Showing papers by "P. B. Armentrout published in 1990"

Electronic State-Specific Transition Metal ION Chemistry

Abstract: The importance of transition metal centers as catalysts for selective trans­ formations of small molecules into useful chemicals has prompted exten­ sive studies in the condensed phase. In the last decade or so , chemical analogues to these processes have also been investigated in the gas phase. The promise of the gas phase research is that more quantitative infor­ mation regarding the dynamics, kinetics, and thermochemistry of these processes can be obtained in a more controlled environment. A particularly active component of this gas phase research is the chemistry of atomic transition metal ions, which are conveniently studied by several types of mass spectrometric techniques (1-7). Although the relationship between this gas phase endeavor and condensed phase organometallic chemistry is still evolving , one area in which gas phase studies have contributed some insight into the reactivity at transition metal centers concerns the influence of the electronic state (8a). The abundance of low-lying electronic statcs is one of the features of transition metals that is integral to their ability to catalyze chemistry , since it allows a metal center to be a versatile reaction template in which many different types of species can bond and subsequently react. Unfortunately, this same feature makes it difficult to generate transition metals (atoms, ions , or complexes) in specific electronic states. Table I shows the extent of the problem for the first row transition metal ions. Since ion generation

Reactions of fourth‐period metal ions (Ca+−Zn+) with O2: Metal‐oxide ion bond energies

Abstract: Reactions of Ca+, Zn+ and all first‐row atomic transition metal ions with O2 are studied using guided ion beam techniques. While reactions of the ground states of Sc+, Ti+, and V+ are exothermic, the remaining metal ions react with O2 in endothermic processes. Analyses of these endothermic reactions provide new determinations of the M+–O bond energies for these eight elements. Source conditions are varied such that the contributions of excited states of the metal ions can be explicitly considered for Mn+, Co+, Ni+, and Cu+. Results (in eV) at 0 K are D0(Ca+–O)= 3.57±0.05, D0(Cr+–O)=3.72±0.12, D0(Mn+–O)=2.95±0.13, D0(Fe+–O)=3.53±0.06 (reported previously), D0(Co+–O)=3.32±0.06, D0(Ni+–O) =2.74±0.07, D0(Cu+–O)=1.62±0.15, and D0(Zn+–O)=1.65±0.12. These values along with literature data for neutral metal oxide bond energies and ionization energies are critically evaluated. Periodic trends in the ionic metal oxide bond energies are compared with those of the neutral metal oxides and those of other related molecules.

Effect of internal excitation on the collision-induced dissociation and reactivity of Co 2 +

Abstract: The kinetic energy dependence of collision-induced dissociation (CID) of dicobalt ion (Co 2 + ) with He, Ar, and Xe has been investigated using guided ion-beam mass spectrometry. The change in efficiency of CID as the target gas is changed is in general agreement with previous CID studies of other systems: the cross section with Ar is ∼0.5 that with Xe, and no product ions are found with He. By varying the conditions under which the reactant ions are formed, the degree of internal excitation of the dicobalt ions is changed. The internal energies can be characterized by a Maxwell-Boltzmann distribution. We find that CID and reactions with O2 and CO are very sensitive to Co 2 + internal energy. The bond-dissociation energy derived from this work is Do(Co 2 + )=2.75±0.10 eV (63.4±2.3 kcal/mol). The Co 2 + results are compared with a previous study of Fe 2 + .

Reactions of cobalt(1+), nickel(1+), and copper(1+) with cyclopropane and ethylene oxide. Metal-methylidene ion bond energies

Abstract: The reactions of atomic cobalt, nickel, and copper ions with cyclopropane and ethylene oxide have been studied by using guided ion beam mass spectrometry. A predominant process in all these systems is formation of MCH{sub 2}{sup +}. Analyses of these endothermic reactions yield the bond energies D{sup 0} (Co{sup +}-CH{sub 2}) = 77.5 {plus minus} 2.3 kcal/mol, D{sup 0}(Ni{sup +}-CH{sub 2}) = 75.2 {plus minus} 1.8 kcal/mol, and D{sup 0}(Cu{sup +}-CH{sub 2}) = 63.9 {plus minus} 1.6 kcal/mol. Differences between these values and those derived from earlier studies for Co{sup +} and Ni{sup +} are discussed. In addition to D{sup 0} (M{sup +}-CH{sub 2}), bond energies for Co{sup +}-H, M-H, (M = Co, Ni, Cu) and M{sup +}-O and M-O (M = Co, Ni) are evaluated and lower limits are placed on D{sup 0} (M{sup +}-C{sub 2}H{sub 4}) and D{sup 0} (M{sup +}-C{sub 2}H{sub 2}) (M = Co, Ni, Cu). The reaction mechanism for these reactions is also discussed in detail.

Ammonia Activation by Sc' and Ti': Electronic and Translational Energy Dependence

Abstract: The reactions of Sc' and Ti' with ammonia are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic energy for the Ti+ reactions is also probed by varying the conditions for forming Ti+. Excited doublet states of Ti' are found to be much more reactive than the a4F ground and b4F first excited states. Both metals form the MH+, MNH', and MNH2+ products, while only Ti+ forms MN+. The results are consistent with reaction that occurs primarily through a covalently bound insertion intermediate, H-M+-NHz, having a singlet and doublet spin state for M = Sc and Ti, respectively. The reactivities of the different electronic states of the reactant ions can be explained by using spin conservation concepts. The thresholds for the cross sections of the endothermic reactions are interpreted to give the 298 K bond energies of Do(Sc+-NH2) = 3.69 i 0.07 eV, Do(Sc+-NH) = 5.16 f 0.10 eV, Do(Ti+-NH2) = 3.69 f 0.13 eV, D"(Ti+-NH) = 4.83 f 0.12 eV, and Do(Ti+-N) = 5.19 f 0.13 eV. The large bond strengths of the M+-NH2 and M+-NH species indicate that the lone pair electrons on the nitrogen atom are involved in the metal-ligand bond.

Dissociative charge transfer reactions of Ar+, Ne+, and He+ with CF4 from thermal to 50 eV

Abstract: Guided ion‐beam techniques are used to measure the cross sections for reaction of CF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Dissociative charge transfer followed by successive loss of F atoms are the major processes observed. Only CF+x (x=1–3) products are observed in the reactions of Ar+ and Ne+. With He+, in addition to the CF+x products, both C+ and F+ are seen at high kinetic energies. Reaction rates for these reactions are also given and compared with previous measurements. It is found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of the CF+4 ion.

Periodic trends in the reactions of atomic ions with molecular hydrogen

Abstract: One of the simplest possible reactions imaginable, that of an atomic ion with molecular hydrogen, shows a remarkable diversity in behaviour as the atomic ion is varied throughout the periodic table. Studies of these reactions reveal the fundamental changes that occur in the chemistry as the number of electrons is systematically varied. Here, we review our work on the kinetic energy dependence of these reactions as studied by using guided ion beam mass spectrometry. Substantial progress has been made in this area and now includes studies of 44 elements. This experimental technique is shown to provide unprecedented detail in reaction excitation functions over an extremely wide kinetic energy range. These studies allow an understanding of such effects as the reaction thermochemistry, electronic degeneracy, adiabatic versus diabatic potential energy surfaces and spin-orbit coupling.

C-H Bond Activation as the Initial Step in the Co+-Mediated Demethanation of Propane: The Critical Role of Angular Momentum at the Rate-Limiting Transition State

Abstract: Exothermic reactions of transition-metal ions with alkanes have in many instances been shown to be facile in the gas phase. Reactions mainly result in the loss of molecular hydrogen and small alkanes to yield metal ion-olefin complexes. A variety of experimental techniques have provided thermochemical, kinetic, dynamic, and mechanistic information for these reactions, with an important focus being the identification of the initial activation step. The question of C-H versus C-C bond activation as the initial step in the formation of C-C bond cleavage products has yet to be resolved. The authors have measured reaction cross sections and kinetic energy release distributions for the exothermic reactions of Co{sup +} with propane, propane-2-d{sub 1}, propane-2,2-d{sub 2}, propane-1,1,1-d{sub 3}, propane-1,1,1,3,3,3-d{sub 6}, and propane-d{sub 8}.

Reactions of Mn+ with i-C4H10, neo-c5H12, (CH3)2CO, cyclo-C3H6, and cyclo-C2H4O : bond energies for MnCH2+, MnH, and MnCH3

Abstract: The reactions of Mn + with isobutane, neopentane, acetone, cyclopropane, and ethylene oxide are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The electronic state of the Mn + ions is varied by altering the ionization technique. The a 5 S and a 5 D excited states are found to be more reactive than the a 7 D ground state, although formation of MnCH 3 shows less of an effect than the other major reactions

Heat of formation of the hydroperoxyl radical HO2. A direct determination from guided ion beam studies of oxygen-methane [O2+(2.PI.g, v = 0) + CH4] reaction

Abstract: The heat of formation of the hydroperoxyl radical, HO{sub 2}, and the ionization energy of methane, IE(CH{sub 4}), are measured directly, by using guided ion beam mass spectrometry to study the reaction of O{sub 2}{sup +}({sup 2}{Pi}{sub g},{nu}=0) + CH{sub 4} The O{sub 2}{sup +} ions are produced in a flow tube source and are shown to be completely thermalized The thermodynamic values derived are IE(CH{sub 4}) = 1254 {plus minus} 007 eV, {Delta}{sub f}H{sub 298}(HO{sub 2}) = 38 {plus minus} 12 kcal/mol, D{sub 298}{degree}(H-OO) = 483 {plus minus} 12 kcal/mol, and D{sub 298}{degree}(H-OOH) = 884 {plus minus} 12 kcal/mol (where the uncertainties represent our 95% confidence limits) The present values are compared to prior experimental and theoretical values

Kinetic energy dependence of dissociative charge-transfer reactions of He+, Ne+, Ar+, Kr+, and Xe+ with silane

Abstract: Guided ion‐beam techniques are used to measure the cross sections as a function of kinetic energy for reaction of SiH4 with He+, Ne+, Ar+, Kr+, and Xe+. State‐specific data for the 2P3/2 ground spin–orbit states of Kr+ and Xe+ are also obtained. The products observed in the He, Ar, and Kr systems are SiH+x for x=0–3. For the Ne system, formation of SiH+x x = 0–2, is seen, while in the Xe system only SiH+3 and SiH+2 are observed. Reactions of He+, Ne+, Kr+, and Xe+ show little dependence on kinetic energy, but for the case of Ar+, the reaction probability and the product distribution are highly sensitive to the kinetic energy of the system. Thermal reaction rates for all of the reactions are derived and compared with previous measurements. The results for these reactions are explained in terms of vertical ionization from the 1t2 and 3a1 bands of SiH4. The relationships of these reactions to plasma deposition and etching are also discussed.

Ion beam studies of the reaction of Si+(2P) with methane. Reaction mechanisms and thermochemistry of SiCHx+ (x=1−3)

Abstract: Guided ion beam mass spectrometry is used to examine the reaction of ground-state silicon ion with methane. Absolute cross sections of all products are measured from near-thermal to 14-eV relative kinetic energy. Only endothermic processes are observed with SiH{sup +} and SiH{sub 3}C{sup +} as the major ionic products. There is evidence that the latter species has two forms, Si{sup +}-CH{sub 3} formed at low energies and a higher energy form that could be a triplet state of SiCH{sub 3}{sup +} or HSiCH{sub 2}{sup +}. Minor ionic products include SiCH{sub 2}{sup +}, CH{sub 3}{sup +}, and SiCH{sup +}. The former product can be formed via the concomitant formation of molecular hydrogen or two hydrogen atoms. The latter process is much more efficient. All observed products are consistent with a reaction that occurs via an HSiCH{sub 3}{sup +} intermediate. From the measured thresholds of the reactions and other information, the 298 K heats of formation (kcal/mol) for the following silicon species are derived: {Delta}{sub f}H{degree}(SiH) = 91.4 {plus minus} 1.8, {Delta}{sub f}H{degree} (SiCH{sup +}) = 339 {plus minus} 7, {Delta}{sub f}H{degree}(SiCH{sub 2}{sup +}) = 285 {plus minus} 3, and {Delta}{sub f}H{degree} (SiCH{sub 3}{sup +}) = 235 {plus minus} 5.

Laser multiphoton ionization and photoelectron spectroscopy of Co(CO) 3NO and Fe(CO)5

Abstract: Laser multiphoton dissociation‐resonance‐enhanced multiphoton ionization (MPD‐REMPI) and time‐of‐flight photoelectron spectra (TOF‐PES) of Co(CO)3NO and Fe(CO)5 have been obtained in the range 445–455 nm. The only ions produced by the pulsed dye laser are Co+ and Fe+. Transitions observed in the MPD‐REMPI spectra are assigned to resonant states of the neutral atoms. Final states of the atomic ions are determined from the TOF‐PES spectra. The multiphoton dissociation process produces metal atoms in a broad distribution of states, ranging in energies up to 33 000 cm−1 for Co, and 32 000 cm−1 for Fe. The most intense REMPI lines are associated with low‐lying electronic states (<8500 cm−1 for Fe and Co). By tuning the laser to appropriate wavelengths, neutral metal atoms in selected electronic states may be ionized. At most laser wavelengths, the atomic metal ions are formed in a distribution of states, only some of which are consistent with preservation of the core configuration of the Rydberg intermediate i...

Ammonia Activation by V+: Electronic and Translational Energy Dependence.

Abstract: The reaction of Ti/sup +/ with methane is studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic excitation is also studied by varying the conditions for forming Ti/sup +/. The a/sup 2/F excited state is found to form the two main products, TiH/sup +/ and TiCH/sub 2//sup +/, substantially more efficiently than the a/sup 4/F ground and b/sup 4/F first excited states. The results indicate that reaction occurs primarily through a doublet H-Ti/sup +/-CH/sub 3/ intermediate. The reactivities of the different electronic states of Ti/sup +/ can be explained by using simple molecular orbital concepts and spin conversation. The thresholds for these reactions and for related reactions in other systems are interpreted to give D/sup 0/(Ti/sup +/-H) = 54.2 +/- 2.5, D/sup 0/(Ti/sup +/-CH/sub 3/) = 57.5 +/- 2.8, D/sup 0/(Ti/sup +/-CH/sub 2/) = 93.4 +/- 3.5, and D/sup 0/(Ti/sup +/-CH) = 121 +/- 4, all in kcal/mol.

Ammonia Activation by Sc+ and Ti+: Electronic and Translational Energy Dependence.

Abstract: The reactions of Sc' and Ti' with ammonia are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic energy for the Ti+ reactions is also probed by varying the conditions for forming Ti+. Excited doublet states of Ti' are found to be much more reactive than the a4F ground and b4F first excited states. Both metals form the MH+, MNH', and MNH2+ products, while only Ti+ forms MN+. The results are consistent with reaction that occurs primarily through a covalently bound insertion intermediate, H-M+-NHz, having a singlet and doublet spin state for M = Sc and Ti, respectively. The reactivities of the different electronic states of the reactant ions can be explained by using spin conservation concepts. The thresholds for the cross sections of the endothermic reactions are interpreted to give the 298 K bond energies of Do(Sc+-NH2) = 3.69 i 0.07 eV, Do(Sc+-NH) = 5.16 f 0.10 eV, Do(Ti+-NH2) = 3.69 f 0.13 eV, D"(Ti+-NH) = 4.83 f 0.12 eV, and Do(Ti+-N) = 5.19 f 0.13 eV. The large bond strengths of the M+-NH2 and M+-NH species indicate that the lone pair electrons on the nitrogen atom are involved in the metal-ligand bond.