Ricardo B. Metz

Vibrational Spectroscopy of Intermediates in Methane-to-Methanol Conversion by FeO+

Gas phase FeO(+) can convert methane to methanol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-CH(3)](+) insertion intermediate and Fe(+)(CH(3)OH) exit channel complex. These intermediates are selectively formed by reaction of laser-ablated Fe(+) with organic precursors under specific source conditions and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H and C-H stretching regions are measured by infrared multiple photon dissociation of Fe(+)(CH(3)OH) and [HO-Fe-CH(3)](+) and by monitoring argon atom loss following irradiation of Fe(+)(CH(3)OH)(Ar) and [HO-Fe-CH(3)](+)(Ar)(n) (n = 1, 2). Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO(+) + CH(4) reaction is developed based on CCSD(T) and CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Vibrationally resolved photofragment spectroscopy of FeO

We report the first vibrationally resolved spectroscopic study of FeO+. We observe the 0←0 and 1←0 bands of a 6Σ←X 6Σ transition at 28 648.7 and 29 311 cm−1. Under slightly modified source conditions the 1←1 transition is observed at 28 473 cm−1. In addition to establishing an upper limit D0o(Fe+–O)⩽342.7 kJ/mol, our results give the first experimental measurements of the vibrational frequencies in both the ground state, ν0″=838±4 cm−1, and the excited electronic state, ν0′=662±2 cm−1. Partially resolved rotational structure underlying the vibrational peaks has been analyzed to measure the predissociation lifetime and estimate the change in molecular constants upon electronic excitation.

Photofragment spectroscopy of covalently bound transition metal complexes: a window into C–H and C–C bond activation by transition metal ions

Transition metal cations M+ and metal oxide cations MO+ can activate C–H and C–C bonds in hydrocarbons. In this review, we discuss our studies of the electronic spectroscopy and dissociation dynamics of the intermediates, reactants and products of these reactions using photofragment spectroscopy. Results are presented on the spectroscopy of the intermediates of methane activation by FeO+, as well as on the spectroscopy of FeO+, NiO+ and PtO+. Resonance enhanced photodissociation allows us to measure the electronic spectroscopy of FeO+ below the dissociation limit with rotational resolution. Complementary time-dependent B3LYP calculations of excited electronic states of FeO+ and NiO+ are in surprisingly good agreement with experiment. Dissociation onsets give upper limits to bond strengths for Fe , Co , Ni , Ta and Au . These results are compared to thermodynamic measurements, and the extent to which rotational energy contributes to dissociation is investigated. The spectroscopy of the π-bonded complexes Pt(C2H4)+ and Au(C2H4)+ is discussed, along with studies of larger systems. Planned studies of the vibrational spectroscopy of covalently bound ions are also discussed.

Direct determination of the ionization energies of FeO and CuO with VUV radiation

Photoionization efficiency curves were measured for gas-phase FeO and CuO using tunable vacuum-ultraviolet radiation at the Advanced Light Source. The molecules are prepared using laser ablation of a metal-oxide powder in a novel high-repetition-rate source and are thermally moderated in a supersonic expansion. These measurements provide the first directly measured ionization energy for CuO, IE(CuO)=9.41 +/- 0.01 eV. The direct measurement also gives a greatly improved ionization energy for FeO, IE(FeO) = 8.56 +/- 0.01 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to a refined bond strength for the FeO cation: D0(Fe(+)-O)=3.52 +/- 0.02 eV. A dramatic increase in the photoionization cross section at energies of 0.36 eV above the threshold ionization energy is assigned to autoionization and direct ionization involving one or more low-lying quartet states of FeO+. The interaction between the sextet ground state and low-lying quartet states of FeO+ is key to understanding the oxidation of hydrogen and methane by FeO+, and these experiments provide the first experimental observation of the low-lying quartet states of FeO+.

Electronic spectroscopy of intermediates involved in the conversion of methane to methanol by FeO

Specific ion–molecule reactions are used to prepare two intermediates of the FeO++CH4 reaction, and photodissociation of the jet-cooled intermediates is examined in the visible and near-ultraviolet using time-of-flight mass spectrometry. The photodissociation spectrum of the aquo iron carbene complex [H2C=Fe–OH2]+ shows transitions to at least four excited electronic states in the FeCH2+ chromophore, with broad vibrational structure. Photoexcitation of the insertion intermediate [HO–Fe–CH3]+ leads to formation of FeOH++CH3 and also triggers the reaction to produce Fe++CH3OH. The photodissociation spectrum of [HO–Fe–CH3]+ presents a vibrationally resolved band involving progressions in the excited state Fe–C stretch, Fe–O stretch, and O–Fe–C bend. The change in the Fe–C bond length in [HO–Fe–CH3]+ and [H2C=Fe–OH2]+ upon photoexcitation is calculated from a Franck–Condon analysis of the vibronic features observed. The analysis of the experimental results is aided by hybrid Hartree–Fock/density-functional (B...

Gas-phase photodissociation of AuCH2 +: The dissociation threshold of jet-cooled and rotationally thermalized ions

Abstract The photofragment spectra of jet-cooled and rotationally thermalized AuCH2+ are reported. Two channels are observed: loss of H2 and loss of CH2 with a branching ratio of 1.4:1 over the region studied. The presence of a threshold at 322 nm for the dissociation of jet-cooled AuCH2+ to Au++CH2 implies the upper limitD0o(Au+–CH2)≤372±3 kJ mol−1. The dissociation threshold of ions rotationally thermalized in an ion trap shifts to lower energy by the amount of parent rotational energy.

The low-lying electronic states of FeO+: Rotational analysis of the resonance enhanced photodissociation spectra of the 6Π7/2←X 6Σ+ system

The resonance enhanced (1+1) photodissociation spectra of the (8,0) and (9,0) bands of the 6Π7/2←6Σ+ system of FeO+ have been recorded. From a rotational analysis, the rotational parameters for the 6Σ+ ground state of FeO+ have been obtained for the first time. The rotational constant B0=0.5020±0.0004 cm−1 is derived, giving r0=1.643±0.001 A. Other molecular parameters determined for the 6Σ+ ground state are the spin–spin coupling constant, λ=−0.126±0.006 cm−1, and the spin–rotational coupling constant, γ=−0.033±0.002 cm−1. The assignment of the upper state as 6Π7/2 is based on the characteristic appearance of the band and on time-dependent density functional (TD-DFT) calculations performed on FeO+. The reliability of the TD-DFT method in the prediction of excited states of FeO+ is corroborated by calculations on CrF and MnO, which have been extensively characterized either by spectroscopy or by high-level theoretical calculations.

Vibrational Spectroscopy of Intermediates in Benzene-to-Pheno Conversion by FeO+

Gas-phase FeO+ can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C6H5]+ insertion intermediate and Fe+(C6H5OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe+ with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe+(C6H5OH), the O-H stretch is observed at 3598 cm−1. Photodissociation primarily produces Fe++C6H5OH; Fe+(C6H4)+H2O is also observed. IRMPD of [HO-Fe-C6H5]+ mainly produces FeOH++C6H5 and the O-H stretch spectrum consists of a peak at ∼3700 cm−1 with a shoulder at ∼3670 cm−1. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO++C6H6 reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Direct determination of the ionization energies of PtC, PtO, and PtO2 with VUV radiation.

Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and IE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +/- 0.07 eV, D0(Pt-O) = 4.30 +/- 0.12 eV, and D0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules DeltaH(0)(f,0)(PtC(g)) = 701 +/- 7 kJ/mol, DeltaH(0)(f,0)(PtO(g)) = 396 +/- 12 kJ/mol, and DeltaH(0)(f,0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.

Vibrational Spectroscopy and Theory of Fe+(CH4)n (n = 1―4)

Vibrational spectra are measured for Fe(+)(CH(4))(n) (n = 1-4) in the C-H stretching region (2500-3200 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring CH(4) fragment loss following absorption of one photon (for n = 3, 4) or sequential absorption of multiple photons (for n = 1, 2). The spectra have a band near the position of the antisymmetric C-H stretch in isolated methane (3019 cm(-1)), along with bands extending >250 cm(-1) to the red of the symmetric C-H stretch in methane (2917 cm(-1)). The spectra are sensitive to the ligand configuration (η(2) vs η(3)) and to the Fe-C distance. Hybrid density functional theory calculations are used to identify possible structures and predict their vibrational spectra. The IR photodissociation spectrum shows that the Fe(+)(CH(4)) complex is a quartet, with an η(3) configuration. There is also a small contribution to the spectrum from the metastable sextet η(3) complex. The Fe(+)(CH(4))(2) complex is also a quartet with both CH(4) in an η(3) configuration. For the larger clusters, the configuration switches from η(3) to η(2). In Fe(+)(CH(4))(3), the methane ligands are not equivalent. Rather, there is one short and two long Fe-C bonds, and each methane is bound to the metal in an η(2) configuration. For Fe(+)(CH(4))(4), the calculations predict three low-lying structures, all with η(2) binding of methane and very similar Fe-C bond lengths. No single structure reproduces the observed spectrum. The approximately tetrahedral C(1) ((4)A) structure contributes to the spectrum; the nearly square-planar D(2d) ((4)B(2)) and the approximately tetrahedral C(2) ((4)A) structure may contribute as well.