Blackbody Infrared Radiative Dissociation of Partially Solvated Mixed Ligand Ru(II) Complex Ions

Abstract

Unimolecular dissociation reactions of gas-phase partially solvated transition metal complex ions, diamminebis(2,2‘-bipyridine)ruthenium(II), [Ru(NH3)2(bipy)2]2+, tetraammine(2,2‘-bipyridine)ruthenium(II), [Ru(NH3)4bipy]2+, and pentaammine(2-methylpyrazine)ruthenium(II), [Ru(NH3)5MePyz]3+, have been investigated with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI−FTICR/MS) under low pressure (<10-8 mbar) conditions. Under these conditions, dissociation of solvent from the complex ions is driven primarily by absorption of blackbody photons from the FTICR analyzer cell walls, that is, via blackbody infrared radiative dissociation (BIRD), with little collisional contribution at pressures below 10-8 mbar. The replacement of ammine hydrogens with deuterium increases measured BIRD rate constants. Optimized geometries, vibrational frequencies, and IR absorption band intensities have been calculated for the complexes by semiempirical and ab initio methods. These calculated parameters have been employed in master equation modeling of the desolvation reactions to extract dissociation energetics. Solvation energies obtained by master equation modeling were found to be in the range 19 ± 1 kcal/mol for the dissociation of acetone from various mixed ligand Ru(II) complex ions studied in this work. This is about 4 kcal/mol higher than for the comparable complexes containing only bipyridyl ligands and no ammine ligands.