Elucidation of Electron Ionization Induced Fragmentations of Adenine by Semiempirical and Density Functional Molecular Dynamics

The gas phase fragmentation pathways of the nucleobase adenine upon 70 eV electron ionization are investigated by means of a combined stochastic and first-principles based molecular dynamics approach. We employ no preconceived fragmentation channels in our calculations, which simulate standard electron ionization mass spectrometry (EI-MS) conditions. The reactions observed compare well to a wealth of experimental and theoretical data available for this important nucleic acid building block. All significant peaks in the experimental mass spectrum of adenine are reproduced. Additionally, the fragment ion connectivities obtained from our simulations at least partially concur with results from previous experimental studies on selectively isotope labeled adenines. Moreover, we are able to assign noncyclic structures that are entropically favored and have not been proposed in nondynamic quantum chemical studies before to the decomposition products, which result automatically from our molecular dynamics procedure. From simulations under various conditions it is evident that most of the fragmentation reactions even at low internal excess energy (<10 eV) occur very fast within a few picoseconds.