Interactions of Dimethyltin(IV) with Uracil As Studied in the Gas Phase

The gas-phase interactions of uracil (Ura) with dimethyltin(IV) were studied by a combined experimental and theoretical approach. Positive-ion electrospray spectra show that the interaction of dimethyltin(IV) with Ura results in the formation of the [(CH 3 ) 2 Sn(Ura-H)] + ion. The tandem mass spectrometry spectrum of this complex is characterized by numerous fragmentation processes, notably associated with elimination of H,N,C,O and C 3 ,H 3 ,N,O moieties, as well as the unusual loss of C 2 H 6 leading to the [Sn(Ura-H)] + complex. In turn, the [Sn(Ura-H)] + complex fragments according to pathways already observed for the [Pb(Ura-H)] + analogue. Sequential losses of ·CH 3 radicals are also observed from the [(CH 3 ) 2 Sn(N,C,O)] + species (m/z 192). Comparison between density functional theory-computed vibrational spectra and the infrared multiple photon dissociation spectrum recorded between 1000 and 1900 cm -1 shows a good agreement as far as the global minimum is concerned. This comparison points to a bidentate interaction with a deprotonated canonical diketo form of uracil, involving both the N3 and O4 electronegative centers. This binding scheme has been already reported for the Pb/uracil system. The bidentate form characterized by the interaction between dimethyltin with N3 and O2 centers is slightly less stable. Interconversion between the two structures is associated with a small activation barrier (56 kJ/mol). The potential energy surfaces were explored to account for the main fragmentations observed upon collision-induced dissociation.