To understand the difference in inhibition efficiency against corrosion of iron surfaces between medium-sized azacycloalkanes and dialkyl amines containing the same number of carbon atoms, the adsorption of azacycloundecane and of dipentyl amine onto a model Fe(111) surface has been studied using the molecular orbital and density functional theoretical methods. B3LYP calculations show that azacycloundecane gives a stronger interaction with the surface from an energy point of view. The difference between the two amines in the strength of interaction originates predominantly from the changes in structure of the carbon chains that take place during the adsorption process. The calculations suggest that a large C-N-C bond angle does not lead to efficient secondary-amine inhibitors that prevent the corrosion of iron surfaces. This conclusion is supported by a paired interaction orbital analysis.