Specific hydration effects on oxo-thio triazepine derivatives.

The specific hydration of 2,7-dimethyl-1,2,4-triazepine oxo-thio derivatives by one water molecule has been investigated at the B3LYP/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The existence of different hydrogen bond (HB) donor and acceptor centers in these molecules led to different kinds of hydrogen bonds (CH-O, OH-S, NH-O, OH-N, and OH-O) and different kinds of complexes. Among them, the most stable structures correspond to complexes where the heteroatom X or Y at positions 3 and 5 behaves as HB acceptor and the hydrogen atom associated with the nitrogen atom at position 4 as HB donor. In accordance with previous studies, it has been shown that the thiocarbonyl group forms stronger HBs than the carbonyl group because the sulfur atom is a better HB acceptor than the oxygen one. With the help of the AIM (atoms in molecules) theory and ELF (electron localization function) analysis, it has been shown that, in the case of 3O5O, 3S5O, and 3S5S, the most basic site is the heteroatom at position 3, while in 3O5S species the most basic center is the sulfur atom.