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Mass spectrometric experiments at extremely low ( 2, as well as some hydrated species. In general, the dipolar water ligand is f...

The reaction of FeS+ with methane is examined by guided ion beam mass spectrometry and density functional theory employing the B3LYP/6-311+G* level of theory. For the FeS+/CD4 system examined in the experiments, two major product ions, Fe+ and FeSD+, are observed along with minor channels leading to FeCD3+, FeSCD3+, and FeSCD+. All products are formed in endothermic processes. The measured thresholds for the formations of Fe+ and FeSD+ are compared with computational data as well as literature thermochemistry. In the theoretical approach, two competing reaction mechanisms for the formation of Fe+, concomitant with neutral methanethiol, are investigated and used to interpret the experimental data. The lowest-energy path involves a formal 1,2-addition of H3C−H across the Fe+−S bond to generate a CH3FeSH+ insertion intermediate. This bond activation step involves spin inversion from the sextet to the quartet surface en route to the products. The occurrence of the second conceivable pathway resulting in forma...

The iron–oxide cluster Fe 2 O 2 + is synthesized in the gas phase from a mixture of Fe(CO) 5 and O 2 , and its gas-phase reactivity is subsequently probed using sector-field and Fourier-transform mass spectrometers. The experimental findings are in accord with, but do not establish that Fe 2 O 2 + has a rhombic structure with two equivalently bound oxo-ligands. Although the reactivity studies conducted with Fe 2 O 2 + are by and large consistent with previous literature data and the few thermochemical data available for ligated iron clusters, a severe discrepancy exists for the Fe 2 O 2 + /C 2 H 4 couple. Although exclusive dehydrogenation to afford Fe 2 C 2 H 2 O 2 + has been reported in a previous work [Gehret and Irion, Chem. Eur. J. 2 (1996) 598], this product constitutes a very minor channel in the present study; instead, various C–H and C–C bond activation processes as well as O-atom transfer from Fe 2 O 2 + to ethene are observed. A possible explanation is that two isomers and/or states of Fe 2 O 2 + cation were probed in the different experiments, thereby highlighting an important structural ambiguity in reactivity studies of transition-metal clusters. Assuming that the most stable Fe 2 O 2 + species is monitored in the present experiments, the reactions observed in combination with complementary thermochemical information imply heats of formation of Δ f H 0 (Fe 2 O 2 − = −32 ± 12 kcal/mol, Δ f H 0 (Fe 2 O 2 ) = 22 ± 12 kcal/mol, and Δ f H 0 (Fe 2 O 2 + ) = 216 ± 9 kcal/mol for anionic, neutral, and cationic di-iron dioxide.

Sector-field mass spectrometry was used to probe the fragmentation patterns of the cationic silicon chlorides Si2Cln+ (n = 1—6). For almost all Si2Cln+ ions, Si—Si fragmentation predominates the Si—Cl bond cleavage both in the metastable ion and collisional activation mass spectra. Analysis of the fragmentation patterns indicates that the long-lived radical cation Si2Cl6·+ corresponds to a complex [SiCl2·SiCl4]·+ rather than the intact molecular ion of hexachlorodisilane. The behavior of Si2Cl5+ is consistent with the formation of the (trichlorosilyl)dichlorosilyl cation Cl3SiSICl2+. Structural aspects are also discussed for the other Si2Cln+ species. A semi-quantitative analysis of the fragmentation patterns in conjunction with the literature thermochemistry data was used to estimate some thermochemical properties of the Si2Cln+ cations.

Consecutive ion/molecule reactions of gold cation Au+ with (deuterated) benzene are examined by Fourier-transform mass spectrometry. Primary products upon reacting Au+ with C6H6 are due to charge transfer to afford C6H6+· concomitant with neutral gold as well as ligand association to yield the Au(C6H6)+ complex. Subsequent reactions of so-formed Au(C6H6)+ with benzene result in exclusive formation of the corresponding bisligated complex Au(C6H6)2+. In contrast to a previous report by Ho and Dunbar [Int. J. Mass Spectrom. 182/183 (1999) 175], occurrence of charge transfer from benzene to Au(C6H6)+ affording C6H6+· concomitant with neutral Au(C6H6) is rigorously excluded. Our results are supported by ab initio calculations which predict a strong interaction in the Au(C6H6)+ cation, whereas neutral Au(C6H6) is best described as a loose van der Waals complex; i.e. D0(Au+–C6H6) = 61.1 kcal/mol versus D0(Au–C6H6) = 2.0 kcal/mol. Interestingly, the reaction kinetics for the sequential additions of benzene to cationic gold show irregularities which point to a non-negligible role of excess energy in consecutive bimolecular associations at low pressures.

Charge stripping of the monocationic counterparts allows for the generation of the dicationic species LaO2+ and LaOH2+ by mass spectrometric means. Energy-resolved measurements establish vertical ionization energies of IEv(LaO+) = 16.0 ± 0.3 eV and IEv(LaOH+) = 10.8 ± 0.3 eV. These figures are in reasonable agreement with IEv(LaO+) = 15.62 eV and IEv(LaOH+) = 10.61 eV predicted by ab initio calculations using the CCSD(T) approach. Further, the calculated properties can be used to convert the experimental IEv values to adiabatic values, i.e., IEa(LaO+) = 15.2 ± 0.4 eV and IEa(LaOH+) = 10.8 ± 0.4 eV. Evaluation of the dication energetics in terms of Born−Haber cycles reveals that the diatomic LaO+ monocation has a proton affinity (PA) similar to that of methane. Accordingly, reactions of LaO+ with strong Bronstedt acids AH+ could provide a route for the generation of gaseous dications in cation/cation reactions. However, for the exothermic model reaction LaO+ + NeH+ → LaOH2+ + Ne, CCSD(T) calculations predi...

Neutral NCN is made in a mass spectrometer by charge stripping of NCN-., while neutral dicyanocarbene NCCCN can be formed by neutralization of either the corresponding anionic and cationic species, NCCCN-. and NCCCN+.. Theoretical calculations at the RCCSD(T)/aug-cc-pVTZ//B3LYP/6-31+G(d) level of theory indicate that the (3)Sigma (-)(g) State of NCCCN is 18 kcal mol(-1) more stable than the (1)A(1) state. While the majority of neutrals formed from either NCCCN-. or NCCCN+. correspond to NCCCN, a proportion of the neutral NCCCN molecules have sufficient excess energy to effect rearrangement, as evidenced by a loss of atomic carbon in the neutralization reionization (NR) spectra of either NCCCN+. and NCCCN-.. C-13 labeling studies indicate that loss of carbon occurs statistically following or accompanied by scrambling of all three carbon atoms. A theoretical study at the B3LYP/6-31+G(d)//B3LYP/6-31+G(d) level of theory indicates that C loss is a consequence of the rearrangement sequence NCCCN --> CNCCN --> CNCNC and that C scrambling occurs within singlet CNCCN via the intermediacy of a four-membered C-2v-symmetrical transition structure.

Based upon mass-spectrometric studies, the activation of ethylsilane by “naked” Fe+ ions is computationally investigated using the B3LYP approach together with a basis set of valence double-ζ quality. In particular, the mechanistic details for the experimentally observed losses of neutral H2, CH4, and SiH4, respectively, are investigated. According to the initial steps of bond activation, this requires the computation of four potential-energy surfaces, corresponding to insertion into the carbonsilicon (CSi), carbonhydrogen [C(1)H and C(2)H], and carboncarbon (CC) bonds; activation of the siliconhydrogen (SiH) bonds is ruled out on experimental grounds. Both quartet and sextet states of Fe+ are taken into account. The agreement between theory and experiments is satisfactory because all products observed in the experiments are also calculated to proceed exothermically on the reaction surface, and the experimentally preferred exit channel is also predicted as the most favored reaction channel by the theoretical approach.

The isomeric dicoordinated borinium ions CH3O–B–H+ and CH3–B–OH+ are generated upon electron ionization of trimethylborate and methyl boronic acid, respectively. The connectivity of the ions is confirmed by collision-induced dissociation experiments on magnetic deflection type tandem mass spectrometers. Neutralization–reionization experiments on these structurally characterized ions indicate that the neutral radicals CH3O–B–H· and CH3–B–OH· are viable species in the gas phase. Calculations at the G2 level of theory were performed to obtain thermochemical data on the title isomers and their main dissociation products. The calculations also provide a rationale for the moderate yield of the neutrals generated in the experiments: the vertical electron transfer processes for both systems are associated with particularly unfavourable Franck-Condon factors.

The ions [Ge,C,H 2 ] + and [Ge,C,H 3 ] + , synthesised using both chemical ionisation (CI) and electron ionisation (EI), have been examined using collisional activation (CA) and neutralisation–reionisation (NR) mass spectrometry. For any method of synthesis, the connectivities Ge-CH n + (n = 2–3) are prevalent. NR experiments establish that transitions to the neutral surface from the cation surface, and vice versa, are survived by a large fraction of [Ge,C,H 2 ] + , and by a much smaller fraction of [Ge,C,H 3 ] + parent ions. The modeling of selected points on the surfaces of [Ge,C,H 2 ] +/0 reveals there is little prospect of synthesising neutral or cationic germaacetylene or germavinylidene in sufficient amounts for spectroscopic detection. A general result for all ions and neutrals is that isomer stabilities increase with an increasing number of C–H bonds, in good agreement with available thermochemical data for Ge–H versus C–H bond strengths and previous experimental results for the analogous Si systems.

The potential-energy surface for the activation of ethylsilane by “naked” Co + ions is investigated by using density functional theory at the B3LYP level of theory and a valence double zeta basis. In particular, the pathways for the expulsion of the neutral molecules SiH 4 , H 2 , and CH 4 are examined, which are also observed in the mass spectrometric experiments. Five conceivable oxidative additions of the substrate to Co + are considered in the computational study, i.e. the primary insertions of the metal ion into the C–Si, C(1)–H, C(2)–H, C–C, and Si–H bonds. Comparison with experimental data shows qualitative agreement in that all experimentally observed products are predicted by theory to be formed in exothermic reactions with thermally surmountable barriers. Quantitatively, the ratio of the losses of SiH 4 , H 2 , and CH 4 is less well reproduced by the applied theoretical approach; however, the energetic differences between the rate determining transition states are too small to be resolved within the accuracy of most, if not all, nowadays theoretical methods applicable to transition-metal compounds. Keywords: B3LYP; C–Si bond activation; H–Si bond activation; Ethylsilane; Co + cation

Mass spectrometric methods are used to examine the H/D equilibrium isotope effects (EIEs) of cationic M(C2X4)+ complexes (X = H, D) of the coinage metals M = Cu, Ag, and Au. Different experimental ...

The mechanism of the degenerate 16O/18O exchange in the reactions of FeO+ and FeOH+ with water is examined by density functional theory. Based on previous experimental work (Chem. Eur. J. 1999, 5, 1176), two possible reaction pathways are investigated for both systems. The first mechanism consists of one (for FeOH+ + H20) or two (for FeO+ + H20) 1,3-hydrogen migrations from one oxygen atom to the other; the iron atom is not directly involved in these OH bond activations. The second route comprises a series of two (for FeOH+ + H20) or four (for FeO+ + H20) 1,2-hydrogen migration steps which involve the intermediate formations of metal-hydrogen bonds. Both mechanisms are evaluated under consideration of the respective low- and high spin potential-energy surfaces. The computational results show a clear preference for the 1,3-routes occurring on the respective high-spin surfaces bypassing the intermediacy of high-valent iron compounds having FeH bonds.

The gas-phase reactivities of the transition-metal sulfides ScS+ and TiS+ are investigated with guided-ion beam (GIB) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. In this work, we study the reactions of bare Sc+ and Ti+ ions with sulfur-transfer reagents, COS and CS2, and of the metal sulfides, ScS+ and TiS+, with oxygen-transfer substrates and with Xe to examine their collision-induced dissociation. The GIB experiments lead to several estimates for D0(M+−S). Further, the reaction MS+ + H2O → MO+ + H2S and its reverse are studied with GIB and FTICR. The equilibrium constants Keq derived from these measurements provide the most accurate values for D0(M+−S). Overall assessment of the results of ion−molecule reactions, collision-induced dissociations, and equilibrium measurements yields the 0 K bond dissociation energies D0(Sc+−S) = 4.97 ± 0.05 eV, D0(Ti+−S) = 4.74 ± 0.07 eV, D0(Sc+−CS) = 1.38 ± 0.08 eV, D0(Ti+−CS) = 1.60 ± 0.06 eV, and ΔfH0(TiOS+) = 9.04 ± 0.18 eV.

The energetics of selected dicationic iron oxides and hydroxides FeOmHn2+ (m = 1, 2; n ≤ 4) are probed by charge-stripping mass spectrometry in conjunction with ab initio calculations employing the...

The interaction of bare iron mono- and dications with hydrogen peroxide in the gas phase is studied by ab initio calculations employing the B3LYP/6-311+G* level of theory. For the monocation, the quartet and sextet coordination complexes Fe(H2O2) are high-energy isomers that easily interconvert to the more stable iron dihydroxide monocation Fe(OH) and hydrated iron oxide (H2O)FeO+ (quartet) or dissociate into FeOH++OH. (sextet). On the dication surface, however, the order of stabilities is reversed in that Fe(H2O2)2+ (quintet) corresponds to the most stable doubly charged species, while the formal FeIV compounds Fe(OH) and (H2O)FeO2+ are higher in energy.

The cation [Si,C,O]+ has been generated by 1) the electron ionisation (EI) of tetramethoxysilane and 2) chemical ionisation (CI) of a mixture of silane and carbon monoxide. Collisional activation (CA) experiments performed for mass-selected [Si,C,O]+, generated by using both methods, indicate that the structure is not inserted OSiC+; however, a definitive structural assignment as Si(+)-CO, Si(+)-OC or some cyclic variant is impossible based on these results alone. Neutralisation-reionisation (+NR+) experiments for EI-generated [Si,C,O]+ reveal a small peak corresponding to SiC+, but no detectable SiO+ signal, and thus establishes the existence of the Si(+)-CO isomer. CCSD(T)@B3LYP calculations employing a triple-zeta basis set have been used to explore the doublet and quartet potential-energy surfaces of the cation, as well as some important neutral states. The results suggest that both Si(+)-CO and Si(+)-OC isomers are feasible; however, the global minimum is 2 pi SiCO+. Isomeric 2 pi SiOC+ is 12.1 kcal mol-1 less stable than 2 pi SiCO+, and all quartet isomers are much higher in energy. The corresponding neutrals Si-CO and Si-OC are also feasible, but the lowest energy Si-OC isomer (3A") is bound by only 1.5 kcal mol-1. We attribute most, if not all, of the recovery signal in the +NR+ experiment to SiCO+ survivor ions. The nature of the bonding in the lowest energy isomers of Si(+)-(CO,OC) is interpreted with the aid of natural bond order analyses, and the ground state bonding of SiCO+ is discussed in relation to classical analogues such as metal carbonyls and ketenes.

The cation\[Si,C,O](+) has been generated by 1) the electron ionisation (EI) of tetramethoxysilane and 2) chemical ionisation (CI) of a mixture of silane and carbon monoxide. Collisional activation (CA) experiments performed for mass-selected \[Si,C,O](+), generated by using both methods, indicate that the structure is not inserted OSiC+; however, a definitive structural assignment as Si+-CO, Si+-OC or some cyclic variant is impossible based on these results alone. Neutralisation-reionisation (+NR+) experiments for EI-generated \[Si,C,O](+) reveal a small peak corresponding to SiC+, but no detectable SiO+ signal, and thus establishes the existence of the Si+-CO isomer. CCSD(T)//B3LYP calculations employing a triple-zeta basis set have been used to explore the doublet and quartet potential-energy surfaces of the cation, as well as some important neutral states The results suggest that both Si+-CO and Si+ - OC isomers are feasible; however, the global minimum is (2)Pi SiCO+. Isomeric (2)Pi SiOC+ is 12.1 kcal mol(-1) less stable than (2)Pi SiCO+, and all quartet isomers are much higher in energy. The corresponding neutrals Si-CO and Si-OC are also feasible, but the lowest energy Si - OC isomer ((3)A") is bound by only 1.5 kcal mol(-1). We attribute most, if nor all, of the recovery signal in the +NR' experiment to SiCO+ survivor ions. The nature of the bonding in the lowest energy isomers of Si+ -(CO,OC) is interpreted with the aid of natural bond order analyses, and the ground stale bonding of SiCO+ is discussed in relation to classical analogues such as metal carbonyls and ketenes.

The dicoordinated borinium ion, dihydroxyborinium, B(OH) 2 + is generated from methyl boronic acid CH 3 B(OH) 2 by dissociative electron ionization and its connectivity confirmed by collisional activation. Neutralization–reionization (NR) experiments on this ion indicate that the neutral B(OH) 2 radical is a viable species in the gas phase. Both vertical neutralization of B(OH) 2 + and reionization of B(OH) 2 in the NR experiment are, however, associated with particularly unfavorable Franck-Condon factors. The differences in adiabatic and vertical electron transfer behavior can be traced back to a particular π stabilization of the cationic species compared to the sp 2 -type neutral radical. Thermochemical data on several neutral and cationic boron compounds are presented based on calculations performed at the G2 level of theory.

The gas-phase reactions of phenyl halides C6H5X (X = F−I) with FeO+ are examined using tandem mass spectrometry. The predominant dissociations involve losses of CO (X = F−I), HX (X = F, Cl), and X (X = I). For chlorobenzene 1, mechanistic insight is obtained from detailed labeling experiments, MS/MS studies, and comparison with structural isomers generated via independent routes. The experiments demonstrate that ring hydroxylation to a phenol, a reaction typical for FeO+ and unsubstituted benzene, does not occur for 1/FeO+. Instead, initial coordination of FeO+ at the functional group (“docking”) predominates, thereby enforcing activation of the adjacent ipso and ortho positions, while meta and para positions do not participate.

The gas-phase oxidations of phenol, anisol, thiophenol, and thioanisol by 'bare' FeO+ are examined by using Fourier transform-ion cyclotron resonance (FT-ICR) and tandem mass-spectrometry. Reaction mechanisms are derived on the basis of isotope-labeling experiments, MS/MS studies, and comparison with structural isomers, that is ions formed by independent routes. The chemistry of all substrates is determined by the functional groups, whereas reactions typical of unsubstituted benzene with FeO+ are suppressed. For phenol and thiophenol, four-membered metallacycles are obtained concomitant with a regioselective loss of water, which involves the O atom from the FeO+ entity and hydrogen atoms originating from the functional group and from the ortho position of the ring. C-H bond cleavage of the methoxy group (kH/kD = 2.0) is rate-contributing for the degradation of metastable anisol/FeO+, which is featured by highly regioselective losses of H2O, HCO, H2CO, and [C,H2,O2]. In the oxidation of thioanisol, two different C-H bond activation mechanisms are operating, resulting in the elimination of [Fe,H,O,S] concomitant with the formation of the benzyl cation (kH/kD = 4.7), and loss of water (kH/kD = 2.5). The reactions of independently generated, formal S- and C-oxidation intermediates of thioanisol indicate the occurrence of extensive structural isomerizations prior to dissociation. For anisol and thioanisol, analogies and differences between oxidation reactions catalyzed by the enzyme cytochrome P-450 in the condensed phase and those observed for the gas-phase model FeO+ are discussed.

[Si,C,N,O]+ and [Si,C,N,S]+ cations formed upon electron ionization of trimethylsilyl isocyanate and isothiocyanate, respectively, are probed by tandem mass spectrometry. In the case of [Si,C,N,O]+, the experimental data suggest that the isocyanato group remains intact upon ionization such that silicon isocyanate SiNCO+ is formed. Neutralization-reionization mass spectrometry indicates that also neutral SiNCO is a viable molecule in the gas phase. Instead, partial isomerization to SiSCN+ is indicated for the sulfur analog. Because of this structural dichotomy, the nature of the [Si,C,N,S]+ species giving rise to the recovery signal in the neutralization-reionization experiments remains uncertain.

Concepts for the activation of methane are derived from ion-molecule reactions of mass-selected, ground-state transition-metal cations M+. Elementary steps of industrially important processes are uncovered (e.g., oxygenation of methane or its coupling with carbon dioxide). In addition, the implications of the electronic structures of M(CH2)+ complexes for their reactions with nucleophiles are discussed, and the crucial role of contemporary quantum mechanical calculations in the elucidation of mechanistic details is emphasized.

The Pt+-mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C−N bond formation is catalyzed efficiently by Pt+. Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt+ dehydrogenates CH4 to yield PtCH2+; in contrast, dehydrogenation of ammonia by Pt+ is endothermic and does not occur experimentally. Starting from PtCH2+ and NH3, C−N bond formation, which constitutes the crucial step in making HCN from CH4 and NH3, is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol-1 and yields neutral PtH and CH2NH2+. The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH2+ (ΔrH = −36 kcal mol-1); dehydrogenation of ...

The potential-energy surface for C−H bond activation of norbornane by `bare' FeO+ is examined at the B3LYP/6-31G** level of theory. The free reactants combine to form norbornane/FeO+ ion-dipole clusters in which the FeO+ unit can bind at either the exo or endo face of norbornane. The transition structures for insertion of FeO+ into the exo and endo C−H bonds are located at least 9 kcal⋅mol−1 below the entrance channel, thus accounting for the observed unit efficiency of the C−H bond activation reported in previous gas-phase ion-cyclotron resonance experiments (Helv. Chim. Acta1995, 78, 1013). Interesting features of the reaction profiles are crossovers of the high-spin sextet (S=5/2) and low-spin quartet (S=3/2) states en route to the transition structures (TS); this type of behavior has been termed two-state reactivity (Helv. Chim. Acta1995, 78, 1393). The branchings between the endo and exo pathways are simulated by Rice-Ramsperger-Kassel-Marcus (RRKM) theory with the calculated harmonic frequencies. Additionally, hydrogen/deuterium kinetic isotope effects are computed using RRKM theory and compared with the experimental data. The simulated KIEs differ for high-spin and low-spin TSs, suggesting that isotope effects can be used as sensitive probes for diagnosing spin-crossover mechanisms.

Charge reversal (+CR−) of cations to anions can be used to structurally differentiate isomeric C6H5+ and C6H6+· hydrocarbon ions by means of tandem mass spectrometry. In view of the manifold of possible isomers, only a few prototype precursors are examined. Thus, charge inversion demonstrates that electron ionization of 2,4-hexadiyne yields an intact molecular ion, whereas the charge inversion spectra of C6H6+· obtained from benzene, 1,5-hexadiyne, and fulvene are identical within experimental error. Similarly, the +CR− spectrum of the C6H5+ cation generated by dissociative ionization of 2,4-hexadiyne is significantly different from the +CR− spectrum of C6H5+ obtained from iodobenzene, suggesting the formation of a 2,4-hexadiynyl cation from the former precursor. Although charge inversion of cations to anions has a low efficiency and requires large precursor ion fluxes, the particular value of this method is that the spectra may not just differ in fragment ion intensities, but these differences can directly be related to the underlying ion structures.

Electron transfer in high-energy collision experiments is used to probe the redox chemistry of the iron chlorides FeCl m n (m = 1–3; n = −1, 0, +1, and +2). These experiments comprise charge inversion of FeCl m − anions ( m = 2–4) to cations, charge inversion of FeCl m + cations ( m = 1–3) to anions, charge stripping of FeCl m + monocations ( m = 1–3) to dications, and charge exchange of FeCl m 2+ dications ( m = 1, 2) to monocations. Ab initio calculations at the B3LYP/6-311+G∗ level of theory are used to evaluate the differences between adiabatic and vertical electron transfers; the accuracy of the calculated absolute energies for the associated electron-transfer processes predicted at this level of theory is doubted, however. The experimentally determined redox properties of the iron chlorides are in fair agreement with literature thermochemistry; new data derived in this work are: IE(FeCl 3 ) = 10.9 eV, IE(FeCl + ) = 15.9 ± 0.4 eV, IE(FeCl 2 + ) = 17.6 ± 0.7 eV, and IE(FeCl 3 + ) = 16.0 ± 0.4 eV. In addition, evidence for the existence of the chlorine complexes Fe(Cl 2 ) + and Fe(Cl 2 ) 2+ is presented. According to the experimental data, diatomic FeCl 2+ is a thermochemically stable dication, whereas FeCl 2 2+ and FeCl 3 2+ are metastable with respect to the dissociations into FeCl ( m −1) + + Cl + and FeCl ( m −2) + + Cl 2 + ( m = 2, 3). Except for the dications, the dissociation behavior of the FeCl m n species ( m = 1–3; n = −1, 0, +1) is dominated by sequential losses of chlorine atoms rather than expulsion of molecular chlorine.

Experimental and computational studies of small, multiply charged ions are of topical interest in chemistry and physics. Moreover, the increasing use of multiply charged ions in the analysis of environmental and biological samples calls for an improvement of the understanding of the fundamental properties of multiply charged ions at a molecular level. Mass spectrometric techniques in conjunction with advanced theoretical studies provide detailed insight into these aspects. This feature article describes the present state of research on small dications, dianions, and trications. Emphasis is given to thermochemically stable, multiply charged ions, with small trications in particular.

Neutralization reionization (NR) mass spectra of mono- and bis-ligated iron cations Fe(L) n + ( n =1, 2) are reported for the simple ligands L=H 2 O, CO, C 2 H 4 , C 4 H 6 , C 6 H 6 , and NH 3 . Most of these iron complexes do not survive the NR process, and dissociation of the neutrals into Fe + n L prevails. Interestingly, those complexes containing CO ligands exhibit much more pronounced recovery signals in the NR spectra. Electronic arguments imply that the carbonyl ligand favors the generation of low-spin complexes of neutral iron which are less prone to dissociate and are not accessed with the other ligands.

The reaction of FeS+ with D2 is examined by guided-ion beam mass spectrometry. Three products, Fe+, FeD+, and FeSD+, are formed in endothermic processes, and thresholds for these reactions are dete...

Structural and thermochemical aspects of the FeS(2)(+) cation are examined by different mass spectrometric methods and ab initio calculations using density functional theory. Accurate threshold measurements provide thermochemical data for FeS(+), FeS(2)(+), and FeCS(+), i.e., D(0)(Fe(+)-S) = 3.06 +/- 0.06 eV, D(0)(SFe(+)-S) = 3.59 +/- 0.12 eV, D(0)(Fe(+)-S(2)) = 2.31 +/- 0.12 eV, and D(0)(Fe(+)-CS) = 2.40 +/- 0.12 eV. Fortunate circumstances allow a refinement of the data for FeS(+) by means of ion/molecule equilibria, and the resulting D(0)(Fe(+)-S) = 3.08 +/- 0.04 eV is among the most precisely known binding energies of transition-metal compounds. The present results agree with previous experimental findings and also corroborate the computed data for FeS(+) and FeS(2)(+). Ab initio calculations predict a sextet ground state ((6)A(1)) for FeS(2)(+) with a cyclic structure. The presence of S-S and Fe-S bonds accounts for the fact that not only reactions involving the disulfur unit but also sulfur-atom transfer can occur. In contrast, the FeS(2)(-) anion is an acyclic iron disulfide. In the gas phase, neutral FeS(2) may adopt either acyclic or cyclic structures, which are rather close in energy according to the calculations.

The gas-phase oxidations of aniline, N-methylaniline, and N,N-dimethylaniline by FeO+ cation are examined by using mass spectrometric techniques. Although bare FeO+ is capable of hydroxylating aromatic C—H bonds, the fate of the oxidation of arylamines is determined by docking of the FeO+ unit at nitrogen. The major reactions of the metastable aniline/FeO+ complex are losses of molecular hydrogen, ammonia, and water, all involving at least one N-H proton. N-alkylation results in a complete shift of the course of the reaction. The unimolecular processes observed can be regarded as initial steps of an oxidative dealkylation of the amines mediated by FeO+. More detailed mechanistic insight is obtained by examining the C—H(D) bond activation of N-methyl-N-([D3]-methyl)aniline by bare and ligated FeO+ species. The gas-phase reactions of FeO+ with methylanilines show some similarities to the enzymatic dealkylation of amines by cytochrome P-450. The kinetic isotope effects observed experimentally suggest an electron transfer mechanism for the gas-phase reaction.Key words: mass spectrometry, gas-phase chemistry, iron, dealkylation, N,N-dimethylaniline.

The composite kinetic energy release (KER) associated with the decomposition of metastable, protonated fluorobenzene is examined in detail by means of mass spectrometry and analyzed theoretically using a combination of RRKM theory and the statistical adiabatic channel model (SACM). A model for the KER is presented, and contributions of rotational, vibrational, and zero-point energies are calculated. The predictions are in reasonable agreement with the experimentally measured data for two of three components of the KER. With respect to the third, the results lead to a reconsideration of earlier mechanistic models and indicate that ring-opening may occur during the unimolecular decomposition of protonated fluorobenzene. This hypothesis is supported by the ion/molecule reactions of C6H5+ cations formed by dissociative protonation of fluorobenzene with strong Bronsted acids.

The kinetic-energy dependence of the V++CS2 reaction is examined using guided ion-beam mass spectrometry. Several different ion sources are used to systematically vary the V+ electronic state distributions and elucidate the reactivities of both the ground and excited state V+ cation. The cross section for VS+ formation from ground state V+(5D) exhibits two endothermic features corresponding to the formation of ground state VS+(3Σ−) and excited state VS+(5Π). The thresholds for these two processes are in good agreement with theoretically determined excitation energies. The cross section for spin-forbidden formation of ground state VS+(3Σ−) exhibits an unusual variation with kinetic energy that is attributed to the energy dependence of the surface-crossing probability. From the thresholds associated with the formation of VS+ and V(CS)+, D0(V+–S)=3.72±0.09 eV and D0(V+–CS)=1.70±0.08 eV are derived. Further, circumstantial evidence for formation of a high-energy isomer of V(CS)+ is obtained.

The reactions of Pt{sup +} and PtCH{sub 2}{sup +} with the nucleophiles H{sub 2}O, PH{sub 3}, H{sub 2}S, HCl, CH{sub 3}NH{sub 2}, and CH{sub 3}OH are studied by Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In the reactions of PtCH{sub 2}{sup +}, carbon-heteroatom bond formation can be accomplished for all substrates except CH{sub 3}OH and HCl. The reaction of PtCH{sub 2}{sup +} with two molecules of water yields Pt(CO)(H{sub 2}O){sup +} and constitutes a gas-phase model for the platinum-mediated generation of water gas according to CH{sub 4} + H{sub 2}O {r_arrow} CO + 3H{sub 2}. In the reactions with PH{sub 3} and H{sub 2}S, carbon-phosphorus and carbon-sulfur bond formation to PtCPH{sup +} and PtCS{sup +} competes with demethanation and dehydrogenation of the substrates to yield PtS{sub n}{sup +} (n = 1--4) and PtP{sub n}H{sub m}{sup +} (n = 1--6; m = 0--3) compounds, respectively. For organic nucleophiles such as CH{sub 3}NH{sub 2} and CH{sub 3}OH, C-N and C-O coupling is much less efficient than platinum-mediated C-H bond activation of the substrates.

Newmechanisticinsight into basic reactions of FeO+ and FeOH+ is obtained by studying the transformations of these ions with the element hydrides CH4, NH3, H2O, SiH4, PH3, H2S, and HCl in a Fourier transform–ion cyclotron resonance (FT–ICR) mass spectrometer. The reaction channels can be classified into four different types: H. abstraction (a), dehydration (b), O-atom transfer (c), and dehydrogenation (d), as depicted.

Although carbon dioxide is well known to have a negative electron affinity, the CO2− · anion radical can be generated in several types of mass spectrometric experiments, such as collisional activation of carboxylate ions or double electron transfer to CO2+ ·. In particular, it is shown that vibrational excitation of the precursor cations increases the yield of the bound anion radicals in charge inversion experiments. Combined application of experimental and theoretical means indicates that for bent geometries, the 2A1 state of the carbon dioxide anion radical is stable against electron detachment in the μs timescale of the experiments. In a chemical sense, the CO2− · anion radical can be regarded as an activated carbon dioxide unit in which the C–O bonds are weakened and the carbon center exhibits distinct radical character. Thus, CO2− · constitutes a new type of distonic anion in which the charge and the unpaired electron are located in different symmetry planes.

Die Neutralisierung von [C,H,Si,X]-Radikalkationen (X=F, Cl) liefert den ersten experimentellen Beleg fur die Existenz der neutralen Siline HC≡SiF und HC≡SiCl, die nach Ab-initio-MO-Rechnungen nichtlineare Strukturen aufweisen (siehe Bild).

Charge-stripping-Massenspektrometrie und Ab-initio-Rechnungen ergaben, das UF3+ das erste zweiatomige, dreifach positiv geladene Ion ist, das gegenuber Coulomb-Explosion in die Fragmente U2++F+ thermochemisch stabil ist. Im Bild ist das zugehorige Energiediagramm gezeigt.

FeNH+ is chosen as a model system to probe the transition-metal-mediated transfer of imine groups in the gas phase by mass-spectrometric means. Ab initio calculations at the MR-ACPF level predict FeNH+ to have a linear sextet ground state (6Σ+); a bent quartet state (4A′) and a linear doublet state (2Δ) are higher in energy by 0.14 eV and 0.51 eV, respectively. The bond-dissociation energy is determined to D(Fe+−NH)=69±2 kcal mol−1 using ion-molecule reactions. Charge-stripping experiments combined with ab initio calculations yield an ionization energy of IE(FeNH+)=15.7±0.5 eV. The chemical behavior of FeNH+ towards oxygen, water, hydrogen, aliphatic hydrocarbons, benzene, and toluene reveals an intrinsically high reactivity of FeNH+. Because a transfer of the 〈NH〉 fragment to the substrate is feasible in most cases, attractive amination reactions like methanemethylamine, benzeneaniline, or toluenebenzylidenamine can be afforded by FeNH+.

The reactions of V+ (5D) with CS2 and COS and the reactions of VS+ with Xe, CO, COS, CO2, and D2 are studied as a function of translational energy in a guided-ion-beam (GIB) mass spectrometer. From these experiments, D0(V+−S) = 3.78 ± 0.10 eV, D0(V+−CS) = 1.70 ± 0.08 eV, and D0(V+−SD) = 2.57 ± 0.15 eV are derived. Verification of D0(V+−S) is achieved by probing reactions of V+ and VS+ in a Fourier transform ion cyclotron resonance mass spectrometer. The good agreement between the thermochemistry obtained in the V+/CS2 system and that from the other systems studied shows that the formally spin-forbidden formation of ground-state VS+ (3Σ-) from V+ (5D) and CS2 has no activation barrier in excess of the reaction endothermicity. At higher energies, the spin-allowed formation of VS+ (5Π) competes efficiently, giving rise to a composite shape of the VS+ cross section. The adiabatic and vertical splittings between the 3Σ- and 5Π states of VS+ are calculated as 1.37 and 1.87 eV at the MR-ACPF level of theory. The...

Three different radical anions of the empirical formula C5H2 have been generated by negative ion chemical ionization mass spectrometry in the gas phase. The isomers C4CH2•-, C2CHC2H•-, and HC5H•- have been synthesized by unequivocal routes and their connectivities confirmed by deuterium labeling, charge reversal, and neutralization reionization experiments. The results also provide evidence for the existence of neutrals C4CH2, C2CHC2H, and HC5H as stable species; this is the first reported observation of C2CHC2H. Ab initio calculations confirm these structures to be minima on the anion and neutral potential energy surfaces.