Two intramolecular rearrangement reactions trans-N 2 H 2 f cis-N 2 H 2 and F 2 S 2 f FSSF have been studied in both gas and solution phases using the ab initio SCF method at the Hartree-Fock level as well as using density functional theory with B3LYP exchange-correlation functionals with 6-311G** and 6-311++G** basis sets. Polarizability calculations have been performed using Sadlej's basis set also. Maximum hardness and minimum polarizability principles have been found to be valid in almost all cases. For the former reaction, the maximum molecular valency principle is obeyed. Reactions become more favorable, thermodynamically and kinetically, in the presence of the solvent. Variation of electrophilicity along the internal reaction coordinate is analyzed in terms of the profiles of the global electrophilicity index as well as condensed electrophilic Fukui functions at different atomic sites. It is found that electrophilicity decreases in the solution phase.
High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono-and dithioderivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono-and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σ * NX antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH · · · O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the π * NN antibondig orbital, and which is significantly perturbed when the IHB is formed.
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