The optimized molecular structures and vibrational frequencies of 4-mercaptopyridine（4MPY） monomer and its dihydrate were studied by means of the density functional theory（ DFT）, viz. B3LYP method with the 6-311 ＋ ＋ G （d,p） basis set. On the basis of the calculations, the assignments of the vibrational spectra of the monomer and the dihydrate were performed, and so were investigated the changes in the structure and the vibrational spectrum of the di- hydrate as well as the intermolecular force resulting in the formation of the dihydrate. The calculated results show that each of the water molecule planes is vertical to the pyridine ring plane in the dihydrate that is formed via the H-bonds between 4MPY and water molecules. Furthermore, the structure and the vibrational spectrum of 4MPY can be considerably affected by the water molecules.
