Activation of CO2 by Actinide Cations (Th+, U+, Pu+, and Am+) as Studied by Guided Ion Beam and Triple Quadrupole Mass Spectrometry.

Reactions of CO2 with Th+ have been studied using guided ion beam tandem mass spectrometry (GIBMS) and with An+ (An+ = Th+, U+, Pu+, and Am+) using triple quadrupole inductively coupled plasma mass spectrometry (QQQ-ICP-MS). Additionally, the reactions ThO+ + CO and ThO+ + CO2 were examined using GIBMS. Modeling the kinetic energy-dependent GIBMS data allowed the determination of bond dissociation energies (BDEs) for D0(Th+-O) and D0(OTh+-O) that are in reasonable agreement with previous GIBMS measurements. The QQQ-ICP-MS reactions were studied at higher pressures where multiple collisions between An+ and the neutral CO2 occur. As a consequence, both AnO+ and AnO2+ products were observed for all An+ except Am+, where only AmO+ was observed. The relative abundances of the observed monoxides compared to the dioxides are consistent with previous reports of the AnOn+ (n = 1, 2) BDEs. A comparison of the periodic trends of the group 4 transition metal, lanthanide (Ln), and actinide atomic cations in reactions with CO2 (a formally spin-forbidden reaction for most M+ ground states) and O2 (a spin-unrestricted reaction) indicates that spin conservation plays a minor role, if any, for the heavier Ln+ and An+ metals. Further correlation of Ln+ and An+ + CO2 reaction efficiencies with the promotion energy (Ep) to the first electronic state with two valence d-electrons (Ep(5d2) for Ln+ and Ep(6d2) for An+) indicates that the primary limitation in the activation of CO2 is the energetic cost to promote from the electronic ground state of the atomic metal ion to a reactive state.

Bond energies of ThO(+) and ThC(+): A guided ion beam and quantum chemical investigation of the reactions of thorium cation with O2 and CO.

Kinetic energy dependent reactions of Th(+) with O2 and CO are studied using a guided ion beam tandem mass spectrometer. The formation of ThO(+) in the reaction of Th(+) with O2 is observed to be exothermic and barrierless with a reaction efficiency at low energies of k/kLGS = 1.21 ± 0.24 similar to the efficiency observed in ion cyclotron resonance experiments. Formation of ThO(+) and ThC(+) in the reaction of Th(+) with CO is endothermic in both cases. The kinetic energy dependent cross sections for formation of these product ions were evaluated to determine 0 K bond dissociation energies (BDEs) of D0(Th(+)-O) = 8.57 ± 0.14 eV and D0(Th(+)-C) = 4.82 ± 0.29 eV. The present value of D0 (Th(+)-O) is within experimental uncertainty of previously reported experimental values, whereas this is the first report of D0 (Th(+)-C). Both BDEs are observed to be larger than those of their transition metal congeners, TiL(+), ZrL(+), and HfL(+) (L = O and C), believed to be a result of lanthanide contraction. Additionally, the reactions were explored by quantum chemical calculations, including a full Feller-Peterson-Dixon composite approach with correlation contributions up to coupled-cluster singles and doubles with iterative triples and quadruples (CCSDTQ) for ThC, ThC(+), ThO, and ThO(+), as well as more approximate CCSD with perturbative (triples) [CCSD(T)] calculations where a semi-empirical model was used to estimate spin-orbit energy contributions. Finally, the ThO(+) BDE is compared to other actinide (An) oxide cation BDEs and a simple model utilizing An(+) promotion energies to the reactive state is used to estimate AnO(+) and AnC(+) BDEs. For AnO(+), this model yields predictions that are typically within experimental uncertainty and performs better than density functional theory calculations presented previously.

Activation of Methane by Os+ : Guided-Ion-Beam and Theoretical Studies.

Activation of methane by the third-row transition-metal cation Os+ is studied experimentally by examining the kinetic energy dependence of reactions of Os+ with CH4 and CD4 using guided-ion-beam tandem mass spectrometry. A flow tube ion source produces Os+ in its electronic ground state and primarily in the ground spin-orbit level. Dehydrogenation to form [Os,C,2 H]+ +H2 is exothermic, efficient, and the only process observed at low energies for reaction of Os+ with methane, whereas OsH+ dominates the product spectrum at higher energies. The kinetic energy dependences of the cross sections for several endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0 (Os+ C)=6.20±0.21, D0 (Os+ CH)=6.77±0.15, and D0 (Os+ CH3 )=3.00±0.17. Because it is formed exothermically, D0 (Os+ CH2 ) must be greater than 4.71 eV, and a speculative interpretation suggests the exothermicity exceeds 0.6 eV. Quantum chemical calculations at the B3LYP/def2-TZVPP level show reasonable agreement with the experimental bond energies and with previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surfaces. Notably, the structure of the dehydrogenation product is predicted to be HOsCH+ , rather than OsCH2 + , in contrast to previous work.

Thermochemistry of alkali metal cation interactions with histidine: influence of the side chain.

The interactions of alkali metal cations (M(+) = Na(+), K(+), Rb(+), Cs(+)) with the amino acid histidine (His) are examined in detail. Experimentally, bond energies are determined using threshold collision-induced dissociation of the M(+)(His) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies of 2.31 ± 0.11, 1.70 ± 0.08, 1.42 ± 0.06, and 1.22 ± 0.06 eV for complexes of His with Na(+), K(+), Rb(+), and Cs(+), respectively. All bond dissociation energy (BDE) determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-neutral collisions. These experimental results are compared to values obtained from quantum chemical calculations conducted previously at the MP2(full)/6-311+G(2d,2p), B3LYP/6-311+G(2d,2p), and B3P86/6-311+G(2d,2p) levels with geometries and zero point energies calculated at the B3LYP/6-311+G(d,p) level where Rb and Cs use the Hay-Wadt effective core potential and basis set augmented with additional polarization functions (HW*). Additional calculations using the def2-TZVPPD basis set with B3LYP geometries were conducted here at all three levels of theory. Either basis set yields similar results for Na(+)(His) and K(+)(His), which are in reasonable agreement with the experimental BDEs. For Rb(+)(His) and Cs(+)(His), the HW* basis set and ECP underestimate the experimental BDEs, whereas the def2-TZVPPD basis set yields results in good agreement. The effect of the imidazole side chain on the BDEs is examined by comparing the present results with previous thermochemistry for other amino acids. Both polarizability and the local dipole moment of the side chain are influential in the energetics.

Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation.

The gas phase structures of cationized histidine (His), including complexes with Li(+), Na(+), K(+), Rb(+), and Cs(+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) (Li(+), Na(+), and K(+) complexes) and B3LYP/HW*/6-311+G(d,p) (Rb(+) and Cs(+) complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li(+)(His) and Na(+)(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N(α),N(1)], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K(+)(His), Rb(+)(His), and Cs(+)(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li(+)(His) and Na(+)(His). For these complexes, the bidentate [CO,N(1)] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.

Guided ion beam and theoretical study of the reactions of Os+ with H2, D2, and HD.

Reactions of the third-row transition metal cation Os(+) with H(2), D(2), and HD to form OsH(+) (OsD(+)) were studied using a guided ion beam tandem mass spectrometer. A flow tube ion source produces Os(+) in its (6)D (6s(1)5d(6)) electronic ground state level. Corresponding state-specific reaction cross sections are obtained. The kinetic energy dependences of the cross sections for the endothermic formation of OsH(+) and OsD(+) are analyzed to give a 0 K bond dissociation energy of D(0)(Os(+)-H) = 2.45 ± 0.10 eV. Quantum chemical calculations are performed here at several levels of theory, with B3LYP approaches generally overestimating the experimental bond energy whereas results obtained using BHLYP and CCSD(T), coupled-cluster with single, double, and perturbative triple excitations, levels show good agreement. Theory also provides the electronic structures of these species and the potential energy surfaces for reaction. Results from the reactions with HD provide insight into the reaction mechanism and indicate that Os(+) reacts via a direct reaction. We also compare this third-row transition metal system with the first-row and second-row congeners, Fe(+) and Ru(+), and find that Os(+) reacts more efficiently with dihydrogen, forming a stronger M(+)-H bond. These differences can be attributed to the lanthanide contraction and relativistic effects.

Guided ion beam and theoretical study of the reactions of Au+ with H2, D2, and HD.

Reactions of the late third-row transition metal cation Au(+) with H(2), D(2), and HD are examined using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au(+) in its (1)S (5d(10)) electronic ground state level. Corresponding state-specific reaction cross sections for forming AuH(+) and AuD(+) as a function of kinetic energy are obtained and analyzed to give a 0 K bond dissociation energy of D(0)(Au(+)-H) = 2.13 ± 0.11 eV. Quantum chemical calculations at the B3LYP∕HW+∕6-311+G(3p) and B3LYP∕Def2TZVPP levels performed here show good agreement with the experimental bond energy. Theory also provides the electronic structures of these species and the reactive potential energy surfaces. We also compare this third-row transition metal system with previous results for analogous reactions of the first-row and second-row congeners, Cu(+) and Ag(+). We find that Au(+) has a stronger M(+)-H bond, which can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals. Results from reactions with HD provide insight into the reaction mechanism and indicate that ground state Au(+) reacts largely via a direct mechanism, in concordance with the behavior of the lighter group 11 metal ions, but includes more statistical behavior than these metals as well.

Vibrational spectroscopy and theory of Fe(+)(CH(4))(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.

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.

Methane activation by cobalt cluster cations, Co(n)+ (n = 2-16): reaction mechanisms and thermochemistry of cluster-CH(x) (x = 0-3) complexes.

The kinetic energy dependences of the reactions of Co(n)(+) (n = 2-16) with CD(4) are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation, Co(n)D(+), dehydrogenation to form Co(n)CD(2)(+), and double dehydrogenation yielding Co(n)C(+). These primary products decompose to form secondary and higher order products, Co(n)CD(+), Co(n-1)D(+), Co(n-1)C(+), Co(n-1)CD(+), and Co(n-1)CD(2)(+) at higher energies. Adduct formation of Co(n)CD(4)(+) is also observed for the largest cluster cations, n > or = 10. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size, although the hexamer cation shows a reduced reactivity compared to its neighbors. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for cobalt cluster cations to D, C, CD, CD(2), and CD(3) are determined. The relative magnitudes of these bond energies are consistent with simple bond order considerations. Bond energies for larger clusters rapidly reach relatively constant values, which are used to estimate the chemisorption energies of the C, CD, CD(2), and CD(3) molecular fragments to cobalt surfaces.

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.

Mode selective photodissociation dynamics in V+(OCO).

The electrostatic V+(OCO) complex has a vibrationally resolved photodissociation spectrum in the visible. Photodissociation produces V+ + CO2 (nonreactive pathway) and VO+ +CO (reactive pathway). Production of VO+ is energetically favored, but spin forbidden. One-photon dissociation studies confirm mode selectivity observed by Lessen et al. [J. Chem. Phys. 95, 1414 (1991)]: excitation of one quantum of rocking motion enhances VO+ production by >30%. Branching ratio measurements in one-photon dissociation are extended to higher energy. The effect of OCO antisymmetric stretch vibrations on reactivity is investigated using vibrationally mediated photodissociation, in which the OCO antisymmetric stretch is excited at 2390.9 cm(-1). Vibrationally excited molecules are then dissociated in the visible. Seven vibronic bands are investigated, involving the antisymmetric stretch alone and in combination with the CO2 bend, the V+(OCO) stretch and rock. Exciting the antisymmetric stretch leads to a approximately 15% increase in the reactive VO+ channel, compared to other states at similar energy. Combination bands involving the antisymmetric stretch all show slightly higher reactivity. Electronic structure calculations were performed to characterize the dissociation pathways and excited electronic states of V+(OCO). The geometries of reactants, products, and transition states and relative energies of quintet and triplet states were determined using hybrid density functional theory; energies were also calculated using the coupled cluster with single, double and perturbative triple excitations method. In addition, time-dependent density functional theory calculations were performed to predict the excited electronic states of quintet and triplet V+(OCO). Spin-orbit coupling of quintet states to triplet states was calculated and used to compute intersystem crossing rates, which reproduce many of the observed mode selective trends. The V+--OCO stretch and OCO antisymmetric stretch appear to enhance reactivity by increasing the intersystem crossing rate.

Electronic and vibrational spectroscopy and vibrationally mediated photodissociation of V+(OCO).

Electronic spectra of gas-phase V+(OCO) are measured in the near-infrared from 6050 to 7420 cm(-1) and in the visible from 15,500 to 16,560 cm(-1), using photofragment spectroscopy. The near-IR band is complex, with a 107 cm(-1) progression in the metal-ligand stretch. The visible band shows clearly resolved vibrational progressions in the metal-ligand stretch and rock, and in the OCO bend, as observed by Brucat and co-workers. A vibrational hot band gives the metal-ligand stretch frequency in the ground electronic state nu3'' = 210 cm(-1). The OCO antisymmetric stretch frequency in the ground electronic state (nu1'') is measured by using vibrationally mediated photodissociation. An IR laser vibrationally excites ions to nu1'' = 1. Vibrationally excited ions selectively dissociate following absorption of a second, visible photon at the nu1' = 1 <-- nu1'' = 1 transition. Rotational structure in the resulting vibrational action spectrum confirms that V+(OCO) is linear and gives nu1'' = 2392.0 cm(-1). The OCO antisymmetric stretch frequency in the excited electronic state is nu1' = 2368 cm(-1). Both show a blue shift from the value in free CO2, due to interaction with the metal. Larger blue shifts observed for complexes with fewer ligands agree with trends seen for larger V+(OCO)n clusters.