Geometry and the degree of aromaticity

FIGURE 5. Optimized structures of Dgh and Da S i IP at (from top to bottom) HF/6-31G, HF/DZ+d, MCSCF/DZ, MP2/6-31G and B3LYP/6-311+G. The D6h values and the D3i MCSCF/DZ values are from Reference 60, the D3d HF values are from Reference 59. The MP2/6-31G and B3LYP/6-311+G Si—Si bond lengths for both structures are from References 52 and 19, respectively 

The HOMO of hexasilabenzene is by ca 2 eV higher than in benzene, while the HOMO-LUMO gap is much smaller in 31 than in benzene60. This suggest that hexasilabenzene should be more reactive than benzene towards both electrophiles and nucleophiles. 

The calculated resonance energies (RE) of 12 (equation 11) and of benzene (equation 2) are 53.1 kcalmoD and 63.8 kcalmoD, respectively (at CCSD(T)/DZP//CISD/DZP), indicating that 12 has 83% of the aromaticity of benzene . Similar results were obtained with lower levels of theory , but this is probably fortuitous. The addition of polarization functions changes the 12/benzene RE ratio from 84% at DZ to 99% at DZP, while electron correlation acts in the opposite direction, decreasing the above ratio to 83%. The existence of aromatic stabilization in 12 can also be concluded from the large difference of 27 kcalmol (MP2/6-31G //MP2/6-31G ) in the stability of the Si=C double bonds in the aromatic 1,3,5-trisilabenzene and the non-aromatic 1,3,5-silacyclohexadiene i.e. AE = —35.3 and 8.5 kcalmoD for equations 12 and 13, respectively. The calculated r-atomic populations and 7r-bond-orders point to a similar degree of aromaticity in CeHe, CsSisHe and SieHe . 

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SINDOl calculations, which successfully reproduced both the geometry and the aromaticity of the cyclooctatetraene dianion, predict a high degree of bond localization and nonplanarity for 1,4-dioxocin and its derivatives (84JOC4475). 

Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected fixjm conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). 

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