Abstract: Ab initio quantum chemical calculations with averaged relativistic effective potentials(AREP),including the effects of electron correlation,are performed on the reactions, M⁺+BrCN→MCN⁺+Br,Where M⁺ is the first-row transition-metal singly charged cation.The highest barrier for the reaction is found for the Mn⁺(485.54 kJ/mol),which is due to the large loss of exchange energy upon bond formation and large repulsion of the 4s electrons.According to the calculated barrier height,the reactivities are in agreement with experimental trends:V>Mn,Fe;Co>Cr.However,contrary to the experimental trends Sc>Ti.In our study,the transition state for scandium cation is considerably higher than that for titanium cation,because titanium cation is in the less repulsive state ⁴F(d²s¹)since it has lower s-population,and therefore it has lower barrier.We can explain why the reaction extent of the first row metal cations with cyanogen bromide was only a few percent,one of the main reasons is that the barrier height is too high.

Key words: Gas-phase reaction, Transition metal cations, Cyanogen bromide, Ab initio study, Relativistic effective potential