Guided-Ion Beam and Theoretical Study of the Potential Energy Surface for Activation of Methane by W⁺

Abstract

A guided-ion beam tandem mass spectrometer is used to study the reactions, W(+) + CH(4) (CD(4)) and [W,C,2H](+) + H(2) (D(2)), to probe the [W,C,4H](+) potential energy surface. The reaction W(+) + CH(4) produces [W,C,2H](+) in the only low-energy process. The analogous reaction in the CD(4) system exhibits a cross section with strong differences at the lowest energies caused by zero-point energy differences, demonstrating that this reaction is slightly exothermic for CH(4) and slightly endothermic for CD(4). The [W,C,2H](+) product ion reacts further at thermal energies with CH(4) to produce W(CH(2))(x)(+) (x = 2-4). At higher energies, the W(+) + CH(4) reaction forms WH(+) as the dominant ionic product with smaller amounts of WCH(3)(+), WCH(+), and WC(+) also formed. The energy dependent cross sections for endothermic formation of the various products are analyzed and allow the determination of D(0)(W(+)-CH(3)) approximately 2.31 +/- 0.10 eV, D(0)(W(+)-CH(2)) = 4.74 +/- 0.03 eV, D(0)(W(+)-CH) = 6.01 +/- 0.28 eV, and D(0)(W(+)-C) = 4.96 +/- 0.22 eV. We also examine the reverse reaction, [W,C,2H](+) + H(2) (D(2)) --> W(+) + CH(4) (CH(2)D(2)). Combining the cross sections for the forward and reverse processes yields an equilibrium constant from which D(0)(W(+)-CH(2)) = 4.72 +/- 0.04 eV is derived. Theoretical calculations performed at the B3LYP/HW+/6-311++G(3df,3p) level yield thermochemistry in reasonable agreement with experiment. These calculations help identify the structures and electronic states of the species involved and characterize the potential energy surface for the [W,C,4H](+) system.

The most reactive third-row transition metal: Guided ion beam and theoretical studies of the activation of methane by Ir+

“…The B3LYP functional was used for all the calculations done here because it provides reasonable results for the analogous Pt + , W + , and Re + with CH 4 systems [19][20][21]. In all cases, the thermochemistry reported here is corrected for zero-point energy effects using unscaled frequencies.…”

“…Such ion beam studies also provide insight into the electronic requirements for the activation of methane by transition metal ions and periodic trends in this reactivity. We have applied this methodology to all the first-row and second-row transition metals except Tc [3,[7][8][9][10][11][12][13][14][15][16][17][18] and more recently to several of the third-row metals as well [18][19][20][21]. A significant advantage of these comprehensive studies is the comparison of bond energies and reactivities of all transition-metal complexes, thereby establishing periodic trends.…”

Gas-phase activation of methane by ligated transition-metal cations

“…More recently, the details of these reactions, including those metals that cannot activate methane under thermal conditions, were analyzed by combined experimental and theoretical approaches, e.g., for M ϭ Fe (8), Ni (9), Zr (10), Hf (11), Mo (12), Ta (13), W (14), Re (15), Author contributions: D.S. and H.S.…”

“…In fact, most of the above-mentioned examples of methane activation lead to species that do not react further with methane; exceptions are found among 5d metals (2,3,13,14,19,20). In addition, bare transition-metal ions correspond to the oxidation state (ϩI), which is unusual in condensed-phase chemistry.…”

Chemie mit Methan: Studieren geht über Probieren!

“…[11b] W, [14] Ir, [15] und Ta [16] ) durchgeführt. Hçchst interessant war dabei der Befund, dass ein durch Spin-Bahn-Kopplung vermittelter Wechsel von einem High-spin-zu einem Low-spin-Zustand im Zuge der Dehydrierung und vor allem bei der Bildung des zentralen Insertionsintermediates H-M + -CH 3 von entscheidender Bedeutung ist.…”