Quantum chemical studies on some imidazole derivatives as corrosion inhibitors for iron in acidic medium G. Bereket * , E. Hu Èr, C. O È g Æretir Department of Chemistry, Faculty of Arts and Science, Osmangazi University, 26020 Eskisehir, Turkey Received 19 June 2001; accepted 30 July 2001 Abstract In this work, computational studies on some imidazole derivatives which behave as corrosion inhibitors at various levels for iron in hydrochloric acid were carried out in gaseous and aqueous phases using semiemprical methods such as MINDO/3, MNDO, PM3 and AM1. Possible correlations between experimentally obtained inhibition ef®ciencies and calculated highest occupied molecular energy level E HOMO ), lowest unoccupied molecular energy level E LUMO ) and the differences between highest occupied molecular orbital HOMO) and lowest unoccupied molecular orbital LUMO) energies E LUMO 2 E HOMO  were studied. Correlation between inhibition ef®ciencies and total charges Z) of molecules, were also investigated. Regression analysis were performed on inhibition ef®ciencies P), E HOMO ) and E LUMO ) energies of the studied molecules. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Imidazole derivatives; Corrosion inhibitor; Semiemprical methods; Quantum chemical calculations 1. Introduction Testing organic inhibitors using experimental methods, has been carried about for a half century. However, increasing number of publications attempt- ing to correlate the structure of corrosion inhibitors their state of adsorption at the metal/solution interface and inhibition effectiveness are appearing in the litera- ture in the last two decades. The semi empirical methods in which macroscopic or quantum chemical properties are used calculated or measured) success- fully enables the correlation of computed data with inhibition ef®ciencies. Studies on the relationship between the molecular structure of organic molecules and their inhibitory effect on corrosion were undertaken [1±10]. The elec- tronic properties of inhibitors, the effect of highest occupied molecular orbital HOMO) and lowest unoc- cupied molecular orbital LUMO) energies and the lypophilic/hydrophilic nature of the studied organic compounds were reported in these studies. Various approximate equations for different organic compounds have been used, which the authors described by different types of chemisorptions depending on mode of adsorption. Many nitrogen atom containing, heterocyclic compounds with polar groups and/or p electrons are ef®cient inhibitors of corrosion in acidic media [11]. These kind of organic molecules can be adsorbed on the metal surface because a bond can be formed between the electron pair of N atoms and/or the p electron cloud, thereby reducing the corrosive attack of metals in acidic media [11,12]. In the present study, the correlations between the Journal of Molecular Structure Theochem) 578 2002) 79±88 0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0166-128001)00684-4 www.elsevier.com/locate/theochem * Corresponding author. Tel.: 190-222-2290433; fax: 190-222- 2393578. E-mail address: gbereket@ogu.edu.tr G. Bereket).