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The high oxidation energy on bond S14-S15 maysuspect a thermal oxidation of the molecule.The relaxation of this molecule on the iron surfacepredicts the reaction of two non consecutive sulfuratoms with iron atoms (fig. 4).Fig. 2. excited state for oxidative reaction on S14 and S15atomsThe bond S14-S15 is the weakest bond for oxidationand for homolytic scission. This part of the moleculeis a clear candidate to react with a metallic surface.3.3 Reaction of DTDP molecule with iron surfaceA direct reaction on an iron surface is also analyzed.A layer of Fe atoms is created and locked torepresent a surface of bcc α-iron, with the samecrystallographic parameters. The iron functional isavailable inside PM5 basis-set and is able to predictreaction with DTDP sulfur based molecule. A DTDPmolecule is then located near the surface withvarious orientations of sulfur atoms. Figure 3presents one initial scheme.Fig. 4. Conformation of a DTDP molecule after reaction withthe iron surface (tilt/iron plane ~ 60°)This type of reaction leads to isolated sulfur-ironbonds distributed on the surface which are weakenedinside the DTDP molecule (fig. 6).Other simulations with the same molecule but with adifferent tilt-angle with the surface lead a differentreaction path (fig. 5).Fig. 5. Conformation of a DTDP molecule after reaction withthe iron surface (side and bottom view) (tilt/plane ~ 80°)Fig. 3. An initial conformation of a DTDP molecule near theiron surfaceAll the sulfur atoms react with an iron atom, ormore, which leads to a cluster of Fe-S bonds on ironsurface. These bonds are strong and weaken theintramolecular bonds, measured by the reducingtheir covalent order (fig. 7).