ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications. © 1998 John Wiley & Sons, Inc., Mass Spec Rev 17, 1–35, 1998

Fourier transform ion cyclotron resonance mass spectrometry: A primer

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Nanochemistry: Synthesis in diminishing dimensions

Electrospray ionization mass spectrometry has been used to study protein interactions driven by noncovalent forces The gentleness of the electrospray ionization process allows intact protein complexes to be directly detected by mass spectrometry Evidence from the growing body of literature suggests that the ESI-MS observations for these weakly bound systems reflect, to some extent, the nature of the interaction found in the condensed phase Stoichiometry of the complex can be easily obtained from the resulting mass spectrum because the molecular weight of the complex is directly measured For the study of protein interactions, ESI-MS is complementary to other biophysical methods, such as NMR and analytical ultracentrifugation However, mass spectrometry offers advantages in speed and sensitivity The experimental variables that play a role in the outcome of ESI-MS studies of noncovalently bound complexes are reviewed Several applications of ESI-MS are discussed, including protein interactions with metal ions and nucleic acids and subunit protein structures (quaternary structure)

Studying noncovalent protein complexes by electrospray ionization mass spectrometry

Investigation of reactions of metal ions and their clusters in the gas phase by laser-ionization fourier-transform mass spectrometry.

The photodissociation of a variety of gas-phase organometallic ions was investigated with Fourier transform mass spectrometry in order to obtain spectroscopic and thermodynamic information on these complexes. The results indicate that these ionic complexes absorb broadly and photodissociate readily in the ultraviolet and visible spectral regions, with cross sections for lambda/sub max/ ranging from 0.02 to 0.30 A/sup 2/. Because of this broad absorption, photodissociation thresholds are attributed to thermodynamic and not spectroscopic factors. Bond energies and heats of formation obtained by monitoring photodissociation onsets show good agreement with those obtained by other techniques. Interestingly, product ions generated by photodissociation are found to differ significantly in a number of instances from those produced by collision-induced dissociation. Finally, differentiation of two FeC/sub 4/H/sub 6//sup +/ and four NiC/sub 4/H/sub 8//sup +/ isomers is demonstrated by observing differences in cross sections, spectral band positions, and neutral losses.

Gas-phase photodissociation of organometallic ions. Bond energy and structure determinations

Selected topics in organometallic ion chemistry

The gas-phase chemistry of a number of bare transition metal-nitrene and -imido ion complexes, MNH/sup +/ are reported herein. Group 3, 4, and 5 atomic metal ions react with NH/sub 3/ at thermal energies to generate MNH/sup +/ via dehydrogenation. A reaction mechanism is proposed involving initial oxidative addition to an N-H bond, in analogy to mechanisms proposed for reactions of gaseous atomic metal ions with hydrocarbons. Cr/sup +/ reacts with NH/sub 3/ via slow condensation to form CrNH/sub 3//sup +/, as do all group 6-11 atomic metal ions investigated. However, excited-state Cr/sup +/ reacts with NH/sub 3/ via bond-insertion reactions to form CrNH/sub 2//sup +/ and CrNH/sup +/. An unidentified metastable electronic state of Cr/sup +/, produced by direct laser desorption of chromium foil, reacts with much higher efficiency than does kinetically excited Cr/sup +/. FeO/sup +/ reacts with NH/sub 3/ to generate FeNH/sup +/ with loss of H/sub 2/O. Thermochemical studies of VNH/sup +/ and FeNH/sup +/ involving ion-molecule reactions indicate values of D/degree/(V/sup +/-NH) = 101 /plus minus/ 7 kcal/mol and D/degree/(Fe/sup +/-NH) = 54 /plus minus/ 14 kcal/mol, the latter value in accord with D/degree/(Fe/sup +/-NH) = 61 /plus minus/ 5 kcal/mol obtained from photodissociation. Themore » high bond strength for VNH/sup +/ indicates multiple bonding, analogous to that in the isoelectronic VO/sup +/, while the weaker bond strength for FeNH/sup +/ indicates a single bond, analogous to that in the isoelectronic FeO/sup +/. Proton-transfer experiments indicate PA(VN) = 220 /plus minus/ 4 kcal/mol from which /delta/H/sub f/(VN) = 111 /plus minus/ 9 kcal/mol and D/degree/(V-N) = 125 /plus minus/ 9 kcal/mol are obtained. VNH/sup +/ is unreactive with ethene and benzene, but FeNH/sup +/ transfers NH to ethene and benzene through metathesis and homologation reactions. 41 references, 2 figures, 2 tables.« less

Gas-phase chemistry of transition metal-imido and -nitrene ion complexes. Oxidative addition of nitrogen-hydrogen bonds in ammonia and transfer of NH from a metal center to an alkene

The gas-phase reactions of Nb{sup +} with small alkanes and alkenes are reported Nb{sup +} sequentially activates C-H bonds in ethene six times to produce NbC{sub 2n}H{sub 2n}{sup +} (n = 1-6) The C{sub 2}H{sub 2} ligands in this reaction couple via migratory insertion of the acetylene units Ethane sequentially reacts with Nb{sup +} five times Ligand coupling is proposed for these reactions as well Comparison of NbC{sub 4}H{sub 6}{sup +} produced from isobutane, n-butane, and ethane indicates the presence of at least two isomeric forms NbC{sub 4}H{sub 4}{sup +} ions produced from ethene, cyclobutane, and 1-butene produced similar CID spectra, supporting the presence of one structure Excited Nb{sup +} produced by direct laser desorption reacts with methane to produce NbCH{sub 2}{sup +} and NbCH{sub 3}{sup +}, but collisional cooling with argon quenches the reaction Primary reactions indicate a strong preference for C-H insertion, and reaction products indicate 1,3-hydrogen migration when {beta}-hydrogens are not present The reactions with alkanes are compared to those with Ta{sup +}, which reacts with CH{sub 4} four times to produce TaC{sub 4}H{sub 8}{sup +} Ta{sup +} and Nb{sup +}, which react similarly, are by far the most reactive metal ions toward C-H bond activation reportedmore » to date CID spectra of the products form the Nb{sup +} reactions indicate that the extensive dehydrogenation reactions occur via ligand coupling and, for larger alkanes (C{sub 5} and C{sub 6}), dehydrocyclization For example, Nb{sup +} undergoes a novel dehydrocyclization reaction with pentane to generate Nb(Cp){sup +} (Cp = cyclopentadienyl) Nb{sup +} reacts with cyclopentane to generate Nb(Cp){sub 2}{sup +} as a second product 34 refs, 3 figs, 2 tabs« less

Ben S. Freiser

Papers

Investigation of reactions of metal ions and their clusters in the gas phase by laser-ionization fourier-transform mass spectrometry.

Gas-phase photodissociation of organometallic ions. Bond energy and structure determinations

Selected topics in organometallic ion chemistry

Gas-phase chemistry of transition metal-imido and -nitrene ion complexes. Oxidative addition of nitrogen-hydrogen bonds in ammonia and transfer of NH from a metal center to an alkene

Gas-phase reactions of niobium(1+) and tantalum(1+) with alkanes and alkenes. Carbon-hydrogen bond activation and ligand-coupling mechanisms