Michael R. Sievers

METAL OXIDE AND CARBIDE THERMOCHEMISTRY OF Y+, ZR+, NB+, AND MO+

Reactions of Y+, Zr+, Nb+, and Mo+ with molecular oxygen and carbon monoxide and the collision induced dissociations of their metal oxides with Xe are studied as a function of kinetic energy using guided ion beam mass spectrometry. A meter‐long flow tube ion source is used to create Zr+, Nb+, and Mo+ ions in their electronic ground state terms and Y+ mostly in its ground state. The kinetic energy dependencies for the reactions of Y+, Zr+, and Nb+ with O2 show exothermic, barrierless behavior, while Mo+ reacts with O2 in a process with a small endothermicity. Reactions with CO lead to formation of MC+ and MO+ in endothermic processes. Analyses of the reaction cross sections obtained in this study yield 0‐K bond dissociation energies (in eV) of D0(Y+–O)=7.24±0.18, D0(Y+–C)=2.91±0.12, D0(Zr+–O)=7.76±0.11, D0(Zr+–C)=4.72 ±0.11, D0(Nb+–O)=7.13±0.11, D0(Nb+–C)=5.16±0.15, D0(Mo+–O)=5.06±0.02, and D0(Mo+–C)=4.31±0.20. There is some question whether the YC+ and YO+ bond energies represent the correct adiabatic val...

POTENTIAL ENERGY SURFACE FOR CARBON-DIOXIDE ACTIVATION BY V+ : A GUIDED ION BEAM STUDY

A guided ion beam tandem mass spectrometer is used to measure the kinetic energy dependence of the V+(5D) + CO2 reaction and a reverse pathway, VO+(3∑−) + CO. Two intermediates along these reaction pathways, V+(CO2) and OV+(CO), are examined by threshold collision‐induced dissociation experiments with Xe. Thermochemical analyses of the cross sections obtained in these systems allow the measurement of D0(OV+–O) = 3.06±0.40 eV, D0(V+–CO2) = 0.75±0.04 eV, D0(OV+–CO) = 1.05±0.10 eV, and speculative characterization of electronic excitation energies for two states of VO+. Combined with literature information on the electronic states of V+ and VO+, these data enable the potential energy surfaces for this reaction system to be mapped out in some detail. We find that coupling between surfaces of different spin is observed, but that spin conservation plays an important role in both the forward and reverse reactions.

Gas phase activation of carbon dioxide by niobium and niobium monoxide cations

Abstract Guided ion beam mass spectrometry is used to investigate the kinetic energy dependence of the reactions of Nb + ( 5 D) and NbO + ( 3 Σ − ) with CO 2 , and the reverse pathways, NbO + ( 3 Σ − ) and NbO 2 + with CO. These systems exhibit complicated behavior because the ground states of the reactants and products have different spins. To further probe the potential energy surfaces for these reaction systems, NbO 2 + and the intermediates, ONb(CO) + , ONb(CO 2 ) + , and O 2 Nb(CO) + , are studied by collisional activation experiments with Xe. Analysis of the reaction cross sections obtained in this study yield (in eV) D 0 (Nb + –CO) = 0.99 ± 0.05, D 0 (ONb + –CO) = 1.10 ± 0.05, D 0 (ONb + –CO 2 ) = 0.88 ± 0.03, D 0 (O 2 Nb + –CO) = 1.11 ± 0.05, and D 0 (ONb + –O) = 5.71 ± 0.17. Speculative determinations of electronic excitation energies for two states each of NbO + and of NbO 2 + are also made. Combining the results obtained from this study and those obtained from the literature, we are able to generate a fairly complete potential energy surface for the Nb + reaction system. We also compare the reactivities of Nb + , NbO + , and NbO 2 + with respect to the interconversion of CO and CO 2 .

Activation of CH4, C2H6, and C3H8 by gas-phase Nb+ and the thermochemistry of Nb-ligand complexes

Abstract The kinetic energy dependence of the reactions of Nb+ (5D) with methane, ethane, and propane have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all three reaction systems. At high energies, products resulting from both C–H and C–C cleavage processes are appreciable. The observation of dihydride and hydrido-methyl niobium cation products provides insight into the reaction mechanisms operating in these processes. The results for Nb+ are compared with those for the first-row transition metal congener V+ and the differences in behavior and mechanism discussed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in electron volts) of D0(Nb–H) > 2.3 ± 0.1, D0(Nb+–2H) = 4.64 ± 0.06, D0(Nb+–C) = 5.28 ± 0.15, D0(Nb+–CH) = 6.02 ± 0.20, D0(Nb+–CH2) = 4.44 ± 0.09, D0(Nb+–CH3) = 2.06 ± 0.11, D0[Nb+–(H)(CH3)] = 4.78 ± 0.11, D0(Nb+–C2H) = 4.34 ± 0.19, D0(Nb+–C2H2) = 2.90 ± 0.06, D0(Nb+–C2H3) = 3.43 ± 0.21, D0(Nb+–C2H4) = 2.8 ± 0.3, D0(Nb+–C2H5) = 2.45 ± 0.12, D0(Nb+–C3H2) = 5.25 ± 0.19, and lower limits for D0(Nb+–C3H3) ≥ 3.76 ± 0.23 and D0(Nb+–C3H5) ≥ 1.4 ± 0.1. The observation of exothermic processes sets lower limits for the bond energies of Nb+ to propyne and propene of 2.84 and 1.22 eV, respectively.

Alkali ion carbonyls: Sequential bond energies of Li+(CO)x (x = 1-3), Na+(CO)x (x = 1, 2) and K+(CO)

The sequential bond dissociation energies for Li+(CO)x (x = 1−3), Na+(CO)x (x = 1,2), and K+(CO) are determined by examining the collision-induced dissociation reactions with argon in a guided ion beam mass spectrometer. Analysis of the kinetic energy dependent cross sections yield values for the (CO)x−1M+CO bond dissociation energies (BDEs) of 0.57 ± 0.13, 0.37 ± 0.04 and 0.36 ± 0.04 eV for M  Li (x = 1–3, respectively), 0.33 ± 0.08 and 0.25 ± 0.03 eV for M  Na (x = 1 and 2, respectively), and 0.19 ± 0.05 eV for K+CO. In addition, the M+Ar BDEs are determined by examining the ligand exchange reaction of M+(CO) with Ar and are found to be 0.34 ± 0.14, 0.16 ± 0.09 and 0.14 ± 0.07 eV for M  Li, Na and K, respectively. The trends in BDEs can be explained readily in terms of electrostatic bonding interactions. A comparison of the alkali metal ion—CO BDEs with those of transition metal ion carbonyls reveals the contributions of s-dσ hybridization and d-π∗ back-donation in the chemical bonding of the latter systems.

Activation of CH4, C2H6, C3H8, and c-C3H6 by gas-phase Pd+ and the thermochemistry of Pd-ligand complexes

The kinetic energy dependences of the reactions of Zr + ( 4 F) with ethane, propane, and cyclopropane have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all three reaction systems. Efficient C-C bond activation is also observed at low energies in the cyclopropane system. At high energies, products resulting from both C-H and C-C cleavage processes are appreciable for all three hydrocarbon systems. The observation of dihydride and hydridomethyl zirconium cation products provides insight into the reaction mechanisms operating in these processes. The results for Zr + are compared with those for the first-row transition metal congener Ti + and the differences in behavior and mechanism discussed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D 0 (Zr + -2H) = 5.02 ′ 0.13, D 0 (Zr + -C) = 4.62 ′ 0.16, D 0 (Zr + -CH) = 5.89 ′ 0.13, D 0 (Zr + -CH 2 ) = 4.61 ′ 0.05, D 0 (Zr + -CH 3 ) = 2.36 ′ 0.10, D 0 [Zr + -(H)(CH 3 )] = 5.43 ′ 0.15, D 0 (Zr + -C 2 H) = 4.57 ′ 0.12, D 0 (Zr + -C 2 H 2 ) = 2.83 ′ 0.15, D 0 (Zr + -C 2 H 3 ) = 3.78 ′ 0.24, D 0 (Zr + -C 2 H 4 ) = 2.84 ′ 0.18, D 0 (Zr + -C 2 H 5 ) = 2.37 ′ 0.17, D 0 (Zr + -C 3 H 2 ) = 5.45 ′ 0.20, and D 0 (Zr + -C 3 H 3 ) ≥ 4.10 ′ 0.23. The observation of exothermic processes sets lower limits for the bond energies of Zr + to propyne and propene of 2.84 and 1.22 eV, respectively.

Activation of C2H6, C3H8, and c-C3H6 by gas-phase Zr+ and the thermochemistry of Zr-ligand complexes

The kinetic energy dependences of the reactions of Zr + ( 4 F) with ethane, propane, and cyclopropane have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all three reaction systems. Efficient C-C bond activation is also observed at low energies in the cyclopropane system. At high energies, products resulting from both C-H and C-C cleavage processes are appreciable for all three hydrocarbon systems. The observation of dihydride and hydridomethyl zirconium cation products provides insight into the reaction mechanisms operating in these processes. The results for Zr + are compared with those for the first-row transition metal congener Ti + and the differences in behavior and mechanism discussed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D 0 (Zr + -2H) = 5.02 ′ 0.13, D 0 (Zr + -C) = 4.62 ′ 0.16, D 0 (Zr + -CH) = 5.89 ′ 0.13, D 0 (Zr + -CH 2 ) = 4.61 ′ 0.05, D 0 (Zr + -CH 3 ) = 2.36 ′ 0.10, D 0 [Zr + -(H)(CH 3 )] = 5.43 ′ 0.15, D 0 (Zr + -C 2 H) = 4.57 ′ 0.12, D 0 (Zr + -C 2 H 2 ) = 2.83 ′ 0.15, D 0 (Zr + -C 2 H 3 ) = 3.78 ′ 0.24, D 0 (Zr + -C 2 H 4 ) = 2.84 ′ 0.18, D 0 (Zr + -C 2 H 5 ) = 2.37 ′ 0.17, D 0 (Zr + -C 3 H 2 ) = 5.45 ′ 0.20, and D 0 (Zr + -C 3 H 3 ) ≥ 4.10 ′ 0.23. The observation of exothermic processes sets lower limits for the bond energies of Zr + to propyne and propene of 2.84 and 1.22 eV, respectively.