Abstract
(1) Dimer formation energy of melamine with two hydrogen bonds between the N atom of triazine ring and H atom of amino group was calculated by the CNDO/2 method. The corresponding energy was evaluated to be 7. 02 kcal/mol. Its value is relatively large among the hydrogen bond energies concerning to methylolmelamine. The hydrogen bond of melamine-dimer type may play an role in the net-work formation of methylolmelamine oligomer. The theoretical UV spectrum curve of melamine-dimer obtained by the CNDO/CI method is similar to those of melamine and methylolmelamine homologues.
(2) The molecular structure without an intra-molecular hydrogen bond is stable for dimethylolmelamine (2MM) with a secondary methylol-group from the COND/2 calculation. The calculation of π-bond order and charge density of the intra-molecular hydrogen bonded model of 2MM suggests that the intra-molecular hydrogen bonded structure is active for the dehydration and deformaldehyde reactions.
(3) The hydrogen bond energy and equilibrium distance between monomethylolmelamine (1MM) were calculated by the CNDO/2 method. The corresponding values were evaluated to be 3. 18 kcal/mol and 1. 57Å, respectively. On decrease of hydrogen bond distance from an equilibrium distance, the 1MM dimer becomes unstable in energy. The π-bond order and charge density calculated for the 1MM dimer model with the hydrogen bond distance of 1. 10Å suggest that the unstable inter-molecular hydrogen bond structure is active for dehydration and deformaldehyde reactions. Unstable intra-molecular and intermolecular hydrogen bonded states may play an role in activated state for dehydration and deformaldehyde reactions of methylolmelamine homologues. Especially, the former state is more effective than the latter one.
