Organometallic ion complexes provide models for catalytic processes, and they have been implicated as important species in astrochemistry, perhaps as carriers of optical signals such as the diffuse interstellar bands (DIBs). In the present work, we investigate the properties of transition metal cation complexes with acetylene or benzene molecules using a combination of infrared and UV-visible laser photodissociation spectroscopy. Ion-molecule complexes of the form M+(C2H2)n and M+(benzene)n are produced by laser vaporization in a pulsed-nozzle supersonic beam source. The ions are analyzed and selected by mass in a reflectron time-of-flight mass spectrometer. Vibrational spectroscopy measurements are performed with infrared photodissociation spectroscopy, using an IR-OPO laser and the method of "tagging" with argon atoms to enhance dissociation yields [1]. Electronic spectroscopy experiments use photodissociation with a UV-visible OPO laser system.
Infrared experiments investigate complexes of vanadium, platinum and iron with acetylene. This work establishes that the M+(C2H2) complexes have cation-π structures, with the metal cation located in a symmetric position above the triple bond of acetylene. Charge-transfer interactions cause the C‒H stretch vibrations to shift to lower frequencies compared to those in the free acetylene molecule. Multiple-acetylene complexes of iron or platinum reveal interesting coordination structures, but no intracluster reactions. Vanadium ion complexes with three or more ligands undergo intracluster reactions to form the metal ion-benzene complexes.
UV-visible spectroscopy on Fe+(C2H2), Fe+(benzene) and Fe+(benzene)2 find spectra that are quasi-continuous throughout the visible wavelength region, eliminating these ions as candidates to explain DIB spectra [2,3]. The energetic threshold where photodissociation first occurs provides the metal ion-ligand bond energy. Additional experiments employ the method of photofragment imaging to investigate the energetics of the cation-π bonds which form in these complexes [2,3]. Photodissociation thresholds and photofragment imaging measurements are in agreement on the cation-molecular bond energies in these complexes, and the values obtained also agree with previous results from collisional dissociation experiments.
References
[1] A. D. Brathwaite, J. H. Marks, I. J. Webster, A. G. Batchelor, T. B. Ward, M. A. Duncan, J. Phys. Chem. A 126, 9680 (2022).
[2] J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, J. Phys. Chem. A 127, 1244 (2023).
[3] J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, J. Phys. Chem. A 127, 2795 (2023).