Hiickel calculations anions

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

The effect of the sulfone dipole on the ESR spectrum of the dibenzothiophene 5,5-dioxide anion has been examined, modified Hiickel calculations which ignored d-orbital conjugation predicting spin distributions which agreed with experimental data. Proton hyperfine splitting constants have also been obtained for the sulfone. Correlation with MO calculations shows that the sulfone group contributes a vacant symmetric orbital to the conjugated system. ... 

Whereas the allyl anion, with a plane of symmetry through the central atom, has a node at that atom in j/ j, amides, esters, enamines, enol ethers and enolate ions do not have a node precisely on the central atom. Taking planar A V-dimethylviny-lamine and the enolate of acetaldehyde as examples, simple Hiickel calculations give the n orbitals in Fig. 2.15, which includes the allyl anion for comparison. 

Fig. 2.15 n Orbital energies and coefficients from simple Hiickel calculations of the allyl anion, enamine and enolate ion (orbital energies in kJ moP1 relative to a)... 

This adjustment to the simple Hiickel calculations explains why the cyclic systems differ from the open-chain pentadienyl anions, which have no alkyl groups at C-l and C-5. These systems are evidently delicately balanced, so much so that quite minor perturbations can lead to high levels of attack at C-3,322 especially with the harder electrophiles like alkyl triflates.323... 

The kinetics of the hydrolysis of D-glucosyl, D-mannosyl, D-galactosyl, and L-fucosyl a- and 3-phosphates between pH 1-6 have been determined. For the mono-anionic species the /3-anomers were hydrolysed 3-7 times faster than the a-anomers. Extended Hiickel calculations have shown that the difference in enthalpy between cyclic AMP and acyclic related phosphate esters is due to larger net exothermic solvation enthalpy in the former, ascribed to extra stabilization of the products of hydrolysis, due to their ability to form more stable hydrogen bonds with water. ... 

Analysis of ESR spectrum on the basis of Hiickel calculation of spin densities. - ESR ) McLachlan calculation of spin densities including perturbation of all a>electrons. spectrum of anion radical of all-tran.s-retinal could not be analyzed because of Assignment based on Hiickel calculation of spin densities, unsuificient resolution. - ) Not assigned to protons in specific positions. ... 

Funabiki et al. have performed a Extended Hiickel calculation study on the relative stability of the probable catecholatoiron complexes [118]. 3,5-Dimethylcatecholato-(tetraammonium)iron(III) complexes are used for simplification. The results support the radical character of the catecholate ligand and suggests that the direct attack of free oxygen on aromatics, if it occurs, can be at the C-2 and C-6 positions from the side perpendicular to the aromatic plane. This parallels the two routes in Scheme 8 and path A in Scheme 9. In the model species of type 50, the most electrodeficient C-1 is favored over C-2 or C-6 for the attack by peroxide if importance is attached to the anionic character rather than the radical character of the peroxide. 

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

J. Bjerrum (1926) first developed the theory of ion association. He introduced the concept of a certain critical distance between the cation and the anion at which the electrostatic attractive force is balanced by the mean force corresponding to thermal motion. The energy of the ion is at a minimum at this distance. The method of calculation is analogous to that of Debye and Hiickel in the theory of activity coefficients (see Section 1.3.1). The probability Pt dr has to be found for the ith ion species to be present in a volume element in the shape of a spherical shell with thickness dr at a sufficiently small distance r from the central ion (index k). 

For aromatic hydrocarbon radical anions, this approach works pretty well. Figure 2.7 shows a correlation plot of observed hyperfine splitting versus the spin density calculated from Hiickel MO theory. It also correctly predicts the negative sign of aH for protons attached to n systems. 

---