DFT study of proton transfer, cooperativity, and tautomerization in 2-pyridineselenol and 2-pyridinethiol ammonia and water clusters

B3LYP/6-311+G(d) method has been used to study 2-pyridineselenol/2-pyridineselenone (2SeHP/2SeP) molecular system complexed with water or ammonia molecules. Other derivatives, 2-pyridinethiol/2-pyridinethione (2SHP/2SP), and 2-hydroxypyridine/2-pyridone (2OHP/2OP) have been studied as well. Trends in changes of molecular geometries and vibrational IR frequencies of molecules on the substitution of oxygen with progressively heavier atoms down the periodic table have been undertaken. Investigations of intermolecular proton transfer from the -SeH and -SH groups to the (NH3)n show that this interesting process occurs for the critical size nc = 3. The selenol-selenone, and thiol-thione intramolecular proton transfer process have been investigated as well as the progressive change in the energy barrier due to the mediation of one and two ammonia (or one, two, and three water) molecules. The results reported here predict that 2-pyridineselenone is more polar and more stable than 2-pyridineselenol, and this order of stability remains unchanged upon complexation with one, two, or three water molecules, or complexation with one or two ammonia molecules. The same conclusion is observed for 2-pyridinethiol/2-pyridinethione as well as 2-hydroxypyridine/2-pyridone molecular systems.