Direct estimate of conjugation and aromaticity in cyclic compounds with the EDA method

Abstract

The nature of the interatomic interactions in cyclic conjugated molecules has been investigated with the Energy Decomposition Analysis (EDA). The focus of this work is on the strength of the π bonding and π conjugation in carbocyclic and heterocyclic molecules. The calculated ΔE_π values suggest that the EDA may be used directly for estimating the strength of π conjugation in aromatic, homoaromatic, homoantiaromatic and antiaromatic compounds. The theoretical data show a pattern which agrees with the 4n + 2 rule. The extra aromatic stabilization energy has been obtained by comparing cyclic conjugation with the strength of π conjugation in acyclic reference compounds. The ΔE_π values indicate that benzene is significantly stabilized by aromatic stabilization, but the total π bonding contribution to the carbon–carbon bonding becomes even more stabilizing when the geometry is distorted toward a D_3h form which has three long and three short C–C bonds. The D_6h equilibrium structure is enforced by the σ orbital interactions and by the quasiclassical electrostatic attraction. The VRE and ASE values suggest that pyridine is more strongly stabilized by aromatic conjugation than benzene. The EDA data of the six-membered cyclic 6π species C₅H₅E (E = CH, N–Bi) and (HB = NH)₃ predicts strength of aromatic stabilization in the order N > CH > P > As > Bi ≫ borazine. The ASE values of the five-membered heterocyclic systems C₄H₄E (E = NH, O, S) are smaller than for the benzene analogues showing the order NH ∼ S > O. The ASE value for Cp⁻ is very small which probably comes from the choice of the reference system. The VRE results indicate that Cp⁻ is strongly stabilized by cyclic conjugation. The ASE values for the cyclic singlet carbenes indicate weak aromatic stabilization in the 2π system cyclopropenylidene which is much stronger in the 6π compound cycloheptatrienylidene while the 4π molecule cylopentadienylidene is clearly antiaromatic. The triplet states of all three cyclic carbenes are antiaromatic. The strength of the π conjugation in the homoconjugated cyclic compounds suggests weak aromatic stabilization in the 6π compounds 1,3-cyclobutene and 1,3-cyclopentadiene but weak antiaromatic destabilization in the 2π compound cyclopropene and in the 8π molecules 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. The 4n + 2 rule thus holds also for homoconjugated systems. A large negative value for the ASE is calculated for 1,3-cyclobutadiene.