Interactions valence isomers

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Compounds 405, which are nitrogen analogs of a,a -dehydro-naphtho[a/]thiapyrans 383, are unknown. However, their valence isomers 404, obtained on interaction of acenaphthylene with N,N-dichlorosulfa-mides, have been described (80MI3). 

On the other hand, shock waves generate high pressures as well as high temperatures, and, consequently, some fector in addition to heat must be involved in the shock reactioa Drickamer [145], for example, has suggested a close relationship between photochemistry and liigh-pressure chemistry. He experimentally showed that high-pressure conditions promoted the formation of pentacene dimers with cross-linked structure, the formation of which usually occurred in the photochemical reaction. If the shock reaction is a type of some reactions in excited states such as a photochemical reaction, many valence isomers such as Dewar benzene and benzvalene would be generated from benzene by shock waves, and the interaction between these isomers would produce various com-poimds such as derivatives of fiilvene. Such valence isomers are imstable and would not have been detected in our study. 

The best studied groups of valence isomers of benzenium ions are those of type D — bicyclop.l.l]hexenyl cations. This type derives from the interaction of the Dewar hexamethylbenzene with electrophilic agents, in particular with acids. Thus, with liquid HCl in CH Clj at —100 C a proton is added to form the 5-endo-H-l,2,3,4,5,6-hexamethylbicyclop.l.l]hexenyl cation (15S) which at a higher temperature is partially converted into the 5-exo-H-isomer (15 )... 

Perfluoropentaethylmethylbenzene isomerizes photochemically to a mixture of Dewar-benzene isomers. Further irradiation converts these into the prismanes. These valence isomers of benzene are remarkably stable, and appear to owe their stability, in part, to destabilization of the parent benzene by non-bonded interactions. 

Tri-f-butylbenzene undergoes photoisomerization to give various products of valence isomers [3]. Possibly bulky fert-butyl group in the aromatic ring induces steric interaction to facilitate this isomerization. It is diflticult to predict the exact mechanism of formation of all these photoproducts. 

H-TaS2 intercalated with hydrated sodium The quadmpole interaction increases upon Na intercalation, and the isomer shift increases to a higher (more positive) value reflecting a lower valence at the Ta site. 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

The electric monopole interaction is a function of the s electron densities at the nucleus. This results in a displacement of the spectrum and is expressed as the velocity of the source (mm s ) necessary to counteract the displacement. This isomer (or chemical) shift, 6, provides information about the coordination number, the valency and spin state of the iron in the compound. 

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