Information Entropy as a New Measure of Aromaticity; Shannon Aromaticity

Aromaticity, the property resulting from cyclic conjugation, is an important concept in chemistry, which is one of the features controlling the stability and reactivity of unsaturated cyclic and polycyclic compounds. Many efforts have been made to quantify aromaticity, but since it is not a directly observable quantity therefore it cannot be defined unambiguously and usually evaluated indirectly by measuring a property that reflects the aromatic character of molecule. For example, Nucleus-Independent Chemical Shift (NICS) [1], Harmonic Oscillator Model of Aromaticity (HOMA), Multicenter Index (Iring) etc have been used extensively to assess the aromaticityand antiaromaticity of many organic and inorganic compounds [2,3]. Unfortunately, all current available indices represent approximations to the problem of measuring aromaticity and no single property existsthat could be taken as a direct measure of aromaticity. In the other hand, all existent indices do not always give consistent results among themselves. In this study an attempt has been made to introduce a new measure ofaromaticity (Shannon Aromaticity) based on the information theory and this index is checked for a range of both aromatic and antiaromatic molecules.