A theoretical study of the structures, energetics, stabilities, reactivities, and out-of-plane distortive tendencies of skeletally substituted benzenes (CH)(5)XH and (CH)(4)(XH)(2) (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+)).
Ab initio molecular orbital theory at Hartree-Fock (HF), post-Hartree-Fock (MP2 and CCSD(T)), and the hybrid density functional theory (B3LYP) calculations were done on the mono-(CH)(5)XH and diskeletally substituted (CH)(4)(XH)(2) benzenes (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+)). The computed relative energies of the disubstituted isomers show interesting trends. While the ortho-isomer is the most stable for X = Ga(-), Ge, and As(+), meta was found to be the most stable for X = B(-), N(+), Al(-) and Si, and para was found to be the most stable for X = P(+). Various intricate factors that govern the relative stabilities, such as the sum of bond strengths in the twin Kekule forms, rule of topological charge stabilization (TCS), and electrostatic repulsion were critically examined. The sum of bond strengths in the twin Kekule forms was proved to be quite a successful measure in predicting the relative stability orders between ortho- and meta-/para-isomers. The rule of TCS breaks down especially in the presence of overwhelming factors such as the differences in the cumulative bond strengths of the two positional isomers; however, the stability ordering between the para- and meta-isomers is successfully predicted in most cases. The tendency for ring puckering increases a great deal especially when the substituents are from 3rd or 4th row. Extension of the popular inverse relationship between the thermodynamic stability and reactivity was found to be inapplicable for this class of compounds. The computed singlet-triplet energy differences and the chemical hardness (eta) values indicate that the skeletal substitution weakens the pi-strength of the benzenoid system and increases their reactivity.
Priyakumar
U Deva
UD
Department of Chemistry, Pondicherry University, Pondicherry-605 014, India.
Sastry
G Narahari
GN