Azo, hydrazone and other tautomers of the azo dye 7-amino-4-hydroxy-3-[(4-methoxy-2-sulfophenyl)azo]-2-naphthalenesulfonic acid: a PM3 study

The compound 7-amino-4-hydroxy-3-[(4-methoxy-2-sulfophenyl)azo]-2-naphthale ne sulfonic acid is an intermediate in the preparation of water-soluble dye s that are used for dyeing and printing of textiles in fast shades. The geo metries and energies of the ground and transition states of tautomers of th is molecule were computed at the PM3 level of theory. The results show that there are two stable conformations of the trans azo configuration and two unique cis azo forms. However, the trans azo forms are more stable than the cis. Among the five possible tautomers, it is seen that the order of stabi lity runs I(azo) > II(hydrazone) much greater than III1 similar to IIIb > I V. Transition states computed for the various equilibria show that the azo- hydrazone (I reversible arrow II) equilibrium is the most favored with the lowest energy. The results from this study closely parallel those reported earlier by us for a related molecule, from which it is concluded that the p osition of the amino group in these molecules has very little influence on their structure and dynamics. (C) 2000 Elsevier Science B.V. All rights res erved.