Potential Energy Surface for Activation of Methane by Pt⁺:  A Combined Guided Ion Beam and DFT Study

Abstract

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.

C–N coupling in the gas-phase reactions of ammonia and [M(CH)]+ (M = Ni, Pd, Pt): a combined experimental/computational exercise

“…17 The validity of this protocol has already been proven by comparing the experimental and theoretical value of BDE(Pt + -CH). 18 For [Ni (CH)] + the calculated BDE value of 311.0 kJ mol −1 is within the uncertainty of the experimental value (301.0 ± 11.6 kJ mol −1 ); 19 BDE(Pd + -CH) has, to the best of our knowledge, not been reported in the literature. All energies (given in kJ mol −1 ) are corrected for (unscaled) zero-point vibrational energy contributions.…”

Vacuum Ultraviolet Photoionization Studies of PtCH2 and H‐Pt‐CH3: A Potential Energy Surface for the Pt+CH4 Reaction

“…[5] In conjunction with the measured IE(PtCH 2 )= 8.78 ± 0.03 eV, this gives D 0 (Pt-CH 2 )=4.62 ± 0.04 eV (446 ± 4 kJ mol -1 ).…”

“…Formation of CH 2 + H 2 from CH 4 is endoergic by 454.7 ± 2.5 kJ mol -1 . [5] Thus, the bimolecular Pt + CH 4 ! PtCH 2 + H 2 reaction is endoergic, but only by 9 ± 5 kJ/mol.…”

“…This study addresses these questions, determining the energetics of H-Pt-CH 3 and PtCH 2 by coupling photoionization measurements of their ionization energy with guided ion beam (GIB) measurements of the thermodynamics of the corresponding cations. [5] In addition, spin-orbit DFT (SODFT) calculations are used to characterize other intermediates and the barriers that separate them. Comparison of the potential energy surfaces for the reactions of Pt and Pt + with CH 4 shows that charge has surprisingly little effect on the potential energy surface for this most fundamental C-H activation reaction.…”

“…SODFT is a two-component approximation to the relativistic Dirac equations, which, when used in conjunction with relativistic effective core potentials, accurately reproduces properties which depend on the valence electrons, such as geometries and bond strengths, with far less computational effort than a full, allelectron relativistic calculation. [3][4] The aug-cc-pVTZ-PP relativistic effective core potential, basis set and spin-orbit parameters [5] was used for platinum. The aug-cc-pVTZ basis set was used for carbon and hydrogen.…”