Structures of the dehydrogenation products of methane activation by 5d transition metal cations

TitleStructures of the dehydrogenation products of methane activation by 5d transition metal cations
Publication TypeJournal Article
Year of Publication2013
AuthorsV.JF Lapoutre, B. Redlich, A.FG van der Meer, J. Oomens, J.M Bakker, A. Sweeney, A. Mookherjee, P.B Armentrout
JournalJournal of Physical Chemistry A

The activation of methane by gas-phase transition metal cations (M +) has been studied extensively, both experimentally and using density functional theory (DFT). Methane is exothermically dehydrogenated by several 5d metal ions to form [M,C,2H]+ and H2. However, the structure of the dehydrogenation product has not been established unambiguously. Two types of structures have been considered: a carbene structure where an intact CH2 fragment is bound to the metal (M +-CH2) and a carbyne (hydrido-methylidyne) structure with both a CH and a hydrogen bound to the metal separately (H-M+-CH). For metal ions with empty d-orbitals, an agostic interaction can occur that could influence the competition between carbene and carbyne structures. In this work, the gas phase [M,C,2H]+ (M = Ta, W, Ir, Pt) products are investigated by infrared multiple-photon dissociation (IR-MPD) spectroscopy using the Free-Electron Laser for IntraCavity Experiments (FELICE). Metal cations are formed in a laser ablation source and react with methane pulsed into a reaction channel downstream. IR-MPD spectra of the [M,C,2H]+ species are measured in the 300-3500 cm-1 spectral range by monitoring the loss of H (2H in the case of [Ir,C,2H]+). For each system, the experimental spectrum closely resembles the calculated spectrum of the lowest energy structure calculated using DFT: for Pt, a classic C2v carbene structure; for Ta and W, carbene structures that are distorted by agostic interactions; and a carbyne structure for the Ir complex. The Ir carbyne structure was not considered previously. To obtain this agreement, the calculated harmonic frequencies are scaled with a scaling factor of 0.939, which is fairly low and can be attributed to the strong redshift induced by the IR multiple-photon excitation process of these small molecules. These four-atomic species are among the smallest systems studied by IR-FEL based IR-MPD spectroscopy, and their spectra demonstrate the power of IR spectroscopy in resolving long-standing chemical questions. © 2013 American Chemical Society.







Alternate TitleJ. Phys. Chem. A

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