The interacting patterns and mechanism of the catechin and guanine have been investigated with the density functional theory B3LYP method by 6-31G* basis set. Fourteen stable structures for the catechin-guanine complexes have been found which form two hydrogen bonds at least. The results indicate that the complexes are mainly stabilized by the hydrogen bonding interactions. At the same time, the number and strength of hydrogen bond play a co-determinant parts in the stability of the complexes which can form two or more hydrogen bonds. Theories of atoms in molecules (AIM) and natural bond orbital (NBO) have been adopted to investigate the hydrogen bonds involved in all systems. The interaction energies of all complexes have been corrected for basis set superposition error (BSSE), ranging from -38.86 to -14.56 kJ/mol. The results showed that the hydrogen bonding contributes to the interaction energies dominantly. The corresponding bonds stretching motions in all complexes are red-shifted relative to that of the monomer, which is in agreement with experimental results.

Ring-opening isomerization from ring-shaped isomers to chain-shaped isomers of N(8)H(8) has been studied by a density function B3LYP method at 6-311+ +G** level. 20 ring-shaped isomers have been found to be able to transform into chain-shaped isomers, with 20 possible transition states got by ring-opening structure optimization. Furthermore, the ring-openings have been found in the longer N-N single bond by analyzing the length change of N-N bond of ring-shaped isomers in ring-opening processes. In addition, with the activation energies in ring-opening processes, the differences of the activation energies in isomerization between the isomers have been found according to the classification of rings. The activation energies in ring-opening isomerization of six-membered ring-shaped isomers are higher than that of the four-membered ring-shaped isomers. It indicates that six-membered ring-shaped isomers difficult in ring-opening in the isomerization are the steadiest ring-shaped isomers of N(8)H(8) while four-membered ring-shaped isomers easy in ring-opening are the most unstable.