Steven W. Buckner

Reactivity, photochemistry and thermochemistry of simple metal—ligand ions in the gas phase

Abstract In this review we discuss recent results from our laboratory on the chemistry, photochemistry and thermodynamics of simple metal—ligand ionic species in the gas phase. Using laser desorption coupled to Fourier transform mass spectrometry, we have successfully generated numerous highly unsaturated metal—ligand ions including bare metal carbenes, methyls, hydrides, nitrenes and amides. The combination of photodissociation, ion—molecule reactions and other techniques has made possible the determination of a wide variety of metal—ligand ion bond dissociation energies. Reactivity studies on these species reveal gas phase catalytic cycles including olefin homologation and Fe+ catalysed production of NC bonds from NH3, olefins and N2O. General trends are observed in the reactions of MOH+, MH+, MCH3+, and MNH2+ with alkanes; MO+ and MS+ with alkanes; and MNH+, MCH2+ and MO+ with alkenes.

Capping and passivation of aluminum nanoparticles using alkyl-substituted epoxides.

We report here on the synthesis and passivation of small (20-30 nm) aluminum nanoparticles using alkyl-substituted epoxides as capping agents. FTIR and 13C NMR spectroscopy indicate that the epoxides polymerize to form a polyether cap on the surfaces of the aluminum nanoparticles. Nanoparticles capped with epoxyhexane and epoxydodecane are stable in air, but particles capped with epoxyisobutane are pyrophoric. TEM images show spherical Al particles. Powder X-ray diffraction shows the presence of crystalline Al. Titrimetric analysis of the core-shell nanostructures in air reveals that 96% of the total aluminum present is active (unoxidized) aluminum.

Gas phase studies of Zn+2, Ag+3, and Ag+5

Laser desorption from ZnO and AgO produces small bare metal cluster ions. Laser desorption from a ZnO/AgO mixture produces an enhancement of the silver cluster ion signal with complete suppression of the zinc signal. The chemistry of Zn+2 indicates IP(Zn2)=9.0±0.2 eV and D0(Zn+–Zn)=0.56±0.2 eV. The reactivity of Zn+2 with alkenes and alcohols is characterized by displacement of a zinc atom and formation of Zn+–B (B=alcohol, alkene). The silver cluster ions are produced with excess kinetic energy; however, collisional cooling is achieved by trapping the cluster ions in a static pressure of argon. Charge transfer reactions indicate IP(Agn) 1.73 eV.

Aluminum nanoparticles capped by polymerization of alkyl-substituted epoxides: ratio-dependent stability and particle size.

We report here on the polymerization of epoxide monomers on incipient aluminum nanoparticle cores and the effects of changing the epoxide-capping precursor and the metallic monomer ratio on the resultant stability and particle size of passivated and capped aluminum nanoparticles. When altering the ratio of aluminum to cap monomer precursor, nanoparticles capped with epoxydodecane, epoxyhexane, and epoxyisobutane show a clear decreasing trend in stability with decreasing alkane substituent length. The nanoparticle core size was unaffected by cap ratio or composition. PXRD (powder X-ray diffraction) and DSC/TGA (differential scanning calorimetry/thermal gravimetric analysis) confirm the presence of successfully passivated face-centered cubic (fcc) aluminum nanoparticles. We also report preliminary results from ATR-FTIR (attenuated total reflectance-Fourier transform infrared), (13)C CPMAS (cross-polarization/magic-angle spinning), and (27)Al MAS solid-state NMR (nuclear magnetic resonance) measurements. The most stable aluminum nanoparticle-polyether core-shell nanoparticles are found at an Al:monomer mole ratio of 10:1 with an active Al(0) content of 94%.

Oxidation reactions and photochemistry of aluminum cluster anions (Al3− to Al23−)

Abstract Reactions of mass-selected Aln− (n=3–23) with O2 produce AlO2−, AlO−, and electron detachment (n≤8); or Aln−4− (n=8–12, 14–22) and sequential dissociation products. All clusters except Al13− absorb at 1064 nm, losing one atom or electron. Several clusters require sequential absorption of two 1064 nm photons to dissociate. Single-photon excitation of Al16− and Al18− retards their oxidation reactions by factors of 2.5±1.0 and 5.0±2.0, respectively. Radiative relaxation of Al16− proceeds with a rate constant of 0.47±0.10 s−1, measured by the return to thermal reaction behavior.

Formation of thermodynamically stable dications in the gas phase by thermal ion–molecule reactions: Nb2+ with small alkanes

The gas‐phase reactions of Nb2+ with small alkanes at thermal energies are reported. For methane and ethane, dehydrogenation is a prominent reaction pathway. For propane and butane, charge transfer is virtually the only reaction pathway observed (>99%). NbCH2+2 and NbC2H2+2 formed in the reactions of Nb2+ with methane and ethane are thermodynamically stable with D(Nb2+–CH2)=197±10 kcal/mol, D(Nb+–CH+2)=107±10 kcal/mol, D(Nb2+–C2H2)≥74 kcal/mol, and D(Nb+–C2H+2)≥7 kcal/mol. The stability of these ions is most likely due to the charge‐stabilizing effect of the metal center. Collision‐induced dissociation of these ions results in charge‐splitting reactions as well as reactions in which both charges remain on the metal center. Hydride transfer is observed to be competitive in the primary reactions of Nb2+ with alkanes. The hydride‐ and charge‐transfer results are in qualitative agreement with a simple curve‐crossing model.

Mechanistic and kinetic aspects of chemical ionization mass spectrometry of polynuclear aromatic hydrocarbons and their halogen-substituted analogues using oxidizing reagents: A gas chromatographic-mass spectrometric and Fourier transform mass spectrometric study

The mass spectral behavior of some substituted polynuclear aromatic hydrocarbons (PAHs) was investigated by using low-pressure chemical ionization conditions with a Fourier transform ion cyclotron resonance spectrometer (FT-MS) and by using gas chromatography-mass spectrometry (GC-MS) under higher pressure chemical ionization conditions. For the reactions of O−· with substituted PAHs, several reaction pathways are observed, including hydrogen atom transfer, hydrogen atom displacement, nucleophilic aromatic substitution, ring cleavage, and abstraction of H+2 to generate (M - 2H)−. Bimolecular rate constants and product branching ratios for the reactions of O−· with benzene and a variety of substituted naphthalenes were obtained using FT-MS. The rate constants vary from (2.1 ± 0.4) × 10−10 cm3 molecule−1 s−1 for naphthalene to (2.1 ± 0.4) × 10−9 cm3 molecule−1 s−1 for 1 − chloronaphthalene, corresponding to reaction efficiencies of 8% and 60%, respectively. In contrast to previous pulsed radiolysis studies on the reactions of O−· with aromatic compounds in solution, O−· is observed to be a powerful gas-phase nucleophile. Nucleophilic substitution reactions are observed even in the absence of activating groups. Competition between this and other reaction pathways is related to the leaving-group abilities and inductive effects of the various substituents. Observation of reactions with the chlorinated naphthalenes implies limits on the heats of formation for the corresponding carbanions of ΔHf(1-ClC10H−6) < 46 ± 5 kcal mol−1 and ΔHf(2-ClC10H−6) < 49 ± 5 kcal mol−1 (1 kcal=4.184 kJ). Although the isomeric PAHs often give very similar mass spectra and their chromatographic peaks are not well resolved, differences in their GC-MS behavior can be used to distinguish anthracene and phenanthrene. For the unsubstituted PAHs reaction pathways are observed in agreement with recent studies and mass-selected FT-MS suggests which reaction pathways are due to ion-molecule reactions. Low ion intensities for many of the oxidation products relative to products formed by electron-transfer reactions of the PAHs suggest oxidation may occur by wall-catalyzed processes, in agreement with recent studies by Stemmler and Buchanan. The positional isomers of chloronaphthalene give clearly different mass spectra under both N2O and N2OO2 chemical ionization conditions whereas the chloroanthracenes are not readily distinguished using GC-MS.

Kinetic energy release in thermal ion-molecule reactions : the Nb2+ (benzene) single charge-transfer reaction

We have adapted the techniques originally developed to measure ion kinetic energies in ion cyclotron resonance (ICR) spectrometry to study the single charge–transfer reaction of Nb2+ with benzene under thermal conditions in a Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS). The partitioning of reaction exothermicity among the internal and translational modes available is consistent with a long‐distance electron‐transfer mechanism, in which the reactants approach on an ion‐induced dipole attractive potential and cross to a repulsive potential at a critical separation of ∼7.5 A when electron transfer occurs. The reaction exothermicity, 5.08 eV, is partitioned to translation of Nb+, 0.81±0.25 eV, translation of C6 H6+, 1.22±0.25 eV, and internal excitation of C6 H6+ to produce the la2u electronic state, which is ∼3 eV above the ground state of the ion. We have also studied the kinetics of the reaction of Nb2+ with benzene and determined the rate constant, k = 1.4×10−9 cm3 molecule−1 s−1...

Decarboxylation in the reactions of O-. with metal carbonyl complexes in the gas phase

Abstract The gas phase reactions of O with some first row transition metal complexes have been investigated using Fourier transform ion-cyclotron resonance spectroscopy. O, generated by dissociative electron capture from N2O, reacts with M(CO)x, (M = V, Cr, Fe) and manganese(methylcyclopentadienyl)(CO)3 to generate ML(CO)n−1, exclusively. Thermochemical arguments indicate the products cannot be generated by a simple dissociative electron-transfer mechanism, but must result from loss of CO2. Although the reactions that generate the metal anion complexes and CO2 are > 95 kcal mol−1 exothermic, the metal carbonyl anion products do not subsequently decompose indicating non-statistical energy disposal in the products. The order of relative rate constants for the reactions of the homoleptic carbonyl complexes [kII(O−./V(CO)6) > kII(O−./Cr(CO)6) > kII(O−./Fe(CO)5)] and the product energy disposal results support an addition/elimination mechanism. ca]†|Author to whom correspondence should be addressed.

Metal and Metal Carbide Nanoparticle Synthesis Using Electrical Explosion of Wires Coupled with Epoxide Polymerization Capping.

In this study, metal-containing nanoparticles (NPs) were produced using electrical explosion of wires (EEW) in organic solvents. The explosion chamber was constructed from Teflon to withstand the shockwave, allow growth and reaction of the incipient NPs in various organic solvents containing dissolved ligands, and allow a constant flow of argon to maintain an inert environment. A survey of different transition d-block metals was conducted with metals from groups 4-8, affording metal carbide NPs, while metals from groups 9-12 gave elemental metallic NPs. Tungsten carbide phase WC1-x, which has not been previously isolated as a single-phase material, was exclusively formed during EEW. We used polymerization initiation by electron-rich metallic nanoparticles (PIERMEN) as a capping technique for the nascent NPs with an alkyl epoxide employed as the monomers. Transmission electron microscopy showed spherical particles with the metallic core embedded in a polymer matrix with predominantly smaller particles (<50 nm), but also a broad size distribution with some larger particles (>100 nm). Powder X-ray diffraction (PXRD) was used to confirm the identity of the metallic NPs. The capping agents were characterized using ATR-FTIR spectroscopy. No evidence is observed for the formation of crystalline oxides during EEW for any metals used. Differential scanning calorimetry/thermal gravimetric analysis was used to study the NPs behavior upon heating under an air flow up to 800 °C with the product oxides characterized by PXRD. The bifurcation between metal-carbide NPs and metal NPs correlates with the enthalpy of formation of the product carbides. We observed PIERMEN capping of elemental metal NPs only when the metal has negative standard electrode potentials (relative to a bis(biphenyl) chromium(I)/(0) reference electrode).