Quantitative Assessment of Aromaticity and Antiaromaticity

7 ABSTRACT: Vibrational frequencies can be measured and calculated with 8 high precision. Therefore, they are excellent tools for analyzing the electronic 9 structure of a molecule. In this connection, the properties of the local 10 vibrational modes of a molecule are best suited. A new procedure is described, 11 which utilizes local CC stretching force constants to derive an aromaticity 12 index (AI) that quantitatively determines the degree of π-delocalization in a 13 cyclic conjugated system. Using Kekule ́ benzene as a suitable reference, the 14 AIs of 30 monoand polycyclic conjugated hydrocarbons are calculated. The 15 AI turns out to describe π-delocalization in a balanced way by correctly 16 describing local aromatic units, peripheral, and all-bond delocalization. When 17 comparing the AI with the harmonic oscillator model of AI, the latter is found 18 to exaggerate the antiaromaticity of true and potential 4n π-systems or to 19 wrongly describe local aromaticity. This is a result of a failure of the Badger 20 relationship (the shorter bond is always the stronger bond), which is only a 21 rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI 22 confirms Clar’s rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads 23 to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. 24 Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.