2. Computational methods

The theoretical calculations were performed at the semiempirical [41] and density functional theory [42, 43] levels using, respectively, the AM1 method and B3LYP [44–46] with 6-31G\* basis set which were implemented in the Gaussian03W program [47, 48]. The molecular geometries were optimized without any symmetry constraints, and the harmonic frequencies were calculated to ensure that the structures really corresponded to a true local minimum energy on the potential energy surface in the first time and to determine the vibrational zero-point energies in the second time.
