Abstract
Mesoionics are neutral compounds that cannot be represented by a fully covalent or purely ionic structure. Among the possible mesomeric structures of these compounds are the diradical electronic configurations. Theoretical and experimental studies indicate that some mesoionic rings are unstable, which may be related to a significant diradical character, that until then is not quantified. In this work, we investigated the diradical character of four heterocycles: 1,3-oxazol-5-one, 1,3-oxazol-5-thione, 1,3-thiazole-5-one, and 1,3-thiazole-5-thione. The oxazoles are known to be significatively less stable than thiazoles. DFT and ab initio single (B3LYP, MP2, CCSD, and QCISD) and ab initio multi-reference (MR-CISD) methods with three basis sets (6-311+G(d), aug-cc-pVDZ, and aug-cc-pVTZ) were employed to assess the diradical character of the investigated systems, in gas phase and DMSO solvent, from three criteria: (i) HOMO-LUMO energy gap, (ii) determination of energy difference between singlet and triplet wave functions, and (iii) quantification of the most significant diradical character (y0, determined in the unrestricted formalism). All of the results showed that the diradical character of the investigated systems is very small. However, the calculated electronic structures made it possible to identify the possible origin of the oxazoles instability, which can help the design of mesoionic systems with the desired properties.
Highlights
Heterocyclic compounds correspond to organic structures that have one or more different carbon atoms in the ring, with oxygen, nitrogen, and sulfur being the most common [1,2,3,4,5]
Among a large number of heterocycles are mesoionic compounds [7,8], a class of molecules composed of five or six-membered rings, usually soluble in common organic solvents [1,9] that can only be represented as mesomeric structures and that have a high charge separation
The structures were reoptimized in the triplet state, on the unrestricted formalism (UMP2, UQCISD, and UCCSD), to verify the geometry effect on the electronic properties
Summary
Heterocyclic compounds correspond to organic structures that have one or more different carbon atoms in the ring, with oxygen, nitrogen, and sulfur being the most common [1,2,3,4,5]. The rings contain from three to six members, which makes it possible to obtain a huge variety of compounds, since a carbocyclic, regardless of its structure and functionality, can in principle be converted into analogous heterocyclic structures by replacing one or more endocyclic carbon atoms [6]. Among a large number of heterocycles are mesoionic compounds [7,8], a class of molecules composed of five or six-membered rings, usually soluble in common organic solvents (such as chloroform and DMSO) [1,9] that can only be represented as mesomeric structures and that have a high charge separation. Type A compounds are more stable (with a possible cleavage of the a-e bond) and have biological activity [12,13], while type B compounds undergo isomerization between their cyclic and acyclic forms (due to cleavage of the b-c bond) [14].
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