A quantitative scale for the degree of aromaticity and antiaromaticity: A comparison of theoretical and experimental enthalpies of hydrogenation

Chemical structures and transition states are often influenced by aromatic stabilization or antiaromatic destabilizing effects, which are not easy to characterize theoretically. The exact description and precise quantification of the aromatic characteristics of ring structures is difficult and requires special theoretical investigation. The present paper suggests a novel, yet simple, method to quantify both aromatic and antiaromatic qualities on the same linear scale, by using the experimentally measured or theoretically computed enthalpy of hydrogenation reaction of the compound examined [Delta H-H2(examined)]. A reference hydrogenation reaction is also considered on a corresponding nonaromatic reference compound [Delta H-H2(reference)] to cancel all secondary structure destabilization factors, such as ring strain or double bond strain. From these data the relative enthalpy of hydrogenation may easily be calculated: Delta Delta H-H2 = Delta H-H2(examined) - Delta H-H2(reference). In the present work concept, the Delta Delta H-H2 value of benzene defines the completely aromatic character (+100%), and the closed shell of the singlet cyclobutadiene represents maximum antiaromaticity (-100%). The component Delta H-H2 values were computed at different levels of theory offering a computational method-independent measure for aromaticity. A total of 28 well-known aromatic, antiaromatic and nonaromatic, neutral and charged compounds were examined to demonstrate the efficiency of this methodology. Finally, a correlation was made between the calculated aromaticity percentage of the compound examined and their popular Schleyers NICS values.