2-MERCAPTOBENZOTHIAZOLE (MBT, C7H5NS2) AND 2-MERCAPTOBENZOXAZOLE (MBO, C7H5NOS) HAVE BEEN WIDELY USED AS A LIGAND FOR PRECONCENTRATION OF SILVER CATION [1, 2], FOR INHIBITION OF THE CORROSION OF CU AND FE AS FUNGICIDAL REAGENTS [3, 4]. BENZOXAZOLIN- 2(3H)-ONE (BOA) WAS FIRST ISOLATED AS A NATURAL ANTIFUNGAL FROM RYE SEEDLINGS. SINCE THEN ITS PROTECTIVE ACTIVITY AND DETOXIFICATION BY FUNGI HAVE BEEN STUDIED BY A NUMBER OF RESEARCHERS AS HAVE ITS ALLELOPATHIC EFFECTS ON THE SEEDLINGS OF VARIOUS SPECIES; AND IT IS ON THIS LATTER ACTIVITY, AND THE RESPONSE OF TARGET PLANTS, THAT RESEARCH ON BOA HAS FOCUSED IN RECENT YEARS. IN THIS WORK, DFT METHODS WERE USED TO INVESTIGATE THE EFFECT OF HYDRATION AND SELF-ASSOCIATION ON THE REACTION MECHANISM OF PROTON TRANSFER IN MBT, MBO AND BOA. GEOMETRY OPTIMIZATIONS OF STABLE SPECIES AND TRANSITION STATE (TS) WERE DONE IN THE GAS PHASE AND AQUEOUS PHASE USING B3LYP/6-311++G(2D, 2P) AND B3LYP/6- 311++G(DF, PD) LEVEL OF THEORY. HARMONIC VIBRATIONAL FREQUENCIES WERE COMPUTED TO VERIFY THE NATURE OF MINIMA AND TRANSITION STATES OF STATIONARY POINT ALONG THE TAUTOMERIZATHION PATH. THE AIM ANALYSIS WAS CARRIED OUT IN B3LYP/6-311++G(2D, 2P) LEVEL OF THEORY. IN THIS WORK, WE INVESTIGATED THE PROTON TRANSFER VIA INTRAMOLECULARMECHANISM, WATER-ASSISTED AND SELF-ASSISTED TAUTOMERIZATION IN MBT AND MBO AND BOA. ON THE BASIS OF THE RESULTS OBTAINED FROM OUR CALCULATIONS, THE THIONE COMPLEXES ARE MORE STABLE THAN CORRESPONDING THIOLS. ALSO THE STABILITY OF COMPLEXES INCREASES WITH THE INCREMENT OF THE NUMBER OF S…H HYDROGEN BONDS. THE H-BOND INTERACTIONS IN THE WATER-ASSOCIATION COMPLEXES ARE STRONGER THAN SELF-ASSOCIATION COMPLEXES. THE BARRIER ENERGY FOR BOTH SELF-ASSISTED AND H2OASSISTED PROTON TRANSFER IS LOWER THAN THAT OF THE DIRECT PROTON TRANSFER. AS A RESULT, H2OASSISTED TAUTOMERIZATION IS ENERGETICALLY A SUITABLE PATH FOR PROTON TRANSFER.