A THEORETICAL-STUDY OF PROTONATION AND TAUTOMERIZATION OF N-SUBSTITUTED AMINOAZOBENZENES

Semiempirical AM1 and ab initio SCF STO-3G calculations with full geom etry optimization were performed on aminoazobenzene (AAB) and its N-me thyl (MAAB), N,N-dimethyl (DMAAB), and N-phenyl (PhAAB) derivatives, a s well as their azonium and ammonium conjugated acids. AM1 calculation s were also performed on hydrated cationic acids, in order to estimate the effect of amphiprotic solvents on tautomerization. In all the cas es studied but DMAAB the AM1 and STO-3G proton affinity of the azo nit rogen was definitely higher than that of the amino nitrogen. For the a mino 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 regar ding aminoazobenzene protonation and tautomerization constants determi ned 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 eff ects can be satisfactorily accounted for by enthalpy contributions in the case of tautomerization, because the order of tautomerization cons tants determined in methanol and dioxane-water mixture generally confo rms with the order of tautomerization enthalpies with hydration includ ed. However, the estimated proton affinities in water are still ranked in the same order as in vacuo which may indicate that entropy contrib utions play a much greater role in the case of protonation than in the case of tautomerization phenomena.