Structures of the Dehydrogenation Products of Methane Activation by 5d Transition Metal Cations

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-05, Vol.117 (20), p.4115-4126
• Main Authors: Lapoutre, V. J. F, Redlich, B, van der Meer, A. F. G, Oomens, J, Bakker, J. M, Sweeney, A, Mookherjee, A, Armentrout, P. B
• Format: Article
• Language: English
• Subjects: Alternative fuels. Production and utilization; Applied sciences; Carbenes; Carbyne; Cations; Cations - chemistry; Dehydrogenation; Energy; Exact sciences and technology; Fuels; Hydrogenation; Mathematical analysis; Methane; Methane - chemistry; Molecular Structure; Quantum Theory; Spectra; Spectroscopy; Transition Elements - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp400305k

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.