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Semiempirical AM1 and ab initio SCF STO-3G calculations with full geometry optimization were performed on aminoazobenzene (AAB) and its N-methyl (MAAB), N,N-dimethyl (DMAAB), and Nphenyl (PhAAB) derivatives, as well as their azonium and ammonium conjugated acids. AM1 calculations were also performed on hydrated cationic acids, in order to estimate the effect of amphiprotic solvents on tautomerization. In all the cases studied but DMAAB the AM1 and STO-3G proton affinity of the azo nitrogen was definitely higher than that of the amino nitrogen. For the amino nitrogen the calculated proton affinity was found to increase in the series AAB < MAAB < PhAAB < DMAAB. The calculated proton affinity of the azo nitrogen increased in the same order with the exception of the STO-3G results of PhAAB. The tautomerization energy/ enthalpy (i.e. difference of the gas-phase proton affinity of the azo and the amino nitrogen) was found to increase in the series DMAAB < MAAB < AAB, the position of PhAAB in the series depending on whether the AM1 or STO-3G method was used. These results contradict the experimental data regarding aminoazobenzene protonation and tautomerization constants determined in various solvents, which indicates a strong effect of solvation on the protonation and tautomerization equilibria of aminoazobenzenes. AM1 calculations on hydrated cationic acids showed that solvation effects can be satisfactorily accounted for by enthalpy contributions in the case of tautomerization, because the order of tautomerization constants determined in methanol and dioxane-water mixture generally conforms with the order of tautomerization enthalpies with hydration included. However, the estimated proton affinities in water are still ranked in the same order as in vacuo which may indicate that entropy contributions play a much greater role in the case of protonation than in the case of tautomerization phenomena.Aminoazobenzenes have three potential centres of protonation: the amino (NY), and two azo (N" and NB) nitrogen atoms (Scheme 1). In our previous papers'.' we investigated, by means of UV-spectroscopy, the proton-exchange phenomena of N-substituted aminoazobenzenes in various solvents, in order to evaluate the relative proton affinity of the possible protonation centres.163, 149.

Hexachlorostannic acid is a precursor of derivatives of an ionic nature. The electrostatic part of the lattice energy in alkali metal salts and nitrogen organic base salts containing the sncls2-ion was determined by adopting the Ewald method. This approach requires knowledge of the complete or at least partial crystal structures of the compounds. In the case of incomplete structures the MNDO geometry optimization procedure was successfully applied to find the unknown positions of atoms, and thus permitted a wider representation of compounds to be considered. The crystal lattice energy calculations were carried out by taking from four different literature sources data regarding charge distribution in sncls2-. It was further assumed that the positive charge in cations was located either directly on certain atoms or distributed between all the atoms in these ions. These latter net atomic charges were evaluated by applying INDO and MNDO methods. The electrostatic energies derived compare well with published values of the crystal lattice energy; this implies that, in the case of the compounds examined, the main contribution to the cohesive forces is made by Coulombic interactions.

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