Dewar benzene, geometry

Dewar benzene has actually been isolated, and found to revert only slowly to benzene (its half life is approximately 2 days at 25°C). This is remarkable given how similar its geometry is to that of benzene, and what is expected to be a huge thermodynamic driving force for the isomerization. The substituted Dewar benzene, 7, formed from photolysis of 1,2,4-tri-tert-butylbenzene, 6, is apparently even... 

Possible examples of the importance of the coincidence of transition-state and excited-state geometries and also the importance of the location of the essential reaction or localization of reaction energy are the chemiluminescent rearrangements of Dewar benzene [ 1 ] and Dewar acetophenone [2] to benzene and acetophenone, respectively (Lechtken et al., 1973 Turro et al., 1974c Turro et al., 1975). The efficiency of excited state production though quite low is approximately the same for both systems. Yet the triplet state of... 

All resonance structures must have identical geometries. Otherwise they do not represent the same molecule. For example, the following structure (known as Dewar benzene) is not a resonance form of benzene because it is not planar and has two less ir electrons. Because molecular geometry is linked to hybridization, it follows that hybridization also is unchanged for the atoms in resonance structures. (Note If it is assumed that the central bond in this structure is a bond, then it has the same number of electrons as benzene. However, in order for the p orbitals to overlap, the central carbon atoms would have to be much closer than they are in benzene, and this is yet another reason why Dewar benzene is an isolable compound rather than a resonance form of benzene.)... 

Benzene still provides fruitful labour for the dedicated photochemist. A three photon excitation study of the So(A, ) - SjfBju) transition in the neat liquid shows that under this condition internal states exist which make the experimentally observed 3 photon allowed pathway. A photochemical study of both benzene and pyridine shows that Dewar benzene structure formation involves S1-S2 state mixing by irradiation at 253.7 nm. For pyridine in a xenon matrix all photochemical reactions are quenched and evidence for the formation triplet state of this molecule is produced for the first time. The photophysical significance of PE surface geometries of the low... 

MINDO/3 calculations of the cycloreversion of vinylcyclobutane to ethylene and butadiene show a biradical transition state to be involved, and the cycloaddition of cyclobutadiene and acetylene, affording Dewar benzene (4), was also examined. The equilibrium geometry of the latter, predicted by semi-empirical INDO calculations, was in good agreement with experimental results and the calculated dipole moment of < 0.04 D suggests that the likelihood of obtaining a microwave spectrum is marginal at best. ... 

A simple example is appropriate. In Fig 3 we show the transition state geometry for the transition structure between Benzene and Dewar Benzene. The active space corresponds... 

Fig 3. Benzene -Dewar Benzene 6 orbital CAS Transition State Geometry... 

It is interesting to mention here that Dewar and Storch (1989) drew attention to the fact that ion-molecule reactions often lack a transition state barrier in theoretical calculations related to the gas phase, but are known to proceed with measurable activation energy in solution. Szabo et al. (1992) made separate calculations at the ab initio Hartree-Fock 3/21 G level for the geometry of the nitration of benzene with the protonated methyl nitrate by two mechanisms, not involving solvent molecules. Both calculations yielded values for the energy barriers. 

Since these five-membered rings, however, differ markedly from benzene in their geometry a more rigorous approach, as for substituent effects in polycyclic systems, would appear to be calculation of appropriate a values by approaches of the Dewar-Grisdale equation [Eq. (2)] or multiparameter form [Eq. (3)] in the former case, the p values are taken as equivalent to that in benzenoid compounds. 

Fig. 3. (a) The singlet states originating from Kekule and Dewar resonances in benzene. The marked points show the CASSCF computed values, the continuous and dashed lines corresponding to the fit with Hamiltonian of equation (14) (left side) (b) the geometry dependence of spin Hamiltonian parameters (right side). 

An inspeetion of Table 2 shows that it is not possible to relate the benzene (Deh symmetry) to Kekule (Dsh) or Dewar (D2k) structures. The ground state of benzene is not degenerate, and there is no theoretical or experimental evidence of a near-degenerate electronic state with same geometry as the ground state. If there is no intersection of degenerate point group states one cannot follow the symmetry deseent path in this case. The only alternative would be that of an ac-... 

A clear-cut distinction is available in the Dewar approacli between the alternative geometries for the Cope reaction (Fig. 5.3(c)). The chair-like transition state is isoconjugate with benzene, and should be substantially... 

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