Study of the solvent effect on the enthalpies of homolytic and heterolytic N–H bond cleavage in p-phenylenediamine and tetracyano- p-phenylenediamine

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In this article, we have studied p-phenylenediamine (PPD) and tetracyano- p-phenylenediamine (TCPPD) molecules in order to study the effect of CN groups and the solvent effect on the enthalpies of homolytic and heterolytic N–H bond cleavage. Geometries of the molecules and reaction enthalpies related to hydrogen atom transfer, single electron transfer–proton transfer (SET–PT) mechanism and sequential proton loss electron transfer (SPLET) mechanisms were studied using DFT/UB3LYP/6-31++G∗∗ method. Ab initio MP2/6-31++G∗∗ method was used as the reference for the geometry calculation of the two molecules in vacuum. Solvent contribution to the enthalpies was computed employing integral equation formalism IEF-PCM method. Obtained results show that solvent is able to cause significant change in the reaction enthalpies of the stepwise SET–PT and SPLET mechanisms of hydrogen splitting-off from NH 2 group. This may result in the change in thermodynamically preferred mechanism. Solvents also attenuate the CN-substituent effect in the case of SET–PT and SPLET mechanisms.