Ab initio study of formazan and 3-nitroformazan

Formazan and 3-nitroformazan have been investigated at ab initio level (MP2/6-31G** and B3LYP/6-31G**) in all their possible conformations, for studying the various possibilities of intramolecular hydrogen bonding formation. The trans-syn-s-cis (TSSC), known also as yellow form, has been found to be the most stable conformer (at least in the gas phase) in both compounds. This particular structure is strongly stabilized by a N-H ... N hydrogen bridge, which gives rise to a hexatomic chelate ring, with the possibility of a proton transfer process. This closely resembles that of malondialdehyde, previously studied, in the evolution of the potential energy shape but with a greater barrier height. Various approaches for obtaining a quantitative estimate of the energy of the hydrogen bridges are discussed. The electronic structures of the most favoured TSSC, TSST (trans-syn-s-trans) and TAST (trans-anti-s-trans) conformations of formazan have been compared with those of the corresponding forms of 1,5-diphenylformazan, in order to account for the UV spectra available in the literature and the different colours exhibited by the molecule on passing from one conformation to another.