Abstract:Metal-benzene complexes of the form M(benzene)n (M ) Ti, V, Fe, Co, Ni) are produced in the gas-phase environment of a molecular beam by laser vaporization in a pulsed nozzle cluster source. These complexes are photoionized with an ArF excimer laser, producing the corresponding cations. The respective mono-and dibenzene complex ions are isolated in an ion-trap mass spectrometer and studied with infrared resonance enhanced multiple-photon dissociation (IR-REMPD) spectroscopy using a tunable free electron laser. Photodissociation of all complexes occurs by the elimination of intact neutral benzene molecules, and this process is enhanced on resonances in the vibrational spectrum, making it possible to measure vibrational spectroscopy for size-selected complexes. Vibrational bands in the 600-1700 cm -1 region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. The spectra in this solvent-free environment exhibit periodic trends in band shifts and intensities relative to the free benzene molecule that varies with the metal. Density functional theory calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The comparison between experiment and theory provides fascinating new insight into the bonding in these prototypical organometallic complexes.
Resonant IR excitation of gas-phase molecules and clusters can lead to superhot species that thermally emit an electron. Monitoring the mass selected ion yield as a function of IR laser frequency yields the IR-REMPI (infrared resonance enhanced multiphoton ionization) spectrum of the molecule or cluster. Although this IR-REMPI spectrum is not the same as the linear absorption spectrum, it can be quite similar and it yields valuable information on the IR optical properties of the species investigated. In this article, the method and the necessary tools are presented. Results from experiments on fullerenes, metal carbide, metal oxide, and metal nitride clusters are shown.
Metal-benzene ions have a special attraction because of their relevance for catalysis and biological processes, 1,2 and for the importance of aromatic π-bonding to organometallic chemistry. 3,4 These complexes are also interesting because they form sandwich structures. Gas-phase complexes can in principle be compared to those produced by conventional synthesis. Unfortunately, there are few studies of spectroscopy that provide insight into the structures and bonding of these species. Infrared (IR) spectroscopy has been applied to condensed-phase complexes, 3,4 but such measurements have not been possible in the gas phase. We report here a new method for the study of the IR spectroscopy of mass-selected metal ion complexes. The results for vanadium-benzene provide the first IR spectroscopy for transition metal ion-benzene complexes in the gas phase.Metal-benzene sandwiches, including bis-benzene vanadium, are familiar in the condensed phase 3,4 and in gas-phase ion chemistry. [5][6][7][8][9][10][11][12][13][14][15] As shown by Kaya, gas-phase V x (benzene) y complexes form multiple-decker sandwiches. 8 Bowers probed these complexes with ion mobility measurements. 9 Kaya reported a partial IR spectrum for the 1:2 complex that was size-selected as a cation, and then trapped in a rare gas matrix and neutralized. 10 Lisy has measured IR spectra of alkali cation-benzene complexes, 11 but there are no vibrational spectra for any ionized transition metal complex with benzene. Low ion densities and poor IR sources have precluded such studies.In this work, V(C 6 H 6 ) 1,2 complexes are produced in a molecular beam by laser vaporization. 6 Neutral complexes enter an ion trap mass spectrometer, 16 where they are ionized with an ArF excimer laser (193 nm; 6.42 eV). Efficient one-photon ionization occurs for both complexes at this wavelength. 8 The cations are trapped for milliseconds, and isolated by mass with the RF potentials of the trap. Tunable IR excitation is accomplished with the Free Electron Laser for Infrared eXperiments (FELIX) 17 that produces intense 10-50 mJ pulses in the 600-1800 cm -1 region of this experiment. After IR excitation, the contents of the ion trap are extracted into a time-of-flight spectrometer for mass analysis. 16 Resonance enhanced multiphoton dissociation (IR-REMPD) produces the V + and V + (C 6 H 6 ) photofragments from V + (C 6 H 6 ) and V + (C 6 H 6 ) 2 , respectively. These ions are recorded as a function of wavelength to obtain IR spectra. From the known bond energies of V + (C 6 H 6 ) (55.8 kcal/mol; ∼19 500 cm -1 ) and V + (C 6 H 6 ) 2 (58.8 kcal/mol; ∼20 600 cm -1 ), 7 it is clear that dissociation is a
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