Vibrational Spectra of the Ruthenium-Tris-Bipyridine Dication and Its Reduced Form in Vacuo.

Experimental IR spectra in the 500-1850 cm fingerprint frequency range are presented for the isolated, gaseous redox pair ions [Ru(bpy)], and [Ru(bpy)], where bpy = 2,2'-bipyridine. Spectra are obtained using the FELIX free-electron laser and a quadrupole ion trap mass spectrometer. The 2+ complex is generated by electrospray ionization and the charge-reduced radical cation is produced by gas-phase one-electron reduction in an ion-ion reaction with the fluoranthene radical anion. Experimental spectra are compared against computed spectra predicted by density functional theory (DFT) using different levels of theory. For the closed-shell [Ru(bpy)] ion, the match between experimental and computed IR spectra is very good; however, this is not the case for the charge-reduced [Ru(bpy)] ion, which demands additional theoretical investigation. When using the hybrid B3LYP functional, we observe that better agreement with experiment is obtained upon reduction of the Hartree-Fock exact-exchange contribution from 20% to about 14%. Additionally, calculations using the M06 functional appear to be promising in terms of the prediction of IR spectra; however, it is unclear if the correct electronic structure is obtained. The M06 and B3LYP functionals indicate that the added electron in [Ru(bpy)] is delocalized over the three bpy ligands, while the long-range corrected LC-BLYP and the CAM-B3LYP functionals show it to be more localized on a single bpy ligand. Although these latter levels of theory fail to reproduce the experimentally observed IR frequencies, one may argue that the unusually large bandwidths observed in the spectrum are due to the fluxional character of a complex with the added electron not symmetrically distributed over the ligands. The experimental IR spectra presented here can serve as benchmark for further theoretical investigations.