Electron Ring

X-Ray structural data and recent high level theoretical calculations confirm that this neutral, diamagnetic dithiadiazole is an aromatic six k-electron ring system. The gas-phase infra-red and photoelectron spectra of S2N2CO have also been reported. ... 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

To relate the pi electron ring current diagram in Figure 5 to the response surfaces of Figure 2, we note that the diamagnetic pi electron response for a field applied perpendicularly to the molecular plane at the tt/3 field point... 

After solving this system of equations for the response orbitals y/ the induced electronic ring current density is obtained from the y/ using the expectation value of the standard quantum mechanical current density operator ... 

Electrochemistry. The redox potentials of 66a and 67b were measured by cyclic voltammetry. Both systems undergo two reversible, one-electron ring reductions. These reductions are compared to data for 47 and H2(pc), Table XVIII. 

FIGURE 28.2 Plot of PDI (electrons), FLU1/2 (values divided by 10), MCI (electrons), /ring (electrons), 7vb. and /NG versus the reaction coordinate (IRP in amu1 2 Bohr). Negative values of the IRP correspond to the reactants side of the reaction path and positive values to the product side of the hutadiene-ethylene DA cycloaddition. 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

Hiickel s rule, based on molecular-orbital calculations,123 predicts that electron rings will constitute an aromatic system only if the number of electrons in the ring is of the form... 

Benzofurazans and benzofuroxans have been included in this chapter because of their heteroaromatic character as lOx-electron ring systems. However, their interactions with nucleophilic reagents seem to be exclusively confined to attachment of the nucleophilic atom to the carbon atoms of the 6-membered ring away from the fused 5-membered heterocyclic moiety, which exerts a strong electron-withdrawing influence. 

An important controlling factor is the conformation of the triene about the two single bonds that link the three double bonds (2.21) an s-c/s-s-cis conformation is required for 6-electron ring-closure, but 4-electron closure can occur also in an s-cis-s-trans conformation. The different conformations interconvert rapidly in acyclic trienes, but because each has different absorption characteristics, the wavelength of irradiation can influence the course of reaction dramatically. 2,5-Dimethylhexa-1,3,5-triene undergoes efficient cis trans isomerization at 254 nm (where the major absorbing species is the s-trans-s-frans conformation), but it cyclizes to a cyclohexadiene (2.22) using 313 nm radiation. 

In Section III,B it was pointed out that 2-phenylpyrimidines did not follow the general reduction path of pyrimidines but were reduced to 2-phenylpyrroles in a four-electron ring-contraction reaction.174 [Eq. (86)]. 

The abstract of Russian work on the application of Hartree-Fock perturbation theory to the calculation of 7r-electron ring currents in inter alia pyrrole, indole, indolizine and carbazole reports agreement between calculated and experimental HNMR shifts... 

These pentapyrrolic macrocycles contain a 22-jr-electron ring system, and are expected to be the largest members of the [An + 2] n aromatics. In fact, their spectroscopic characteristics, i.e. the intense absorption band in visible region, the large diamagnetic shift of peripheral H and inner NH NMR signals, and relatively strong parent (M+) and dicationic (M2+) mass peaks, indicate the existence of a delocalized cyclic aromatic n system (Table 11). 

Phospholes as two electron donors and four n electron ring donors... 

Because open-shell molecules are ordinarily highly reactive, the An electron rings should be chemically unstable. 

---