Electron transfer driven decomposition of adenine and selected analogs as probed by experimental and theoretical methods

9 pags., 6 figs., 1 tab.We report on a combined experimental and theoretical study of electron-transfer-induced decomposition of adenine (Ad) and a selection of analog molecules in collisions with potassium (K) atoms. Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6-68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine (Pu), adenine (Ad), 9-methyl adenine (9-mAd), 6-dimethyl adenine (6-dimAd), and 2-D adenine (2-DAd). Following our recent communication about selective hydrogen loss from the transient negative ions (TNIs) produced in these collisions [T. Cunha et al., J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process.T.C., M.M., and F.F.d.S. acknowledge the Portuguese National Funding Agency FCT-MCTES through Nos. SFRH/BD/52538/2014, PD/BD/106038/2015, and researcher position IF-FCT IF/00380/2014, respectively, and together with PLV the research Grant No. UID/FIS/00068/2013. This work was also supported by Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, No. PD/00193/2010); No. UID/Multi/04378/2013 (UCIBIO). G.G. acknowledges partial financial support from the Spanish Ministerio de Economia, Industria y Competitividad (Project No. FIS2016-80440). S.E. acknowledges the support of the British EPSRC through a Career Acceleration Fellowship (No. EP/J002577/1) and a Research Grant (No. EP/L002191/1). M.-C.B.-M. acknowledges support from the computational resources from the CCIN2P3 in Villeurbanne and CCRT/CINES/IDRIS by GENCI (Grand Equipement National de Calcul Intensif) under the Allocation No. x201708766