Estudo do caráter dirradicalar de compostos da classe 1,3-Oxazol e 1,3-Tiazol.

Abstract

The mesoionic compounds can not be represented by a purely ionic or covalent structure. Are neutral compounds that have well-separated regions of positive and negative charges, leading to a high dipole moment. They show wide range of applications, through biology and medicinal chemistry, as well in the obtaining products of technological interest. In this study, the stability and diradicalar character of four mesoionic rings were investigated: 1,3-oxazol-5- one, 1,3-oxazol-5-thione, 1,3-thiazol-5-one and 1,3-thiazol-5-thione. Many criteria to quantify the diradicalar character were used: (I) HOMO-LUMO energy difference (EHL, inversely proportional to the diradicalar character; EHL = 0 corresponding to a pure diradical), (ii) determination of the energy difference due to the broken symmetry in the unrestricted formalism (ESS), (iii) determination of the energy difference between the singlet and triplet wave functions (EST); and (iv) diradicalar character (Y0, calculated from the occupation numbers n of HOMO and LUMO natural orbitals, in the unrestricted formalism). Various methods of electronic structure (DFT/B3LYP, MP2, CCSD and QCISD) with different basis sets (6-311+ G* and aug-cc-pVDZ) were employed. It was observed that: (i) a suitable choice of the substituent at R position (the outer ring) is important for the structural stability; (ii) substitution of oxygen by sulfur in the R and R positions (into the ring) do a little contribution for the diradicalar character (Y0 < 1% in all cases, indicating pronounced mesoionic character in the investigated structures); and (iii) the use of an electron donating group at R position and an electron withdrawing group at R position contribute to maximize the mesoionic character. Thus, it was possible to describe the dependencies of the molecular structure on the electronic properties, and the effect of substitutions, which is of great importance for the control of mesoionic character.