Showing papers by "P. B. Armentrout published in 1989"

The chemistry of atomic transition-metal ions: insight into fundamental aspects of organometallic chemistry

Abstract: For species as simple as atomic transition-metal ions, organometallic transformations usually involve multistep processes. These transformations have been studied with the entire arsenal of experimental techniques developed for the study of ion-molecule chemistry: conventional tandem mass spectrometry, flowing afterglow (FA) techniques, and ion cyclotron resonance (ICR) mass spectrometry and its Fourier transform adaptation (FT-ICR). In this Account, the authors limit ourselves to those methods in which motion along the reaction coordinate is the key dynamic variable that can be controlled as reactants approach and measured as the products recede. In our laboratories, guided ion beam techniques have been developed and highly refined for studies of the variation of reaction probabilities with E{sub T}. Complementing this work are measurements of product kinetic energy release distributions (KERDs).

Collision‐induced dissociation of Fe+n (n=2–10) with Xe: Ionic and neutral iron binding energies

Abstract: Cross sections for collision‐induced dissociation (CID) of Fe+n with Xe, 2≤n≤10, are presented Experiments were performed on a newly constructed guided ion beam mass spectrometer, the design and capabilities of which are described in detail The single mechanism for dissociation of iron cluster ions is sequential loss of iron atoms with increasing collision energies There is no evidence for fission to molecular neutral products The cross section threshold energy dependences are analyzed to give the bond dissociation energies (BDEs), D0(Fe+n−1–Fe) Data analysis employs an empirical model that incorporates RRKM theory to account for inefficient dissociation on the time scale of the experiment Results show that Fe+6 has the strongest BDE, D0(Fe+5–Fe) =344±018 eV, while Fe+3 is the most weakly bound, D0(Fe+2–Fe) =164±015 eV Neutral cluster BDEs are derived from ionic binding energies and known ionization potentials Branching ratios and other cross section features are also discussed with respect to

State-specific reactions of iron(1+)(6D, 4F) with oxygen and oxirane: D.degree.0(iron(1+)-oxygen) and effects of collisional relaxation

Abstract: Etude de la reaction de Fe + avec O 2 et c-C 2 H 4 O en utilisant des techniques de jets moleculaires: formation de FeO + . Mecanisme de la reaction

Neutral and ionic metal-hydrogen and metal-carbon bond energies: Reactions of Co+, Ni+, and Cu+ with ethane, propane, methylpropane, and dimethylpropane

Abstract: The reactions of Co{sup +}, Ni{sup +}, and Cu{sup +} with a series of alkanes are examined by guided ion beam mass spectrometry. The emphasis of this study is on C-H and C-C bond cleavage channels from which bond dissociation energies for M-H, M-CH{sub 3}, and M{sup +}-CH{sub 3} are derived from the endothermic reaction thresholds. For these three bond energies, we find values (in kilocalories per mole) of 46 {plus minus} 3, 46 {plus minus} 3, and 49.1 {plus minus} 3.5, respectively, for M = Co; 58 {plus minus} 3, 55 {plus minus} 3, and 45.0 {plus minus} 2.4, respectively, for M = Ni; 61 {plus minus} 4, 58 {plus minus} 2, 29.7 {plus minus} 1.7, respectively, for M = Cu. Trends in the thermochemistry of these species and M{sup +}-H are briefly discussed. The reactivity of Cu{sup +}, which has not been previously studied, is compared with that for other transition-metal ions. Also, unusual features in the reactivity of Co{sup +} and Ni{sup +} that have not previously been commented on are discussed.

Oxidation reactions at variably sized transition metal centers: Fe+n and Nb+n +O2 (n=1–3)

Abstract: Cross sections for the reactions of Fe+n and Nb+n (n=1–3) with O2 are measured as a function of kinetic energy over a range of 0 to >10 eV. In all systems, analysis yields insight into the kinetics and thermochemistry of the oxidation processes. Nb+n reaction with O2 exothermically near the Langevin–Gioumousis–Stevenson close‐collision limit, driven by formation of strong NbO+ and NbO bonds. Fe+n are less reactive, although oxidation becomes progressively more facile as the size of the reactant increases from Fe+ to Fe+3. In contrast to the Nb+n systems, Fe+n (n=2,3) react at elevated energies by simple cluster fragmentation processes. Quantitative limits are established for ionic and neutral cluster oxide bond dissociation energies. Cross sections for formation of MnO+ from reaction of M+n (Fe+3, Nb+2, and Nb+3) are observed to have both an exothermic and an endothermic feature. Since there is only one chemical pathway to form this product, it is suggested that there are activation barriers to formation ...

Collision-induced dissociation of niobium cluster ions: transition-metal-cluster binding energies

Abstract: The cross sections for collision-induced dissociation (CID) of Nb{sub n}{sup +} (n = 2-6) with Xe are presented. Experiments are conducted on a recently constructed guided ion beam mass spectrometer, which produces intense beams of thermalized, mass-selected, niobium cluster ions. Nb{sub n}{sup +} are observed to fission to all possible ionic fragments and the largest possible neutral fragments at collision energies < 10 eV. Evidence is presented for loss of multiple Nb atoms from the cluster at energies higher than 10 eV. This fragmentation pattern differs markedly from that previously observed for small iron cluster ions. CID thresholds are used to derive D{degree}(Nb{sub n{minus}1}{sup +}-Nb) for n = 2-6, along with D{degree}(Nb{sub m}) and ionization potentials (IPs) of Nb{sub m} for m = 2 and 3. By using known IPs, D{degree}(Nb{sub n}) for n = 4,5 and 6 are also obtained. Nb{sub 2}{sup +} is found to be the most strongly bound cluster ion, D{degree}(Nb{sup +}-Nb) = 6.15 {plus minus} 0.15, and Nb{sub 3}{sup +} is the most weakly bound cluster ion, D{degree} (Nb{sub 2}{sup +}-Nb) = 4.60 {plus minus} 0.15 eV.

Periodic trends in chemical reactivity: reactions of Sc+, Y+, La+, and Lu+ with H2, D2, and HD

Abstract: The reactions of Sc{sup +}, Y{sup +}, La{sup +}, and Lu{sup +} with H{sub 2}, D{sub 2}, and HD are examined by use of guided ion beam mass spectrometry. Sc{sup +}, Y{sup +} and La{sup +} are found to react primarily via an insertion mechanism, while Lu{sup +} reacts impulsively at threshold and in a direct manner at higher energies. A simple molecular orbital model coupled with adiabatic surface crossings is used to explain the reactivity seen. The results are analyzed to give the O K bond energies D{degree}(Sc{sup +}-H) = 2.44 {plus minus} 0.09 eV, D{degree} (Y{sup +}-H) = 2.66 {plus minus} 0.06 eV, D{degree} (La{sup +}-H) = 2.48 {plus minus} 0.09 eV, and a more tentative value of D{degree} (Lu{sup +}-H) = 2.11 {plus minus} 0.16 eV (48.6 {plus minus} 3.7 kcal/mol). The results suggest that intrinsic M{sup +}-H bond dissociation energies for third-row metals are about 60 kcal/mol, similar to values for the first and second rows.

Guided ion beam studies of the state-specific reactions of iron(1+)(6D,4F) with methyl halides (CH3X; X = Cl, Br, I)

Abstract: Reactions of Fe{sup +} with CH{sub 3}X (X = Cl, Br, I) are studied by guided ion beam techniques. State-specific reaction cross sections for production of FeCH{sub 3}{sup +} and FeX{sup +} are presented for the {sup 6}D ground and the {sup 4}F first excited states of Fe{sup +}. The overall behavior seen in these reactions is similar to that seen in the analogous reactions of Co{sup +} and Ni{sup +}, discussed in the preceding paper in this issue. The two states of Fe{sup +} exhibit large differences in reactivity, with the {sup 4}F state generally being more reactive than the {sup 6}D state for production of FeX{sup +} and FeCH{sub 3}{sup +}. The only exception is for the exothermic formation of FeCH{sub 3}{sup +} below 0.7 eV in the CH{sub 3}I system. We postulate that this is due to a potential energy surface crossing that is avoided at low kinetic energies due to spin-orbit interactions and is permitted at higher energies. Analysis of the threshold behavior of the endothermic reactions provides two determinations of D{degree} (Fe{sup +}-CH{sub 3}) = 2.49 {plus minus} 0.13 and 2.47 {plus minus} 0.07 eV, in good agreement with previous values. Lower limits are placedmore » on the bond energies for Fe{sup +}-X.« less

Spin–orbit state‐selected reactions of Xe+(2P3/2 and 2P1/2) with H2, D2, and HD

Abstract: Spin–orbit state‐selected reactions of Xe+(2PJ ), J=3/2 and 1/2, with isotopic molecular hydrogen (H2, D2, and HD) to form XeH+ and XeD+ are studied using guided ion beam mass spectrometry. Reaction cross sections are determined as a function of reactant kinetic energy from near thermal energy to 15 eV c.m. Although the reaction of Xe+(2P1/2)+H2 to form ground state products XeH++H is exothermic, no reaction is observed at low energies and the reaction cross section is zero or small at higher energies. The Xe+(2P3/2)+H2 reaction has an apparent threshold near its endothermicity, but the cross section rises slowly above this threshold. The reaction energetics and isotope effects indicate two separate mechanisms for Xe+(2P3/2). The dominant mechanism is a direct, impulsive process with an apparent activation barrier. The second mechanism allows reaction near the thermochemical threshold and involves more interaction among all three atoms. These results are interpreted in relation to the spin–orbit coupled p...

Guided ion beam studies of the reactions of Co+ and Ni+ with CH3X (X = Cl, Br, I). Implications for the metal-methyl ion bond energies

Abstract: Une etude sur les reactions de Co + et Ni + avec les halegeneurs de methyle par les techniques de faisceau ionique guide. Les energies de liaison pour les systemes du type MCH 3 + et MCl + sont determinees a partir de toutes les six reactions

Reactions of Atomic Metal Ions with H2, CH4, and C2H6: Electronic Requirements for H—H, C—H, and C—C Bond Activation

Abstract: One of the obligations (and difficulties) faced by gas phase inorganic chemist is relating their work to the general field of condensed phase inorgani chemistry. One specific area of common interest to both gas phase and con densed phase inorganic chemists is the activation of C—H bonds by meta centers. Research into homogeneous systems which activate alkanes has seei a flurry of activity in recent years.(1) Similarly, ever since the first observatioi by Allison, Freas, and Ridge(2) in 1979 that atomic transition metal ions cai activate C—H and C—C bonds, a tremendous amount of experimental worl has centered on the gas phase reactions of M+ with alkanes. Unfortunately the connection between these gas phase and condensed phase systems ha been elusive. This relationship is critical if the data of the gas phase chemis are to be most meaningful.

Reactions of Ar+, Ne+, and He+ with SiF4 from thermal energy to 50 eV c.m.

Abstract: Guided ion‐beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF+x (x=0–4) products are observed, except for SiF+4 from reaction with Ne+ and He+, and Si+ from reaction with Ar+. At high energies, the dominant products are SiF+3 in the Ar system, and SiF+ in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF+3 at 5.7 eV. In the Ar+ and Ne+ reactions, the observed energetics are consistent with literature thermochemistry, but with He+, reaction barriers are observed. A value of ΔH0f,298 (SiF+3)=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions ...

Neutral and Ionic Metal-Hydrogen and Metal-Carbon Bond Energies: Reactions of Co+, Ni+, and Cu+ with Ethane, Propane, Methylpropane, and Dimethylpropane.

Abstract: The reactions of Co{sup +}, Ni{sup +}, and Cu{sup +} with a series of alkanes are examined by guided ion beam mass spectrometry. The emphasis of this study is on C-H and C-C bond cleavage channels from which bond dissociation energies for M-H, M-CH{sub 3}, and M{sup +}-CH{sub 3} are derived from the endothermic reaction thresholds. For these three bond energies, we find values (in kilocalories per mole) of 46 {plus minus} 3, 46 {plus minus} 3, and 49.1 {plus minus} 3.5, respectively, for M = Co; 58 {plus minus} 3, 55 {plus minus} 3, and 45.0 {plus minus} 2.4, respectively, for M = Ni; 61 {plus minus} 4, 58 {plus minus} 2, 29.7 {plus minus} 1.7, respectively, for M = Cu. Trends in the thermochemistry of these species and M{sup +}-H are briefly discussed. The reactivity of Cu{sup +}, which has not been previously studied, is compared with that for other transition-metal ions. Also, unusual features in the reactivity of Co{sup +} and Ni{sup +} that have not previously been commented on are discussed.

Reactions of Ar+, Ne+, and He+ with SiF4 from Thermal Energy to 50 eV c.m.

Abstract: Guided ion‐beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF+x (x=0–4) products are observed, except for SiF+4 from reaction with Ne+ and He+, and Si+ from reaction with Ar+. At high energies, the dominant products are SiF+3 in the Ar system, and SiF+ in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF+3 at 5.7 eV. In the Ar+ and Ne+ reactions, the observed energetics are consistent with literature thermochemistry, but with He+, reaction barriers are observed. A value of ΔH0f,298 (SiF+3)=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions ...