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 [(CH3)2Sn(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 C3,H3,N,O moieties, as well as the unusual loss of C2H6 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 ·CH3 radicals are also observed from the [(CH3)2Sn(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.