Coincidence method for measuring the mass of neutral fragments emitted in a delayed fragmentation process from a singly charged molecule: Fragmentation pathway of adenine

We have studied by mass spectrometry the fragmentation pathways of a molecule of biological interest: adenine H₅C₅N₅0.3em0ex(Ad). The adenine molecules are ionized and excited in a single collision event mode with slow highly charged Ar⁸⁺ ions. A method based on the detection in coincidence of neutral and charged fragments has been developed for studying delayed fragmentation channels. The measured spectra show that the well-stated process, i.e., the successive emission of HCN fragments, is not the only fragmentation pathway. Although in the first fragmentation step the dominant channel is the emission of a neutral HCN⁰0.3em0ex(88%), in the second fragmentation step the branching ratios for the delayed emission of an HCN⁰ and an H₂CNCN⁰ from the intermediate parent ion H₄C₄N₄⁺0.3em0ex[(Ad-HCN)⁺] are surprisingly similar, 54% and 46%, respectively. As a consequence, the role of these two latter channels in the reversal process, i.e., the formation of adenine in gas phase, should be considered. Ab initio calculations of molecular energies show that the energy levels of these two fragmentation channels are very close. This is in qualitative agreement with the measured comparable branching ratios.