Document Type


Subject Area(s)

Chemical Physics, Physical Chemistry


Formazan and 3-nitroformazan have been investigated at abinitio 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 asyellowform, 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.