Vibrational Spectroscopy of Co+(CH4)n and Ni+(CH4)n (n = 1–4)

IR Spectroscopic Characterization of Methane Adsorption on Copper Clusters Cun+ (n = 2-4)

The interaction of CH₄ with cationic copper clusters has been studied with infrared-multiple photon dissociation (IRMPD) spectroscopy. Cu_n⁺ (n = 2-4) formed by laser ablation were reacted with CH₄. The formed complexes were irradiated with the IR light of the free-electron laser for intracavity experiments (FELICE), and the fragments were mass-analyzed with a reflectron time-of-flight mass spectrometer. The structures of the Cu_n⁺-CH₄ complexes are assigned on the basis of comparison between the resulting IRMPD spectra to spectra of different isomers calculated with density functional theory (DFT). For all sizes n, the structure found is one with molecularly adsorbed CH₄. Only slight deformations of the CH₄ molecule have been identified upon adsorption on the clusters, which results in redshifts of the spectroscopic bands. This deformation can be explained by charge transfer from the cluster to the adsorbed methane molecule.

Ion spectroscopy in methane activation

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C-H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η³ type coordination of methane. In contrast, binding governed by orbital interactions results in a η² type coordination of methane. We further review the spectroscopic characterization of activation products of metal-methane reactions, such as the metal-carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

Structures of M+(CH4)n (M = Ti, V) Based on Vibrational Spectroscopy and Density Functional Theory

Photofragment spectroscopy is used to measure the vibrational spectra of M⁺(CH₄)(Ar) and M⁺(CH₄)_n (M = Ti, V; n = 1-4) in the C-H stretching region (2550-3100 cm^-1). Spectra were measured by monitoring the loss of Ar from M⁺(CH₄)(Ar) and loss of CH₄ from the larger clusters. The experimental spectra are then compared to simulations done at the B3LYP/6-311++G(3df,3pd) level of theory to identify the structures of the ions. The spectra all have a peak near 2800 cm^-1 due to the symmetric C-H stretch of the hydrogens adjacent to the metal. Some complexes also have a smaller peak due to the corresponding antisymmetric stretch. Most complexes also have a peak near 3000 cm^-1 due to the C-H stretch of hydrogens pointing away from the metal. The symmetric proximate C-H stretches of M⁺(CH₄)(Ar) to M⁺(CH₄)₄ are red-shifted from the symmetric stretch in bare CH₄ by 149, 152, 128, and 107 cm^-1 for the titanium complexes and 164, 175, 158, and 146 cm^-1, respectively, for the vanadium complexes. In M⁺(CH₄)(Ar) (M = Ti, V), the heavy atoms are collinear. Ti⁺(CH₄)(Ar) has η³ methane hydrogen coordination (∠M-C-H = 180°), while V⁺(CH₄)(Ar) has η² (∠M-C-H = 124°). The n = 2 complexes have C-M-C linear. Ti⁺(CH₄)₂ has C_2h symmetry with η³ CH₄ while V⁺(CH₄)₂ has methane coordination intermediate between η² and η³ (∠M-C-H = 156°). Both the M⁺(CH₄)₃ (M = Ti, V) complexes have C_2v symmetry with one methane farther away from the metal in an η² binding orientation and two methanes close to the metal with a nearly η² methane for vanadium and coordination between η² and η³ CH₄ for titanium (∠M-C-H = 150°). In Ti⁺(CH₄)₄ and V⁺(CH₄)₄ all of the methanes have η² coordination. The titanium complex has a distorted square planar geometry with two different Ti-C bond lengths and the vanadium complex is square planar.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by association and carbon-carbon bond cleavage of isoprene

La atom reaction with isoprene is carried out in a laser-vaporization molecular beam source. The reaction yields an adduct as the major product and C-C cleaved and dehydrogenated species as the minor ones. La(C₅H₈), La(C₂H₂), and La(C₃H₄) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of all three species exhibit a strong origin band and several weak vibronic bands corresponding to La-ligand stretch and ligand-based bend excitations. La(C₅H₈) is a five-membered metallacycle, whereas La(C₂H₂) and La(C₃H₄) are three-membered rings. All three metallacycles prefer a doublet ground state with a La 6s¹-based valence electron configuration and a singlet ion. The five-membered metallacycle is formed through La addition and isoprene isomerization, whereas the two three-membered rings are produced by La addition and insertion, hydrogen migration, and carbon-carbon bond cleavage.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions

A combined spectroscopic and computational study of gas-phase Au⁺(CH₄)_n (n = 3–8) complexes reveals a strongly-bound linear Au⁺(CH₄)₂ core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH₄a₁ and t₂ fundamental transitions reveals essentially free internal rotation of the core ligands about the H₄C–Au⁺–CH₄ axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t₂ vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au⁺(CH₄)_n=2,3 complexes corresponding, presumably, to the loss of two H₂ molecules.

Activation of C-H Bonds in Pt(+) + x CH4 Reactions, where x = 1-4: Identification of the Platinum Dimethyl Cation

Activation of C-H bonds in the sequential reactions of Pt(+) + x(CH4/CD4), where x = 1-4, have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theoretical calculations. Pt(+) cations are formed by laser ablation and exposed to controlled amounts of CH4/CD4 leading to [Pt,xC,(4x-2)H/D](+) dehydrogenation products. Irradiation of these products in the 400-2100 cm(-1) range leads to CH4/CD4 loss from the x = 3 and 4 products, whereas PtCH2(+)/PtCD2(+) products do not decompose at all, and x = 2 products dissociate only when formed from a higher order product. The structures of these complexes were explored theoretically at several levels of theory with three different basis sets. Comparison of the experimental and theoretical results indicate that the species formed have a Pt(CH3)2(+)(CH4)x-2/Pt(CD3)2(+)(CD4)x-2 binding motif for x = 2-4. Thus, reaction of Pt(+) with methane occurs by C-H bond activation to form PtCH2(+), which reacts with an additional methane molecule by C-H bond activation to form the platinum dimethyl cation. This proposed reaction mechanism is consistent with theoretical explorations of the potential energy surface for reactions of Pt(+) with one and two methane molecules.

Vibrational spectroscopy and theory of Fe2+(CH4)n (n = 1–3)

Vibrational spectroscopy of size selected clusters reveals the extent to which interactions with Fe₂⁺ weaken proximate C–H bonds in methane ligands.