Category: 2003

Ab initio (HF and MP2) and hybrid density functional theory (B3LYP) calculations were performed on a series of skeletally mono- and di-substituted benzenes with substituents from the II to IV row of the periodic table. The aromaticity in this class of compounds is evaluated based on the nucleus independent chemical shift (NICS) values. Routine classification of 6π-electronic benzenoid systems as aromatic molecules solely based on NICS criterion is questioned in heteroatomics with III and IV row substituents based on the calculated NICS values. However, the NICS criterion is found to be satisfactory in those compounds involving substituents from II row.

B3LYP/6-311+G** calculations on alkali metal ion (Li⁺ and Na⁺) complexation with corannulene and sumanene indicate stronger binding compared to [5]-radialene or benzene. The dependence of binding to the convex and concave site is marginal, albeit the preference was consistent for convex binding in the range of 1–4 kcal/mol. The bowl-to-bowl inversion barriers are only marginally affected, below 2 kcal/mol, by metal ion complexation.

Metal ions bind strongly to both π-faces of buckybowls in comparison with benzene and the binding from the convex side is marginally preferred over the concave side.

Monosubstituted tricyclo[2.1.0.0^2,5]pentan-3-one derivatives have been synthesised and subjected to hydride reduction. Quite unexpectedly and in contrast to earlier observations in related systems, anti-selectivity has been encountered in these substrates. Computational studies employing various models reproduce the observed facial preference and indicate that the causative factor for the anti-preference could be the polarization of the C1C5 strained σ bond

Density functional theory (B3LYP) calculations were performed to examine the effect of Si substitution on the aromaticity of some polycyclic hydrocarbons using geometric criterion (HOMA), isodesmic isomerization reactions, homodesmotic equations, NICS values, chemical hardness, and out-of-plane distortive tendencies. The HOMA values are lower and the NICS values are higher in the Si-substituted rings compared to those in the hydrocarbon counterpart, whereas the homodesmotic equations predict little loss of aromaticity upon Si replacement in polycylic systems. The chemical hardness values decrease and the out-of-plane distortive tendency increases upon silicon substitution. The relative energies of the positional isomers and the causative factors are analyzed. The high reactivity of some silaaromatics toward dimerization is explained based on local softness indices.

Ab initio HF, MP2, CCSD(T) and hybrid density functional B3LYP calculations were performed on a series of skeletally mono- and di-substituted benzenes, (CH)₅Z and (CH)₄Z₂, Z = C^–, N, O⁺, Si^–, P, S⁺, Ge^–, As, Se⁺, BH^–, NH⁺, AlH^–, SiH, PH⁺, GaH^–, GeH and AsH⁺. Various measures of aromaticity such as the bond length equalization, homodesmic equations, singlet-triplet energy difference (AE _s-t), chemical hardness (η) and out-of-plane distortive tendency are critically analysed. The relative energy ordering in skeletally disubstituted benzenes displays trends that are inexplicable based on conventional wisdom. In general, the orthoisomer is found to be the least stable when the substituent is from the second row, whereas if the substituent is from the fourth row, the ortho-isomer is the most stable. Various qualitative arguments, including (a) lone pair-lone pair repulsion, (b) the sum of bond strengths in the twin Kekule forms, and (c) the rule of topological charge stabilization (TCS), are used to explain the observed relative energy trends. The rule of TCS in conjunction with the sum of bond strengths is found to predict the relative energy ordering reasonably well. The reactivity of this class of compounds is assessed based on their singlet-triplet energy differences, chemical hardness and the frequencies corresponding to out-of-plane skeletal distortions. These reactivity indices show less kinetic stability for the compounds with substituents from the fourth row and point to the fact that the thermodynamically most stable compounds need not be the least reactive ones. The ‡E_s-t values indicate that the π-framework of benzene weakens upon skeletal substitutions.