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

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 pi bonding and pi conjugation in carbocyclic and heterocyclic molecules. The calculated deltaE(pi) values suggest that the EDA may be used directly for estimating the strength of pi 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 pi conjugation in acyclic reference compounds. The deltaE(pi) values indicate that benzene is significantly stabilized by aromatic stabilization, but the total pi 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 sigma 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 6pi 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 approximately 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 2pi system cyclopropenylidene which is much stronger in the 6pi compound cycloheptatrienylidene while the 4pi molecule cylopentadienylidene is clearly antiaromatic. The triplet states of all three cyclic carbenes are antiaromatic. The strength of the pi conjugation in the homoconjugated cyclic compounds suggests weak aromatic stabilization in the 6pi compounds 1,3-cyclobutene and 1,3-cyclopentadiene but weak antiaromatic destabilization in the 2pi compound cyclopropene and in the 8pi 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.