Computational details

All calculations were performed with Gaussian 09 suite of programs [57] and UV–visible curves were generated by GaussSum [58]. Geometries were optimized at hybrid B3LYP method using 6-31G(d) basis set. The B3LYP mode provides a good balance between cost and precision [59, 60]. A preliminary predict of the geometry of compound 2 is based on the stereochemistry ascribed through 1D and 2D NMR characterizations. Compound 2 has conformational flexibility around the symmetric dihedral angles H–C(5)–N(6)–H and C(5)–N(6)–C(7)–N(6). The dihedral angles around the C(5)–N(6) axes were scanned at 15 degrees step and a minimum at 22.9° around the C(5)–N(6) axes was retained because of its conformation closed to the structure of allantoin (1). Afterwards, dihedral angles around the N(6)–C(7) axes were scanned at 10 degrees step, to find the lowest energy conformer. It was then submitted to geometry optimization at B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels of theory to provide the optimized geometries of 2 (Fig. 4). The optimized structure was in each case confirmed by frequency analysis at the same levels as a true minimum (no imaginary frequency). Six methods were evaluated for the simulation of ¹H- and ¹³C-NMR spectra; B3LYP/6-31G(d), B3LYP/6-31G(d,p), B3LYP/6-31G+(d,p), MPW1PW91/6-31G(d), MPW1PW91/6-31G(d,p) and MPW1PW91/6-31G+(d,p). The electronic properties at isodensity 0.02 such as Ionization Potential (IP), Electron Affinity (EA), HOMO, LUMO and band gaps were calculated at B3LYP/6-31G(d) and B3LYP/6-311G++(d,p). The band gap was taken as the difference in energies of HOMO and LUMO. Meantime, thermodynamic properties like enthalpy, entropy or molar capacity at constant volume along with reactivity descriptors like chemical potential, electronegativity, hardness, softness were also calculated by means of the same methods. TD-DFT studies were evaluated at B3LYP/6-31G+(d,p) in CHCl₃ and in EtOH after a geometry optimization at the same level of theory of the iminolate in Fig. 8. Six steps were applied to get excitation energies.