Structures and energetics of electrosprayed uracilnCa2+ clusters (n = 14–4) in the gas phase

Clusters of uracil (U) about a calcium dication, UnCa2+ (n = 14–4), have been studied in the gas phase by both experimental and theoretical methods. Temperature dependent blackbody infrared radiative dissociation (BIRD) experiments were performed on UnCa2+ clusters with n = 14–5 and the observed Arrhenius parameters are reported here. Master equation modeling of the BIRD kinetics data was carried out to determine threshold dissociation energies. Initial geometry calculations were performed using the B3LYP density functional and 3-21G(d) basis set. A sample of ten conformations per cluster was obtained through a simulated annealing study. These structures were optimized using B3LYP/6-31G(d) level of theory. Fragment-based hybrid many body interaction (HMBI) MP2/6-311++G(2df,2p)/Amoeba calculations were performed on representative conformations to determine theoretical binding energies. Results were examined in relation to cluster size (n). A significant increase in the energy required to remove uracil from U6Ca2+ when compared to larger clusters supports previous reports that the calcium ion is coordinated by six uracil molecules in the formation of an inner shell. For clusters larger than n = 6, an odd-even alternation in threshold dissociation energies was observed, suggesting that the outer shell uracil molecules bind as dimers to the inner core. Proposed binding schemes are presented. Multiple structures of U5Ca2+ are suggested as being present in the gas phase where the fifth uracil may be either part of the first or second solvation shell.