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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 NwHÉ É ÉN 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 di †erent colours exhibited by the molecule on passing from one conformation to another.

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The unexplored carbon rich cationic closo carboranes, C3Bn−3Hn+1 (n=5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calculated at the B3LYP/6‐31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring‐cap orbital overlap compatibility along with the number of BC, CC, and BB bonds help in explaining the stability order in each category. The most stable isomer is the one with maximum ring‐cap orbital overlap and largest number of BC bonds. The order of relative stability among the trigonal bipyramid is 1c>1b>1a′, where the stability is proportional to the number of CH caps over the small three‐membered ring. The C3B3H6+ isomer with the one allyl C3 group (2b) is more favorable than the one with a cyclopropenyl group (2a). Among the C3B4H7+ isomers the stability order is 3e>3d>3c>3b>3a, which mostly depends on the ring‐cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of ring cap compatibility and the number of BC bonds. The order of 5e>5d>5c>5b>5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1)–(5) devised for estimating the stability of isomers of C3Bn−3Hn+ indicate an increase in stability with cage size. The mono‐positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1542–1551, 2001