Ionization-induced proton transfer in thymine–ammonia van der Waals clusters

Ion intensity distribution of thymine–ammonia clusters produced in a supersonic jet was investigated using the resonant 2-photon ionization technique. The mass spectrum of Thym(NH3)n (m = 1–7) exhibited an anomalously strong ion intensity for n = 1 in contrast to the nearly negligible ion signals for n > 1. We suggest that proton transfer from the thymine radical cation to an ammonia molecule following the ionization of the clusters is responsible for the observed anomaly. It is also proposed that charge migration occurring with the proton transfer leads to an ion core switch from the thymine radical cation to the newly formed ammonium ion. The subsequent evaporation of other ammonia molecules in the cluster ion as a consequence of the energy released from the reaction results in extensive loss of ion signals for n > 1, and at their expense, an anomalously large ion intensity for n = 1. This mechanism is supported by density functional theory calculations on the thymine–ammonia 1:1 complex ion performed along the reaction coordinate of the proton transfer. The formation of the ammonium ion in the cluster is also confirmed by the fragmentation feature of metastable Thym(NH3)1+ (m = 1–4) obtained using reflectron time-of-flight mass spectrometry.