The barrier for the radical isomerization CH3Oo --> oCH2OH is calculated by CBS-QB3 to be 29.7 kcal mol-1 and lies higher (by 5.7 kcal mol-1) than the dissociation limit CH2O+Ho. Hence, CH3Oo does not isomerize to the more stable oCH2OH on its own. However, this barrier is reduced to 15.8 kcal mol-1 when the CH3Oo radical is coordinated with protonated methanol (CH3-Oo...H-O(H)-CH3+) and the CH3Oo -->oCH2OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation oCH2-O(H)...H-O(H)-CH3+ can be viewed as an acid catalyzed rearrangement. The ion CH3-Oo...H-O(H)-CH3+ represents the most stable form of the methanol dimer radical cation. The ion oCH2-O(H)...H-O(H)-CH3+ can fragment directly to CH3OH2+ + oCH2OH or it can rearrange further to produce the hydrogen-bridged radical cation oCH2-O+(CH3)-H...OH2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or tho CH2=O...H+...OH2+CH3o via an SN2 type reaction. Alternatively, oCH2-O+(CH3)-H...OH2 may undergo proton-transport catalysis to produce the complex ion CH3-O-CH3o+...OH2 which then dissociates. Our calculations confirm for the most part recent experimental findings on the methanol dimer radical cation [Y.-P. Tu, J.L. Holmes, J. Am. Chem. Soc. 112 (2000) 3695] but they also provide a different mechanism for the key isomerization reaction observed in that study.
Four hyperbranched synthetic polyesteramides were synthesized by the polycondensation of the trifunctional diisopropanolamine (D) and difunctional anhydrides (X) of succinic acid, glutaric acid, 1,2-cyclohexane dicarboxylic acid, and phthalic acid. The polymers were analyzed with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The most intense oligomer series observed was XnDn+1 containing diisopropanolamine end groups as expected from the polycondensation conditions. A series of oligomers XnDn+1H2O is observed as well, which can have its origin in the polymerization process or alternatively could result from in-source fragmentation of XnDn+1. Breakdown diagrams of the protonated parent ions X3D4 and additional MSn (n = 1, 2, 3) measurements gave insight in the fragmentation behavior of the polymers. Three main fragmentation pathways have been observed for all polymers of which the loss of H2O to oxazolonium ions has the lowest onset energy followed by the rearrangement of the amide and ester bonds also leading to oxazolonium ions. The loss of a second H2O to allylic or morpholine end groups has highest onset energy. MS3 experiments demonstrated that the presence of a series of oligomers XnDn+1H2O can be attributed to the polymerization process. Most probably an allylic end group has formed from one of the alcohol end groups. The formation of allylic end groups partly terminates the polymerization reaction and results in a change of the composition of the molecular weight distribution and decrease of the number average molecular weight.
Detection of the blister agent HD [bis(2-chloroethyl)sulfide] or distilled mustard directly on the surface of soil particles using ion trap secondary ion mass spectrometry in the static mode is demonstrated. HD by its very nature is adsorptive; this attribute makes detection of surface adsorbed HD by gas-phase approaches difficult, but renders the compound amenable to surface detection. Two different ion trap (IT) mass spectrometers, modified to perform secondary ionization mass spectrometry using a ReO4- primary ion beam, were employed in the present study. Sputtered ions were trapped in the gas phase in the IT, where they could be scanned out (MS1), or isolated and fragmented (MS2). The intact HD molecular ion was not observed, however an abundant ion corresponding to [HD - Cl]+ was formed, as were lower mass fragment ions, and ions derived from the chemical background. Ab initio calculations were used to propose structures of the fragment ions. At 0.5 monolayers surface coverage, [HD - Cl]+ and lower mass HD fragment ions were significantly more abundant than the background. At lower concentrations, however, the HD secondary ion signal became masked by the background. Sensitivity and selectivity were significantly improved in the MS2 mode of operation. MS2 of [HD more » - Cl]+ resulted in production of analytically diagnostic C2H4SH+ and other S- and Cl-bearing fragment ions. HD was detected at 0.07 monolayers using the MS2 approach, which corresponds to 108 ppm on a mass/mass basis. « less
Experimental and computational methods were applied to study the double ionization of the molecules CH 3 Cl, CH 2 Cl 2 , and CHCl 3 . Double-charge-transfer spectroscopy was applied to measure the double-ionization energies of the molecules. The translational energies of the negative ions generated when fast-moving singly charged positive projectile ions acquire two electrons in collisions with the molecules provides information on the states of the dications populated. The use of H + , OH + , and F + projectile ions allowed transitions to singlet and triplet electronic states of the dications to be studied. Double-ionization energies to those states were calculated using the second order algebraic diagrammatic construction Green’s function method. The calculated and measured values for CH 3 Cl were found to be in good agreement. The computed data predicted that the density of states for CH 2 Cl 2 2+ and CHCl 3 2+ were much higher than for CH 3 Cl 2+ . On grouping the calculated double-ionization energies to close-lying states together, however, good agreement with the measured values was evident. The combined experimental and computational study reported here thus provides a detailed understanding of the double ionization of the three molecules to singlet and triplet electronic states of their dications.
Gaseous oxygen and nitrogen bases, both singly and as binary mixtures, have been introduced into ion mobility spectrometers to study the appearance of protonated molecules, and proton-bound dimers and trimers. At ambient temperature it was possible to simultaneously observe, following the introduction of molecule A, comparable intensities of peaks ascribable to the reactant ion (H2O)nH+, the protonated molecule AH+ and AH+ · H2O, and the symmetrical proton bound dimer A2H+. Mass spectral identification confirmed the identifications and also showed that the majority of the protonated molecules were hydrated and that the proton-bound dimers were hydrated to a much lesser extent. No significant peaks ascribable to proton-bound trimers were obtained no matter how high the sample concentration. Binary mixtures containing molecules A and B, in some cases gave not only the peaks unique to the individual compounds but also peaks due to asymmetrical proton bound dimers AHB+. Such ions were always present in the spectra of mixtures of oxygen bases but were not observed for several mixtures of oxygen and nitrogen bases. The dimers, which were not observable, notable for their low hydrogen bond strengths, must have decomposed in their passage from the ion source to the detector, i.e. in a time less than ∼5 ms. When the temperature was lowered to −20 °C, trimers, both homogeneous and mixed, were observed with mixtures of alcohols. The importance of hydrogen bond energy, and hence operating temperature, in determining the degree of solvation of the ions that will be observed in an ion mobility spectrometer is stressed. The possibility is discussed that a displacement reaction involving ambient water plays a role in the dissociation.
The results of a combined experimental and theoretical study of the double ionization of sulphur hexafluoride are reported. Double-charge-transfer (DCT) spectroscopy was used in the experimental investigation. Double-electron-capture (DEC) reactions, on which DCT spectroscopy is based, are known to be subject to spin conservation. Consequently, double-ionization energies to singlet and triplet electronic states of SF62+ were measured by using H+ and F+ projectile ions, respectively. In the theoretical investigation, ADC(2) Green’s function calculations indicated a high density of singlet and triplet electronic states for SF62+. It was found, however, that groups of states could be identified that were well separated from adjacent groups. The average double-ionization energies (DIEs) for these groups are in good agreement with those measured. The combined experimental and theoretical approach has thus provided a considerably improved understanding of the double ionization of the SF6 molecule.
For five 12- to 17-mer multiply charged peptide cations, capture of low energy electrons yields unique products, mainly c and z· ions from amine bond cleavage. Their mass values for m > 400 define the complete sequence for two peptides, all but the ordering of a doublet in another, and all but the partial ordering of a triplet in the other two. The mass values from collisionally activated dissociation (CAD), on the other hand, indicate cleavages of 33 amide bonds (b and y ion products) of the 68 possible bonds between the amino acids of these peptides. Because the other common methods for ion dissociation yield products similar to those from CAD, electron capture dissociation (ECD) should provide a valuable complementary technique for sequencing of multiply charged peptide cations.
The gas-phase acidities of the C5-C9 normal alcohols, along with some more highly branched alcohols, have been measured by the equilibrium method in an ICR spectrometer. The acidities obtained for the 1-alkanols are consistently weaker than those obtained by the kinetic method, implying that some effect is altering the structure or dynamics of the transition state in the kinetic method.
Cu2+(CH3CN)n (n = 2–4) and Cu2+(CH3CN)3(H2O) have been formed by electrospray ionization of solutions of CuSO4 in aqueous acetonitrile. The ions have been identified by CAD and their reactivates have been investigated by ion-molecule reactions with acetonitrile-d3, acetone and acetone-d6, ammonia and benzene and benzene-d6 at low collision energy in the central collision cell of a quadrupole/hexapole/quadrupole instrument. Ammonia is the least reactive molecule with all the ions. The degree to which Cu(II) is reduced to Cu(I) depends on its degree of ligation and also on the collision gas. CD3CN exchanges for CH3CN with the tetra and tri-ligated ions with no charge reduction, while little charge conservation occurs in reaction with Cu2+(CH3CN)2. Similarly, acetone exchanges for CH3CN in the tetracoordinated ions but reacts almost exclusively by charge reduction with the less ligated ions. Both CD3CN and acetone can react with the tetracoordinated ions to form Cu(II) ions with up to six ligands, but although the tetracoordinated products have partially to fully exchanged ligand shells, the penta- and hexacoordinated products have ligands exclusively from the collision gas. This is attributed to the way in which the ions traverse the hexapole collision cell. Ammonia reacts by charge reduction, CH3CN replacement and addition. Benzene reacts with all ions solely by charge reduction, the benzene cation being a prominent product. The presence of H2O rather than CH3CN as a ligand leads to enhanced reactivity. In general, susceptibility to charge reduction decreases with the number of CH3CN ligands on the precursor ion.
Molecules containing a benzene ring and an oxygen atom typically have two types of protonation sites: on the ring (where facile intramolecular hydrogen transposition from carbon to carbon probably takes place) or on an oxygen lone pair. Four aryl ethers are compared: the isomers phthalan (1, a cyclic benzylic ether) and coumaran (2, a cyclic phenyl ether), as well as isochroman (3) and isopropyl phenyl ether (iPrOPh). The proton affinities of 1–3 have been measured using FT-ICR techniques as 830, 855 and 838 kJ/mol, respectively. Comparison with model compounds and Hartree-Fock-based SCF calculations indicate that protonated phthalan (1H) and protonated isochroman (3H) have O-protonated structures. By contrast, the conjugate acids of coumaran and iPrOPh prefer ring-protonated structures. Acidification/neutralization experiments in the ICR, as well as MIKE spectra, demonstrate that chemical ionization of iPrOPh produces noninterconverting O- and ring-protonated forms. Metastable ion decompositions of protonated phthalan and protonated isochroman give evidence of separate decomposition pathways for both types of tautomers. Protonated coumaran exhibits complete randomization of hydrogen between oxygen and the ring, which is attributed to high barriers for expulsion of neutral fragments.
CnSi+ (n = 4–15) clusters have been studied by means of the density functional method. As a general rule, when 4 ≤ n ≤ 9, the energetically most favorable isomers are found to be the linear (C∞v) structures with the silicon atom located at the very end of the carbon chain. The ground state is predicted to be 2Σ+ or 2Π alternately depending on whether n (the number of carbon atoms) is even or odd. Conversely, for 10 ≤ n ≤ 15, the linear arrangement (2Π) is found less stable than the Si-capped pure carbon monocyclic configuration. Published by Elsevier Science B.V.
The reactivity of the dimethylene ketene radical cation (a distonic ion) toward organic disulfides was examined inside a Fourier-transform ion cyclotron resonance mass spectrometer. The radical cation efficiently cleaves the disulfide bond in all the disulfides studied. Hence, this radical cation provides a potentially useful tool for the mass spectrometric characterization and location of disulfide bonds in neutral substrates without the requirement of prior derivatization.
Electronic excitation of NH+ ions by impact on rare gas atoms as well as H2, N2, O2, NO molecules was investigated by means of emission spectroscopy in the 10–1000 eVlab collision energy range. The NH+ BX and CX transitions were observed, while AX emission appears to be absent. The excitation functions for populating the B and the C states of NH+ are quite different. The C-state excitation sets in at ≈ 100 eVCM and rises monotonically thereafter. The B-state excitation function indicates two distinct mechanisms. One is active from the thermodynamic threshold on, but has low efficiency. At medium energies (50–200 eVCM) it is superseded by a much more efficient mechanism, which has an energy dependence similar to that for C-state excitation, but a ≈15–20 times larger cross section than the latter. The vibrational excitation in both states is moderate. It can be explained by Franck-Condon transitions from NH+ (X) reactant ions, even at low collision energy, assuming that the latter are vibrationally hot (Tvib = 5000 K). Published by Elsevier Science B.V.
Homogeneous dimers of phenylsilane, formed in a rare-gas seeded supersonic expansion have been studied by laser resonant two-photon ionization combined with a time-of-flight mass spectrometer. The resonant intermediate states are the S1 (270 nm) and S2 (210 nm) ones. The ionization of phenylsilane monomer is inefficient at 210 nm whereas phenylsilane homo-dimers are resonantly ionized with high efficiency at this wavelength region. The wavelength dependence of the dimer at S1← S0 origin region implies the existence of at least two, almost isoenergetic, dimer conformers in the molecular beam. Photoionization of phenylsilane dimer induces chemical reactions within the dimer. The detected dissociation channels have to do with −SiH3 and −C6H6 loss and proton-transfer. Van der Waals fragmentation (evaporation of a neutral phenylsilane) is also taking place.
The gas-phase reactions of FeCF2+ and CoCF2+ with selected linear and branched alkanes and olefins, and cyclic alkanes are studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. For reactions with alkanes, condensation and initial CC insertion are dominant, in contrast to the CH insertion and carbene-alkyl coupling of FeCH2+ and CoCH2+. The reactions of FeCF2+ and CoCF2+ with alkenes are characterized mainly by CF2H2 elimination and CF2 ligand displacement. No metathesis products are observed, while these are the predominant products in the reactions of FeCH2+ and CoCH2+ with alkenes. For cyclic alkanes, however, similar reaction pathways are observed for both MCF2+ and MCH2+ (M Fe, Co). Competitive CID and ion-molecule reaction bracketing gave D0(Fe+CF2) = 48 ± 4 kcal/mol and D0(Co+CF2) = 50 ± 6 kcal/mol, which can be compared to the literature values of D0(Fe+CH2) = 81.5 ± 0.9 kcal/mol and D0(Co+CH2) = 75.9 ± 1.2 kcal/mol. Density functional calculations indicate that like FeCH2+ and CoCH2+, both FeCF2+ and CoCF2+ have planar C2v structures, and that a double bond is formed between the M+ and the C atom. The bond energy calculated for FeCF2+ is 57.2 kcal/mol, which is somewhat higher than the experimental value. For CoCF2+, however, the calculated bond energy of 50.4 kcal/mol is in good agreement with the experimental bond energy. The structures, chemical bonding and reactivities of the difluorocarbene ligand in MCF2+ are significantly different from those of the ion-dipole complex FM+···F2C (M Fe, Co) reported earlier.
The reactions of H2O+, H3O+, D2O+, and D3O+ with neutral H2O and D2O were studied by tandem mass spectrometry. The H2O+ and D2O+ ion reactions exhibited multiple channels, including charge transfer, proton transfer (or hydrogen atom abstraction), and isotopic exchange. The H3O+ and D3O+ ion reactions exhibited only isotope exchange. The variation in the abundances of all ions involved in the reactions was measured over a neutral pressure range from 0 to 2 x 10(-5) Ton. A reaction scheme was chosen, which consisted of a sequence of charge transfer, proton transfer, and isotopic exchange reactions, Exact solutions to two groups of simultaneous differential equations were determined; one group started with the reaction of ionized water, and the other group started with the reactions of protonated water. A nonlinear least-squares regression technique was used to determine the rate coefficients of the individual reactions in the schemes from the ion abundance data. Branching ratios and relative rate coefficients were also determined in this manner.A delta chi-squared analysis of the results of the model fitted to the experimental data indicated that the kinetic information about the primary isotopic exchange processes is statistically the most significant, The errors in the derived values of the kinetic information of subsequent channels increased rapidly, Data from previously published selected ion flow tube (SIFT) study were analyzed in the same manner, Rigorous statistical analysis showed that the statistical isotope scrambling model was unable to explain either the SIFT or the tandem mass spectrometry data. This study shows that statistical analysis can be utilized to assess the validity of possible models in explaining experimentally observed kinetic behaviors. Published by Elsevier Science B,V.
Polyatomic ion-induced desorption of adsorbates is an important effect in secondary ion mass spectrometry, surface-induced dissociation and both ion beam and plasma processing of materials. We compared the adsorbate desorption cross-sections for a model ion-adsorbate system, NH3/CO/Ni(111) exposed to the isobaric atomic and polyatomic ions, Xe+ and SF5+, with kinetic energies of 10–1000 eV. The depletion of the adsorbate layer and the formation of new surface chemical species were monitored by X-ray photoelectron spectroscopy. Two to three times higher desorption cross-sections for both NH3 and CO were observed for SF5+ ion bombardment compared with Xe+ at the same incident energy. TRIDYN Monte Carlo simulations generally gave good agreement with the experimental results. TRIDYN implicated the larger reflected ion contributions for S and F compared with Xe as the source of the enhanced desorption yields by the polyatomic ion. Substrate damage was also found to be lower for SF5+ than Xe+ due to the former's lower energy per atom and lighter constituent atom masses.
Isotopic and elemental fractionation occurs as an inevitable and often frustrating factor in SIMS although the potential and demonstrated capabilities of ion probes make the solution of such problems well worth attaining. This paper describes a source of isotopic fractionation which may arise from the crystallographic structure of the sample. This effect, observed in magnetite, is potentially a source of error in acquiring reproducible isotopic ratios during SIMS of magnetite although no similar effect has been observed during several years of measurements on quartz, olivine or carbonates. The measurements were acquired using an Isolab 54 ion probe by rotating the sample about an axis normal to the surface and measuring 18O/16O ratios as a function of angle. The primary ion beam impacts the sample at an angle of 45° so that rotation of the sample should bring lattice planes approximately into and out of coincidence with the primary beam. Repeated 18O/16O measurements from magnetite crystals as a function of orientation show maxima in measured 18O/16O values which can be correlated with channelling along lattice planes. The measured ratio has been observed to increase by approximately 7‰ as a function of angle, variations which are very significant compared with isotopic fractionations produced by geological processes. The maxima correlate well with the [1 1 0], [1 0 1] and [0 1 1] lattice vectors of magnetite and are interpreted as channelling of the primary ions along the lattice vectors. Since it is extremely difficult, if not impossible, to determine the crystallographic orientation of magnetite in thin section, we believe that it is therefore necessary to measure magnetite samples in at least two orientations (separated by an angle that is not equal to the difference between two channelling orientations) against a standard in a known orientation.
The Bronsted acidities of several neutral NH-acids (substituted diphenylamine, substituted anilines and imides) were measured in the gas phase (pulsed FT ICR spectrometry), dimethyl sulfoxide and aqueous solution. Comparison of the Bronsted acidities of neutral NH-acids in the gas phase, dimethyl sulfoxide and water was also carried out. It was shown that substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into dimethyl sulfoxide and water. The strongest solvent-induced attenuation of the substituent effects is characteristic of the meta-substituted anilines whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by 2.83 times and with transfer into water by 4.13 times. At the same time, the reaction series of para and/or ortho-pi-acceptor substituted anilines, amides, imides and substituted diphenylamines are less sensitive to a change in gas phase for DMSO or water.In the special case of para-acceptor substituted anilines it was demonstrated that the specific solvation induced an increase in the acidity of the para- and/or ortho-acceptor substituted anilines as compared with the behavior of the corresponding meta-substituted anilines by amounts up to 10 pK(a) units. (C) 1998 Elsevier Science B.V. All rights reserved.
