons, Nonalternant Compounds with More Than Five Fused Rings Indeno(l,2,3-crf)pyrene... 

Fluoranthene under Polycyclic Hydrocarbons, Nonaltemant Compounds with Four Fused Rings Fluorene under Polycyclic Hydrocarbons, Nonaltemant Compounds with Two or Three Fused Rings... 

Aromatic hydrocarbons can be divided into two types alternant and nonalternant.In alternant hydrocarbons, the conjugated carbon atoms can be divided into two sets such that no two atoms of the same set are directly linked. For convenience, one set may be starred. Naphthalene is an alternant and azulene a nonaltemant hydrocarbon ... 

For nonaltemant hydrocarbons the energies of the bonding and antibonding orbitals are not equal and opposite and charge distributions are not the same in... 

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

Table 2. Energies and symmetries of the second excited stales of nonaltemant hydrocarbons...

The best-known exception to Kasha s rule is the anomalous fluorescence displayed by azulene and its derivatives (nonaltemant hydrocarbons) and some aliphatic and aromatic thioketones. 

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-... 

The new nonaltemating aromatic hydrocarbon (4, benzo[4.5]cyclohepta[[Pg.131]

For nonaltemant hydrocarbons, however, a bathochromic or a hypso-chromic shift may result, depending on the absolute magnitude of the LCAO coefficients of the HOMO and LUMO. This is also shown in Figure 2.26 and is clearly illustrated by the absorption spectra of azulene, 4-azaazulene, and S-azaazulene, given in Figure 2.28. 

Most of the compounds described here are nonaltemant and heterocyclic. A rigorous theoretical treatment of such compounds is extremely difficult and, even for the related isoconjugate hydrocarbons, far from conclusive.From the chemistry of cyclazines, however, it emerges that many questions in which experimentalists are interested can be answered in a satisfactory way by a PMO treatment based on simple HMO calculations. 

The first absorption band of nonaltemant hydrocarbons and the band shifts induced by substitution are generally well described by HMO theory (Section 4.7). Absorption to Si corresponds to the HOMO LUMO transition. Nonradiative decay often dominates the photophysical properties of nonaltemant hydrocarbons and also alternant hydrocarbons with a 4 -membered ring (biphenylene), so that they generally have short singlet lifetimes and low triplet yields and are less prone to undergo photoreactions upon direct irradiation. 

Pandey, S., Acree, W. A., and Fetzer, J. C., Cetylpyridinium chloride rrticelles as a selective fluorescence quenching solvent media for discrirrtinating between alternant versus nonaltemant polycyclic aromatic hydrocarbons, Talanta, 45, 39-45, 1997. 

There are four classical nonaltemant jr-electron hydrocarbons which have long been the subject of many studies of their aromatic character fulvene, heptafulvene, azulene, and pentalene. " Eulvene, heptafulvene, and pentalene have been considered for a long time as nonaromatic (or even antiaromatic), both for their chemical Instability and the negative REPE values. More recent studies strongly support this hypothesis. The X-ray determined geometry of heptafulvene" allowed us to estimate the aromaticity the HOMA value for the ring is equal to 0.257. The case of fulvene is more complicated—only geometries for the substituted species are known. The mean HOMA value calculated for 11 exocyclically substituted fulvenes is 0.001. ... 

Methylenecyclopropene (structure b in Problem 4.4) is a nonaltemant hydrocarbon. What did the results of the HMO calculation for methylenecycloprojjene indicate about the charge density on each atom Do those results agree with the statement on page 195 about the nature of alternant and nonaltemant systems How could you have predicted the polarity of methylenecyclopropene on the basis of resonance theory ... 

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