Structures, energetics, relative stabilities, and out-of-plane distortivities of skeletally disubstituted benzenes, (CH)(4)X-2 (X = N, P, C-, Si-, O+, and S+): An ab initio and DFT study

The positional isomers of disubstituted benzenes, (CH)(4)X-2 (X = N, P. C-, Si-, O+, and S+), are studied using ab initio molecular orbital theory at Hartree-Fock (HF), MP2, and CCSD(T) levels and also using density functiona l theory (B3LYP). All of the planar structures are characterized as minima, and they show fully delocalized geometric parameters, with no significant sign of bond fixation. The ortho isomer is computed to be the least stable in all cases except when X = P and Si-. While meta isomer is more stable th an para in general, exceptions are seen for C- and S+ substitutions. Variou s factors that affect the relative stabilities of positional isomers, viz., (a) lone pair-lone pair (1p-1p) repulsion, (b) electrostatic interaction, (c) bond strengths in the corresponding valence isomers, and (d) topologica l charge stabilization (TCS), are invoked to explain the computed pattern o f relative stabilities. The strengths and limitations of each of these aspe cts in controlling the geometries and energetics of the three isomeric form s are assessed. The 1p-1p interaction is found to be insignificant in the i somers bearing third row atom substituents. The rule of TCS excellently acc ounts for the distinction between meta and para isomers. The sum of the bon d strengths of the constituent Kekule forms derived from bond increment sch emes successfully explain the differences in the stability between ortho an d meta/para isomers. The effect of skeletal disubstitution on the out-of-pl ane distortivity in the three isomeric forms is discussed by examining the changes in the frequencies of normal modes. The tendency for ring-puckering increases as more heteroatom substituents from the third row replace the C H groups in the ring. Comparisons were made with the monosubstituted analog ues, and the effect of adding one more heteroatom substituent on the out-of -plane distortive modes is analyzed.