A DFT and Natural Resonance Theory investigation of the electronic structure of mesoionic compounds
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Mesoionic compounds are a class of heterocyclic molecules which have an unusual electronic structure. Although there are reports about their synthetic, optical and biological applications, some of their properties are not fully understood. In this work, an electronic structure investigation of three different mesoionic rings was made by means of DFT, Natural Bond Orbitals and Natural Resonance Theory calculations; we varied the substituents at three positions, totalizing 54 structures. Our results showed that C2–X bond is the longest endocyclic bond and the most susceptible to undergo cleavage. In addition, 1,3-oxazol-5-one (NOO) rings are more likely to open than 1,3-diazole-4-thione (NNCS) and 1,3-thiazole-5-thione (NSS) ones. Natural resonance analysis provided us a better understanding of the important canonical forms for those compounds which could be a good starting point for higher-level multi-reference calculations. We also found out that substituent groups may affect the electronic structure of those compounds as much as the ring structure itself.