Non-covalent interactions play an important role in chemical and biochemical processes. The interplay of the 1,5 intramolecular Y -> X (Y=O, S and X=S-H) bonds in quasi-cyclic systems (1(Z) and 2(Z)) to govern the equilibrium ratio has been examined with quantum-chemical studies. The S center dot center dot center dot O interaction energy has been found to be 6.7 KJ/mol calculated at CBS-QB3 level in the gas phase, and stronger than S center dot center dot center dot S interaction (5.9 KJ/mol). The interactions of S center dot center dot center dot X (X= O, S) are less affected with the polarity of solvents than hydrogen bonds. The AIM analysis reveals that the S center dot center dot center dot O interaction is mainly governed by electrostatic effects than hyperconjugative orbital interactions. (C) 2015 Elsevier B.V. All rights reserved.