Blackbody Infrared Radiative Dissociation of Partially Solvated Tris(2,2‘-bipyridine)ruthenium(II) Complex Ions

Abstract

Electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry has been utilized to study solvates of tris(2,2‘-bipyridine)ruthenium(II). Spontaneous dissociation of solvent (acetonitrile, acetone, or methyl ethyl ketone) from the solvation shell occurs when these ions are trapped for extended periods of time in the ICR cell. The pressures employed in these experiments are low enough (10-9 to 10-8 mbar) to neglect (or partially neglect) collisional activation as a means for dissociation. Therefore, it is suggested that the solvated ruthenium species undergo dissociation following the absorption of blackbody infrared radiation. Solvent−complex dissociation has been studied at several pressures ranging from 10-9 to 10-8 mbar to provide a range of dissociation data in the low-pressure regime. The results reported here demonstrate the consistency of the dissociation rate constants at pressures that differ by an order of magnitude. Temperature dependence studies were performed to extract zero-pressure activation energies from Arrhenius analyses. Given the number of degrees of freedom and the magnitude of the rate constants at a given temperature of the ruthenium complex ion solvates, the experimental Arrhenius activation energies are likely to be substantially lower than the true bond dissociation energies. ZINDO semiempirical methods, which were calibrated against DFT and experimental values, have been used to determine optimized structures and vibrational frequencies for bipyridine-containing ruthenium(II) solvates. These parameters were then used both for master equation modeling and the truncated Boltzmann/modified Tolman approach, each of which provide calculated binding energies of the solvents to the ruthenium complex ion. Solvation energies in the range 15−20 kcal/mol were found for binding of solvent molecules in the first solvation shell of tris(2,2‘-bipyridine) ruthenium(II) ions.