IRMPD spectroscopic and computational study of gas phase [M(Ura-H)(Ura)]+ and [M(Ura-H)(H2O)n]+ (M = Sr, Ba; n = 1, 2) complexes

Infrared multiple photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer is used to investigate the structures of Sr2+ or Ba2+ complexes with uracil in the gas phase. [Sr(Ura-H)(Ura)]+, [Ba(Ura-H)(Ura)]+, [Sr(Ura-H)(H2O)]+, [Ba(Ura-H)(H2O)]+, and [Sr(Ura-H)(H2O)2]+ were examined in the NH/OH stretching region from 3200 to 3900 cm−1. Calculations for the corresponding structures and infrared spectra were carried out using the B3LYP density functional theory in conjunction with the 6-31+G(d,p) basis set for the non-metal atoms and with the LANL2DZ basis set with relativistic core potential on Ba and Sr. The results were in good agreement with similar studies involving Pb2+ which showed that all lowest energy monomer structures involved an N3 deprotonated uracil molecule with the metal cation bound between the N3 and O4 positions. The [Sr(Ura-H)(Ura)]+ and [Ba(Ura-H)(Ura)]+ complexes were found to have a tetracoordinate metal ion, and both singly and doubly solvated complexes proved to have the water molecules coordinated to the metal ion. For all the complexes studied, the computed spectra for the lowest energy structures agreed very well with the experimental IRMPD spectra obtained. The structures of the Sr2+- and Ba2+-centred complexes are compared with those of the Pb2+-centred complexes.

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► The structures of [Sr(Ura-H)(Ura)]+ and [Ba(Ura-H)(Ura)]+ were investigated using IRMPD spectroscopy and are found to be similar to that found for [Pb(Ura-H)(Ura)]+ in a previous study, except that the latter was very much non-planar due to the lone pair of electrons.
► All three species are indistinguishable in the 3200–3900 cm−1 region.
► Singly solvated [Sr(Ura-H)]+ and [Ba(Ura-H)]+ are also found to be identical to [Pb(Ura-H)]+, however the doubly hydrated complexes are very different.
► All complexes are found to have a [M(Ura-H)]+ core which is deprotonated at N3 and the metal is bound to N3 and O4 or to N3 and O2.