Acta Chimica Slovaca, Vol.1, No. 1, 2008, 72 – 84 Quantum-chemical study of N,N´-diphenyl-p-phenylenediamine (DPPD) dehydrogenation Anton Gatial*, Júlia Polovková a , Martin Breza Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology, SK-81237 Bratislava, Slovakia a Institute of Measurement Science, Slovak Academy of Sciences, SK-84104 Bratislava, Slovakia * anton.gatial@stuba.sk In the memory of Professor Vojtech Kellö (1919–2007) Abstract Using B3LYP/6-31G* treatment, the optimal geometries and IR spectra of N,N´-diphenyl-p- phenylenediamine antioxidant (DPPD) and of N,N´-diphenyl-p-quinonediimine (DQDI) as its double dehydrogenated oxidation product have been obtained. The complete conformation analysis predicts the existence of four stable conformers of each of the systems under study. Experimental IR spectra of DPPD sample heated on air at 140 ºC confirmed the DQDI formation even at increased temperatures. Keywords: Antioxidants; N,N´-substituted p-phenylenediamines; dehydrogenated structures; IR spectra; DFT geometry optimization. Introduction Aromatic secondary amines, particularly N,N´-substituted p-phenylenediamines (PPD), represent the most important group of antioxidants used in rubber industry (Cataldo 2001, 2002). Antioxidants reduce the concentration of radicals that determines the rate of the oxidation reaction. It is supposed that benzoquinonediimines are reaction products of PPD degradation. EPR spectroscopy confirmed the formation of aminyl radicals and especially the nitroxide radicals originating from the -NH- bridge oxidation by ROO • radicals (Landolt- Börnstein 1980a, 1980b, Male 1988, Petr 1988, Omelka 2001, Burian 2003). On the other hand, non-isothermal DSC studies of the antioxidant effectiveness of a series of N-alkyl-N´-aryl-p-phenylenediamines in polyisoprene rubber (Cibulková 2005a,