Infrared Ion Spectroscopy of Gaseous [Cu(2,2'-Bipyridine)]: Investigation of Jahn-Teller Elongation Versus Compression.

Symmetry breaking is ubiquitous in chemical transformations and affects various physicochemical properties of materials and molecules; Jahn-Teller (JT) distortion of hexa-coordinated transition-metal-ligand complexes falls within this paradigm. An uneven occupancy of degenerate 3d-orbitals forces the complex to adopt an axially elongated or compressed geometry, lowering the symmetry of the system and lifting the degeneracy. Coordination complexes of Cu are known to exhibit axial elongation, while compression is far less common, although this may be due to the lack of rigorous experimental verification. Here, we present the gas-phase vibrational spectrum of the archetypal [Cu(2,2'-bipyridine)] ionic complex, obtained by infrared multiple-photon dissociation (IRMPD) spectroscopy using the widely tunable IR free-electron laser FELIX. Predicted vibrational spectra at the density functional level of theory are nearly─but not entirely─identical for the two JT-distorted geometries. We compare experimental and theoretical spectra and address the question of an axially elongated or compressed geometry of the complex, or a mixture thereof, in the gaseous ion population.