Aromaticity of monosubstituted derivatives of benzene. The application of out-of-plane ring deformation energy for a quantitative description of aromaticity

The degree of aromaticity of monosubstituted derivatives of benzene has been investigated using different aromaticity indices calculated on the basis of molecular structure obtained at the MP2/cc-pvdz level of theory. It is demonstrated that, due to the violation of symmetry in π-systems, the introduction of any substituent in the benzene ring results in a decrease in the degree of aromaticity. It has been found that structural Bird and HOMA aromaticity indices do not reflect the strength of substituents and their ability to perturb ring electronic density in the benzene ring. The application of magnetic indices NICS0 and NICS1 results in an overestimation of aromaticity of substituted molecules with respect to benzene. Modifications of these indices (NICS-z and NICS-anis) reproduce well the character of the substituents except for functional groups containing carbonyl bonds. The values of NICS-z and NICS-anis correlate well with the ring out-of-plane deformation energy. However, the values of the ring deformation energy agree very well with the strength of all substituents including those containing carbonyl. Therefore, out-of-plane ring deformability is a more universal and sensitive aromaticity index than the known structural and magnetic indices. The values of ring deformation energy demonstrate linear correlation with the substituent that is constant for electron-donation and electron-withdrawing substituents. However, the former functional groups perturb more significantly the aromatic ring electron density.