Ab initio GB study of the acid-catalyzed cis-trans isomerization of methyl yellow and methyl orange in aqueous solution

Ab initio GB calculation that includes solvent effects by the generalized Born formula (GB) was applied to methyl yellow (4-dimethylaminophenylazobenzene, MY) and methyl orange (4-[4'-dimethylaminophenylazo]-benzenesulfonate, MO) and their protonated species, and the acid-catalyzed cis-trans isomerization of MY and MO was analyzed. The HF/3-21G((*))/GB calculations determined the structures and relative energies of MY and MO, their ammonium and azonium ions, and the transitions state for the cis-trans isomerization of the azonium ions in aqueous solution. The calculations indicated that (i) the energy difference between the cis and trans forms of the azonium ion is much smaller than those of the ammonium ion and the unprotonated species, (ii) the cis-azonium ion is much more stable than the cis-ammonium ion, and (iii) the cis-trans isomerization occurs easily in the azonium ion which is produced by the protonation of the azo compound or by the acid-catalyzed proton transfer from the cis-ammonium ion. (C) 2001 Elsevier Science B.V. All rights reserved.