title: Structures of the dehydrogenation products of methane activation by 5d transition metal cations.
abstract: 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 C(2v) 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.
authors.affiliation: FOM Institute for Plasma Physics Rijnhuizen, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands.
authors.forename: V J F
authors.initials: VJ
authors.lastname: Lapoutre
authors.forename: B
authors.initials: B
authors.lastname: Redlich
authors.forename: A F G
authors.initials: AF
authors.lastname: van der Meer
authors.forename: J
authors.initials: J
authors.lastname: Oomens
authors.forename: J M
authors.initials: JM
authors.lastname: Bakker
authors.forename: A
authors.initials: A
authors.lastname: Sweeney
authors.forename: A
authors.initials: A
authors.lastname: Mookherjee
authors.forename: P B
authors.initials: PB
authors.lastname: Armentrout
mesh_terms: D002412:Cations / Q000737:chemistry; D006865:Hydrogenation; D008697:Methane / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory; D028561:Transition Elements / Q000737:chemistry*
chemical_list: D002412:Cations; D028561:Transition Elements; D008697:Methane