2013. 54, 2 UDC 541.6:547.12 QUANTUM CHEMICAL CALCULATION FOR THE INHIBITORY EFFECT OF COMPOUNDS A.Y. Musa1, W. Ahmoda1, A.A. Al-Amiery1,2, A.A.H. Kadhum1, A.B. Mohamad1 1Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 2Biotechnology Division, Applied Science Department, University of Technology, Baghdad, Iraq E-mail: dr.ahmed@eng.ukm.my; dr.ahmed1975@gmail.com Received October, 27, 2011 Revised — January, 19, 2012 The effects of the molecular structure on the corrosion inhibition efficiency are investigated by nine methods of calculations. <...> The selected thio compounds were previously identified as corrosion inhibitors for mild steel in the 1.0 M HCl solution. <...> The electronic properties such as highest occupied molecular orbital (EHOMO) energy, lowest unoccupied molecular orbital (ELUMO) energy, dipole moment (), and Fukui indices are calculated and discussed. <...> Results show that the corrosion inhibition efficiency increase with the increase in both EHOMO and values, respectively, and decrease in ELUMO. <...> QSAR approach is utilized in this study; a good relationship is found between the experimental corrosion inhibition efficiency (IEExp%) and the theoretical corrosion inhibition efficiency (IETheo%). <...> The calculated inhibition efficiency is found closer to the experimental inhibition efficiency with a coefficient of correlation (R2) of 0.875. <...> Corrosion is the destructive attack of metal or alloy chemically or electrochemically against its environment which leads to the loss of useful properties of materials. <...> Corrosion inhibitor can effectively reduce the corrosion rate of metallic materials in acid solution and has been widely applied in acidic cleanout, crude oil refiner, electrochemical and chemical etching [ 2 ]. <...> Organic compounds, mainly containing oxygen, nitrogen and sulfur atoms and having multiple bonds, are recognized as effective inhibitors of the corrosion of many metals and alloys [ 3 ]. <...> Inhibitors can adhere to a metal surface to form a protective barrier against corrosive agents in contact with metal [ 4 ]. <...> The effectiveness of an inhibitor to provide corrosion protection depends to a large extent on the interaction between the inhibitor and the metal surface. <...> The most efficient corrosion inhibitors are organic compounds containing electronegative functional groups and electrons in their triple or conjugated double bonds [ 5 ]. <...> Some thio <...>