MOLECULAR MODELING OF REDUCTION REACTIONS OF DICYANDIAMIDE ON ZINC - A THEORETICAL-STUDY OF EPOXY ADHESIVE GALVANIZED STEEL ADHESION

Molecular orbital calculations have been used to model the interfacial reactions that occur during bond formation of epoxy adhesives to galvanized steel. The theoretical calculations were applied to interfacial reactions of generic dicyandiamide-crosslinked epoxy adhesives on zinc surfaces. Semi-empirical MNDO molecular orbital calculations were used to model the reduction reactions. The calculations were used to predict the most stable tautomer of dicyandiamide as well as to suggest the structure of the most likely zinc-dicyandiamide reduction products. The results of the calculations are consistent with infrared spectroscopic studies of dicyandiamide reduction on zinc surfaces that suggest the formation of carbon-nitrogen multiply bonded moieties. An MNDO geometry optimization of dicyandiamide suggests that one tautomeric form of dicyandiamide, the diamino form, is the most stable structure by about 9.3 kcal/mol. The optimized geometry of the diamino form of dicyandiamide is nearly planar, suggesting that the molecule has a π-system largely isoelectronic with butadiene. The symmetry and magnitude of the coefficients for the atomic orbitals of the lowest unoccupied molecular orbital (LUMO) of the diamino form of dicyandiamide are consistent with the formation of a radical anion reduction product which would afford a carbodiimide intermediate on the zinc surface. © VSP 1990.