Theoretical study of Schiff base compounds as antiwear lubricant additives: A quantum chemical calculation approach

To correlate antiwear properties of Schiff bases derived from condensation of salicylaldehyde with 1,2-phenylenediamine, [2,2′-(1,2-phenylenebis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)diphenol; H₂Saloph], 1,4-phenylenediamine [2,2′-(1E,1′E)-(1,4-phenylenebis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)diphenol; H₂Salpph] and 4,4′-diaminodiphenylmethane, [2,2′-(1E,1′E)-(4,4′-methylenebis(4,1-phenylene)bis(azan-1-yl-1-ylidene)diphenol; H₂Saldphm] with their chemical structures, quantum chemical calculations based on density functional theory have been performed. The interaction between the additive molecules and metal surfaces has also been discussed. In order to compare the antiwear behavior, various parameters such as frontier molecular orbital energy, energy of highest occupied molecular orbital, energy of lowest unoccupied molecular orbital, energy gap and Mulliken charges have been calculated. The quantum chemical calculations have shown that wear reducing behavior of Schiff bases increases with increase in energy of highest occupied molecular orbital, decrease in energy of lowest unoccupied molecular orbital and also decrease in energy gap between energy of lowest unoccupied molecular orbital and energy of highest occupied molecular orbital of the additives. The quantum chemical calculations results have been found to be in good agreement with those of experimental results.