1023-1935/03/3902- $25.00 © 2003 åÄIä “Nauka /Interperiodica” 0177 Russian Journal of Electrochemistry, Vol. 39, No. 2, 2003, pp. 177–181. From Elektrokhimiya, Vol. 39, No. 2, 2003, pp. 197–202. Original English Text Copyright © 2003 by Lukovits, Shaban, Kálmán. INTRODUCTION The term “quantitative structure–activity relation- ships” (QSAR) was first used in pharmacology in 1964, when Hansch and his co-workers [1] discovered that variation of pharmacological efficiency of (closely- related) drugs might be explained in terms of simple variables, the substituent constants. Substituent con- stants were invented earlier [2] and are in fact numbers characterizing various groups (like –OH, –Cl, –NO 2 , -COOH, –CH 3 , etc.) and in principle do not depend on the fact which parent structure they are attached to. In pharmacology, QSAR research has produced thousands of papers, and there is even a scientific journal (besides others devoted to QSAR) bearing this abbreviation. The aim of these theoretical studies is to reduce the cost of research, since in many cases based on the results of a QSAR analysis, many prospective derivatives can be safely excluded from further screening, even without the necessity to synthesize them! In this paper the term “QSAR” will denote any relationship between quanti- tative measures of efficiency and indices related to the molecular structure. The relationship between efficiency and the struc- ture of corrosion inhibitors became, soon after Han- sch’s discovery, the topic of electrochemical research [3]. Nevertheless the number of papers published so far on QSAR is much smaller in corrosion research than in pharmacology. The aim of the present investigation is to summarize the results obtained in the field of QSAR of corrosion inhibitors. The following review is struc- tured as follows: results in terms of the parameters used to account for variations in the molecular structure will be discussed first (Sections 2–4, table), then the results obtained by nonlinear regression analysis (Section 5) will be reviewed briefly. INDICATOR VARIABLES The simplest way to account for variations in the molecular structure is to apply indicator variables. According to this approach, which was first used by Free and Wilson [4], the activity η (that is the inhibition efficiency, in our case) is equal to the sum of group con- tributions. Throughout this paper the following defini- tion of η will be used: (1) where i 0 denotes the corrosion current density in the inhibitor absence, and i denotes the same quantity in the inhibitor presence. As an example, consider a series of phenyl deriva- tives, where one derivative contains an ortho methyl, and a para –NO 2 . Then, (2) where [X] denotes the group contribution of substituent X, and the constant is the expected activity of the non- substituted molecule. Different positions of the same substituent (in our case, CH 3 in ortho- or para-posi- tions) have to be considered separately. The actual val- ues of the group contributions are easily obtained by using linear regression analysis [5]. It has to be noted that Eq. (2) (and Eq. (3), too) is related to constant con- centrations of the inhibitors, in disagreement with QSAR investigations, where pharmacological activity is constant, and the concentration, needed to evoke a (standard) response, varies. In a series of aromatic compounds, Dupin et al. [6, 7] were the first to perform a Free–Wilson type analysis for corrosion inhibitors. Using the Free–Wilson approach for a series of eighteen secondary amines, Kálmán et al. [8], have demonstrated a moderate corre- lation coefficient (R = 0.89) between experimental effi- ciencies and the estimated values. In both cases the results indicated that the optimal molecule was already a member of the set of molecules used to derive QSAR η i 0 i – ( 29 / i 0 , = η o -CH 3 [ ] n -NO 2 [ ] const, + + = Corrosion Inhibitors: Quantitative Structure–Activity Relationships* I. Lukovits, A. Shaban, and E. Kálmán Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary Received April 1, 2002; in final form, April 30, 2002 Abstract—Quantitative structure–activity relationships (QSAR) between corrosion inhibition efficiency and molecular structure may help promote the discovery of new, more efficient corrosion inhibitors. In this work various methods used in the QSAR studies of corrosion inhibitors are reviewed and discussed briefly. * This article was submitted by the authors in English. Dedicated to the seventieth anniversary of R.R. Dogonadze’s birth