The role of structural chemistry in the inhibitive performance of some aminopyrimidines on the corrosion of steel M.S. Masoud a , M.K. Awad b , M.A. Shaker c , M.M.T. El-Tahawy c, * a Chemistry Department, Faculty of Science, Alexandria University, Egypt b Chemistry Department, Theoretical Applied Chemistry Unit, Faculty of Science, Tanta University, Egypt c Chemistry Department, Faculty of Science (Damanhour), Alexandria University, Egypt article info Article history: Received 5 December 2009 Accepted 12 April 2010 Available online 4 May 2010 Keywords: A. Aminopyrimidine derivatives B. Corrosion inhibition C. Structural parameters C. Molecular orbital calculations abstract Corrosion inhibitive performance of 2-aminopyrimidine (APr), 2,4-diaminopyrimidine (dAPr), 2,4-dia- mino-6-hydroxy-pyrimidine (dAHPr) and 2,4,6-triaminopyrimidine (tAPr) during the acidic corrosion of steel surface was investigated using three methods, MP2, ab initio Hartree–Fock and density functional theory (DFT). Quantum chemical parameters such as the highest occupied molecular orbital energy (E HOMO ), the lowest unoccupied molecular orbital energy (E LUMO ), energy gap (DE), dipole moment (l), sum of the total negative charge (TNC), molecular volume (MV), electronegativity (v), chemical potential (Pi), global hardness (g), softness (r) and the fraction of electrons transferred (DN), were calculated. Fur- thermore, the interaction energies of the inhibitors with the iron surface were studied. A good correlation was found between the theoretical data and the experimental results. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Study of corrosion inhibition is a very active field of research. Several classes of organic compounds are widely used as corrosion inhibitors for metals in acid environments [1–8]. Experimental means are useful to explain the inhibition mechanism but they are often expensive and time-consuming. Ongoing hardware and software advances have opened the door for powerful use of theo- retical chemistry in corrosion inhibition research. Several quantum chemical methods and molecular modeling techniques have been performed to correlate the inhibition efficiency of the inhibitors with their molecular properties [9–15]. Using theoretical parame- ters helps to characterize the molecular structure of the inhibitors and to propose their interacting mechanism with surfaces [16]. Some studies have shown that the inhibition of the corrosion pro- cess is mainly described by the formation of donor–acceptor sur- face complexes between free or p-electrons of an organic inhibitor, mostly containing nitrogen, sulphur or oxygen atoms, and a vacant d-orbital of a metal [17–26]. Pyrimidines which are important compounds in many fields [27,28] have been reported earlier as corrosion inhibitors for steel [29]. Recently, Abdallah et al. [30] experimentally found that the aminopyrimidine derivatives show inhibitive properties for 1018 carbon steel in nitric acid solution. The relationships between the structural parameters and corrosion inhibition of those compounds have not been studied yet. This present work is designed to corre- late the observed inhibition efficiency with the quantum chemical parameters of the investigated aminopyrimidine inhibitors (Fig. 1). The calculated quantum chemical parameters are E HOMO , E LUMO , DE, l, and those parameters that give valuable information about the reactive behavior such as TNC, MV, v, Pi, g, r and DN. Further- more, the interaction energies of the investigated inhibitors on the iron surface were studied to discuss the inhibition mechanism. 2. Theory and computational details The molecular structures of the investigated compounds were optimized initially with PM3 semiempirical method so as to speed up the calculations. The resulting optimized structures were fully re-optimized using: (i) ab initio MP2/3-21g * (ii) ab initio Hartree– Fock method (HF) using 6-31++G(d,p) basis set at Restricted Hartree–Fock level (RHF) [31], (iii) DFT method using 6-31++G(d,p) basis set. The B3LYP function is a hybrid method (HF wave func- tion + DFT energy calculation) that adds the exchange and electronic correlation in DFT terms including the Lee, Yang and Parr (LYP) func- tional [32–34]. The molecules were built with the GaussView 3.0 implemented in Gaussian 03 package [35]. Pyrimidine and its deriv- atives are easily protonated in acidic medium; therefore the calcula- tions were extended to the protonated forms of the pyrimidines under study. These molecules have imin-enamine tautomerism. All possible tautomeric forms were calculated at gaseous and solvent phases. The calculations showed that the most stable forms of 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.04.011 * Corresponding author. Tel.: +20 109679305; fax: +20 453368757. E-mail address: mohsen_taha26@yahoo.com (M.M.T. El-Tahawy). Corrosion Science 52 (2010) 2387–2396 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci