Theoretical study for a complex of 1,2,5-thiadiazole with formic acid

Ab initio and DFT calculations have been performed on a complex of 1,2,5-thiadiazole with formic acid. Fifteen local minima of the complex are found using the MP2/6-31G method. In the ten planar structures, 1,2,5-thiadiazole and formic acid connect by two non-covalent bonds, such as 0-H...N, C-H...O, C-H...N and S...O, to form ring structures, and the other five structures contain only one non-covalent bond. The three stable structures with the lowest total energies are examined at the HF/6-311++G(2d, 2p), MP2/6-311++G(2d,2p) and B3LYP/6-311++G(2d, 2p) theory levels. The first and second stable structures contain intermolecular 0-H...N/C-H...O and S...O/O-H...N interactions, respectively. Their binding energies by MP2 corrected BSSE and ZPE are -7.0 to -7.2 kcal/mol, which indicate a great stabilization of the complex. The NBO analysis has revealed that the n(N) -> sigma (center dot)(O-H) interaction gives the strongest stabilization to the complex and the lone pairs of carbonyl oxygen in formic acid play an important role in the weak intermolecular interactions in the complex. (C) 2008 Elsevier B.V. All rights reserved.