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 OH···N, CH···O, CH···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 OH···N/CH···O and S···O/OH···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) → σ∗(OH) 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.