Examination of barriered and barrierless hydrogen atom abstraction reactions by organic radical cations:: the cytosine radical cation

The radical cations of the nucleobases cytosine and adenine are easily generated in a dual cell Fourier transform ion cyclotron resonance mass spectrometer by using the technique of laser-induced acoustic desorption coupled with electron ionization. The cytosine radical cation is found to undergo facile hydrogen atom abstraction reactions with a variety of neutral reagents even when barrierless, exothermic electron transfer reactions are expected to dominate. In contrast, the adenine radical cation undergoes hydrogen atom abstraction only slowly in most of the cases studied. The ionic curve-crossing model is used to rationalize these results. Some of the thermodynamic and molecular properties of a radical cation that are favorable for efficient hydrogen atom abstraction are described in this article. Based on this analysis, dimethyl sulfoxide radical cation was predicted to abstract hydrogen atoms efficiently. Indeed, it was found to abstract hydrogen atoms at rates similar to cytosine radical cation. The transition from barriered to barrierless hydrogen atom abstraction that occurs as the ionization energy of the neutral hydrogen atom donor is lowered was used to "bracket" the recombination energy of cytosine radical cation at 8.9 +/- 0.2 eV. (C) 2001 Elsevier Science B.V.