Kinetic-energy dependence of competitive spin-allowed and spin-forbidden reactions: V++CS2

Activation of CH4 by gas-phase Mo+, and the thermochemistry of Mo-ligand complexes

The kinetic-energy dependence of the reactions of Mo(+) ((6)S) with methane has been studied using guided ion beam mass spectrometry. No exothermic reactions are observed in this system, as also found previously, but efficient dehydrogenation occurs at slightly elevated energies. At higher energies, MoH(+) dominates the product spectrum and MoC(+), MoCH(+), and MoCH(3)(+) are also observed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D(0)(Mo(+)-C) = 4.55 +/- 0.19, D(0)(Mo(+)-CH) = 5.32 +/- 0.14, D(0)(Mo(+)-CH(2)) = 3.57 +/- 0.10, and D(0)(Mo(+)-CH(3)) = 1.57 +/- 0.09. The results for Mo(+) are compared with those for the first- and third-row transition-metal congeners, Cr(+) and W(+), and the differences in behavior and mechanism are discussed. Theoretical results are used to elucidate the geometric and electronic structures of all product ions as well as the complete potential-energy surface for reaction. The efficiency of the coupling between the sextet and quartet spin surfaces is also quantified.

Gas-Phase Infrared Photodissociation Spectroscopy of Tetravanadiumoxo and Oxo–Methoxo Cluster Anions

The infrared spectra of the binary vanadium oxide cluster anions V(4)O(9)(-) and V(4)O(10)(-) and of the related methoxo clusters V(4)O(9)(OCH(3))(-) and V(4)O(8)(OCH(3))(2)(-) are recorded in the gas phase by photodissociation of the mass-selected ions using an infrared laser. For the oxide clusters V(4)O(9)(-) and V(4)O(10)(-), the bands of the terminal vanadyl oxygen atoms, nu(V-O(t)), and of the bridging oxygen atoms, nu(V-O(b)-V), are identified clearly. The clusters in which one or two of the oxo groups are replaced by methoxo ligands show additional absorptions which are assigned to the C-O stretch, nu(C-O). Density functional calculations are used as a complement for the experimental studies and the interpretation of the infrared spectra. The results depend in an unusual way on the functional employed (BLYP versus B3LYP), which is due to the presence of both V-O(CH(3)) single and V=O double bonds as terminal bonds and to the strong multireference character of the latter.

Ion mobility studies of electronically excited States of atomic transition metal cations: development of an ion mobility source for guided ion beam experiments

The design of an ion mobility source developed to couple to a guided ion beam tandem mass spectrometer is presented. In these exploratory studies, metal ions are created continuously by electron ionization of the volatile hexacarbonyls of the three group 6 transition metals. These ions are focused into a linear hexapole ion trap, which collects the ions and then creates high intensity pulses of ions, avoiding excessive ion losses resulting from the low duty cycle of pulsed operation. The ion pulses are injected into a six-ring drift cell filled with helium where ions having different electronic configurations can separate because they have different ion mobilities. Such separation is observed for chromium ions and compares favorably with the pioneering work of Kemper and Bowers (J. Phys. Chem.1991, 95, 5134). The results are then extended to Mo(+) and W(+), which also show efficient configuration separation. The source conditions needed for high intensities and good configuration separation are discussed in detail and suggestions for further improvements are also provided.

Gas-phase reactions of atomic lanthanide cations with CO2 and CS2: room-temperature kinetics and periodicities in reactivity

Gas-phase reactions of atomic lanthanide cations (excluding Pm+) have been surveyed systematically with CO2 and CS2 using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Observations are reported for reactions with La+, Ce+, Pr+, Nd+, Sm+, Eu+, Gd+, Tb+, Dy+, Ho+, Er+, Tm+, Yb+, and Lu+ at room temperature (295 +/- 2 K) in helium at a total pressure of 0.35 +/- 0.02 Torr. The observed primary reaction channels correspond to X-atom transfer (X = O, S) and CX2 addition. X-atom transfer is the predominant reaction channel with La+, Ce+, Pr+, Nd+, Gd+, Tb+, and Lu+, and CX2 addition occurs with the other lanthanide cations. Competition between these two channels is seen only in the reactions of CS2 with Nd+ and Lu+. Rate coefficient measurements indicate a periodicity in the reaction efficiencies of the early and late lanthanides. With CO2 the observed trends in reactivity across the row and with exothermicity follow trends in the energy required to achieve two unpaired non-f valence electrons by electron promotion within the Ln+ cation that suggest the presence of a kinetic barrier, in a manner much like those observed previously for reactions with isoelectronic N2O. In contrast, no such barrier is evident for S-atom transfer from the valence isolectronic CS2 molecule which proceeds at unit efficiency, and this is attributed to the much higher polarizability of CS2 compared to CO2 and N2O. Up to five CX2 molecules were observed to add sequentially to selected Ln+ and LnX+ cations.

Reactions of Group 3 Transition Metal Atoms with CS2 and OCS: Matrix Isolation Infrared Spectra and Density-Functional Calculations of SMCS, SM-(η2-CS), SMCO, and SM-(η2-CO) in Solid Argon

Laser-ablated scandium, yttrium, and lanthanum atoms were reacted with CS2 and OCS molecules in an argon matrix. Products of the type SMCX and S-M(eta2-CX) (X = S or O) were formed on sample deposition. Photolysis favored the S-M(eta(2)-CX) complex, while annealing increased the more stable SMCX isomer. Product absorptions are identified by density-functional frequency calculations and isotopic substitutions. This work reports the first vibrational spectroscopic characterization of Sc, Y, and La reaction products with CS2 and OCS and the subsequent interconversion between SMCX and S-M(eta2-CX) structural isomers.

Degradation of Ionized OV(OCH3)3 in the Gas Phase. From the Neutral Compound All the Way down to the Quasi-terminal Fragments VO+ and VOH+

The consecutive fragmentation of ionized trimethyl vanadate(V), OV(OCH3)3 (1), is examined by experiment and theory. After an elimination of formaldehyde from the molecular ion 1+, subsequent dissociations proceed via losses of first H2 and then two molecules of formaldehyde to finally yield the VOH+ cation; these redox reactions involve the V(II)/V(IV) manifold. At elevated energies, expulsion of CH3O* from 1+ can efficiently compete to afford OV(OCH3)2+, a formal V(V) compound, from which subsequent losses of H2 and two units of CH2O lead to bare VO+, thereby exploring the V(III)/V(V) redox manifold. Experiments using complementary mass spectrometric techniques, i.e., neutralization-reionization experiments and ion/molecule reactions, in conjunction with extensive computational studies provide deep insight into the ion structures and the relative energetics of these dissociation reactions. In particular, a quantitative energetic scheme is obtained that ranges from neutral OV(OCH3)3 all the way down to the quasi-terminal fragment ions VOH+ and VO+, respectively.

Gas-phase ion chemistry in organometallic systems

This review essentially deals with positive ion/molecule reactions occurring in gas-phase organometallic systems, and encompasses a period of time of approximately 7 years, going from 1997 to early 2004. Following the example of the excellent review by Eller & Schwarz (1991; Chem Rev 91:1121-1177), in the first part, results of reaction of naked ions are presented by grouping them according to the neutral substrate, while in the second part, ligated ions are grouped according to the different ligands. Whenever possible, comparison among similar studies is attempted, and general trends of reactivities are evidenced.

Carbon Disulfide Reactions with Atomic Transition-Metal and Main-Group Cations: Gas-Phase Room-Temperature Kinetics and Periodicities in Reactivity

The reactions of 46 atomic-metal cations with CS2 have been investigated at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and products were measured for the reactions of fourth-period atomic ions from K+ to Se+, of fifth-period atomic ions from Rb+ to Te+ (excluding Tc+), and of sixth-period atomic ions from Cs+ to Bi+. Primary reaction channels were observed leading to S-atom transfer, CS2 addition and, with Hg+, electron transfer. S-atom transfer appears to be thermodynamically controlled and occurs exclusively, and with unit efficiency, in the reactions with most early transition-metal cations (Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+) and with several main-group cations (As+, Sb+) and less efficiently with Se+, Re+ and Os+. Other ions, including most late transition and main-group metal cations, react with CS2 with measurable rates mostly through CS2 addition or not at all (K+, Rb+, Cs+). Traces of excited states (< 10%) were seen from an inspection of the observed product ions to be involved in the reactions with Mo+, Te+, Ba+ and Au+ and possibly Pt+ and Ir+. The primary products YS+, ZrS+, NbS+, HfS+, TaS+, WS+, ReS+ and OsS+ react further by S-atom transfer to form MS2(+), and TaS2(+) reacts further to form TaS3(+). CS2 addition occurs with the cations MCS2(+), MS+, MS2(+), CS2(+), and TaS3(+) to form M+(CS2)(n) (n < or = 4), MS+(CS2)(n) (n < or = 4), MS2(+)(CS2)(n) (n < or = 3), (CS2)2(+) and TaS3(+)(CS2). Up to four CS2 molecules add sequentially to bare metal cations and monosulfide cations, and three to disulfide cations. Equilibrium constant measurements are reported that provide some insight into the standard free energy change for CS2 ligation. Periodic variations in deltaG degrees are as expected from the variation in electrostatic attraction, which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CS2.

Guided ion-beam studies of the reactions of Con+ (n=2–20) with O2: Cobalt cluster-oxide and -dioxide bond energies

The kinetic-energy dependence for the reactions of Co(n)+ (n=2-20) with O2 is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co(m)+, Co(m)O+, and Co(m)O2+ (m < or = n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co(n)+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co(n)+-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters.

Exploring Two-State Reactivity Pathways in the Cycloaddition Reactions of Triplet Methylene

Spin forbidden 1,2-cycloadditions of triplet methylene to alkenes have been theoretically studied as an example of the two-state reactivity paradigm in organic chemistry. The cycloadditions of triplet methylene to ethylene and the (E)- and (Z)-2-butene isomers show spin inversion after the transition state and therefore with no effect on the reaction rate. A local analysis shows that while triplet methylene addition to alkenes leading to the formation of a biradical intermediate is driven by spin polarization, the ring closure step to yield cyclopropane is a pericyclic process. We have found that at the regions in the potential energy surface where the spin crossover is likely to occur, the spin potential in the direction of increasing spin multiplicity, mu(+)(s), tends to equalize the one in the direction of decreasing spin multiplicity, mu(-)(s). This equalization facilitates the spin transfer process driven by changes in the spin density of the system.

Mass spectrometry--not just a structural tool: the use of guided ion beam tandem mass spectrometry to determine thermochemistry

Guided ion beam tandem mass spectrometry has proved to be a robust tool for the measurement of thermodynamic information. Over the past twenty years, we have elucidated a number of factors necessary to make such thermochemistry accurate. Careful attention must be paid to the reduction of the raw data, ion intensities versus laboratory ion energies, to a more useful form, reaction cross sections versus relative kinetic energy. Analysis of the kinetic energy dependence of cross sections for endothermic reactions can then reveal thermodynamic data for both bimolecular and collision-induced dissociation (CID) processes. Such analyses need to include consideration of the explicit kinetic and internal energy distributions of the reactants, the effects of multiple collisions, the identity of the collision partner in CID processes, the kinetics of the reaction being studied, and competition between parallel reactions. This work provides examples illustrating the need to consider this multitude of effects along with details of the procedures developed in our group for handling each of them.

Noncovalent metal-ligand bond energies as studied by threshold collision-induced dissociation

This review focuses on noncovalent metal ion-ligand complexes and measurements of the bond energies of such species. The method utilized in this work is threshold collision-induced dissociation (CID), as achieved using a guided ion beam tandem mass spectrometer. Accurate determination of bond energies requires attention to many details of the experiments and data analysis. These details are discussed thoroughly and compared to other methods. A comprehensive listing of metal-ligand bond dissociation energies determined by threshold CID is provided. This list includes a variety of metals (alkalis, magnesium, aluminum, and first and second row transition metals), many different types of ligands, and variations in the number of ligands. The trends in these values are discussed, and we elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, different conformers and tautomers, steric interactions, solvation phenomena, and electronic effects such as hybridization and promotion.

Two-state reactivity as a new concept in organometallic chemistry

It is proposed that spin-crossing effects can dramatically affect reaction mechanisms, rate constants, branching ratios, and temperature behaviors of organometallic transformations. This phenomenon is termed two-state reactivity (TSR) and involves participation of spin inversion in the rate-determining step. While the present analysis is based on studies of transition metals under idealized conditions, several recent reports imply that TSR is by no means confined to the gas phase. In fact, participation of more than a single spin surface in the reaction pathways is proposed as a key feature in organometallic chemistry.