A fourier transform ion cyclotron resonance study of the gas phase negative ion chemistry of benzaldehyde

Fourier transform ion cyclotron resonance mass spectrometry has been used to investigate some aspects of the gas phase negative ion chemistry of benzaldehyde. It is shown that all protons are nearly equally acidic. Exothermic proton abstraction from the aldehydic position by NH₂^- leads to the formation of C₆H₅^-, ions. Reactions of several nucleophiles and benzaldehyde are discussed. Some of them proceed via a tetrahedral intermediate. Arguments are put forward to show that one of the reactions of the conjugate base of benzaldehyde involves benzaldehyde-hydrate molecules formed in the inlet lines towards the cell.     

Fourier transform ion cyclotron resonance study of the dehydrogenative polymerization of allene and propyne induced by W+ in the gas phase

A Fourier transform ion cyclotron resonance study of the gas-phase reaction of W⁺ with allene is reported. W⁺ successively dehydrogenates at least nine allene molecules, leading to the formation of a series of WC_3nH ions, with WC₂₇H as the largest product observed after a reaction delay of 40 s. Starting from the third reaction, there is a competition between elimination of H₂ and C₂H₂ at each step. The activation of propyne was also investigated and found to lead to essentially the same sequence as with allene. The reaction of W⁺ with 1,1-d₂-allene and collision-induced dissociation spectra of product ions are used to discuss a plausible mechanism for the formation and structures of some of the species observed.     

Gas phase studies of N-nitrosamines by ion cyclotron resonance spectroscopy

The gas phase basicities of a series of cyclic and acyclic N-nitrosamines were determined by ion cyclotron resonance techniques. The fragmentation pattern and ion-molecule reactions of these compounds are discussed in terms of high resolution mass spectrometry and deuterium labeling. The presence of a unique rearrangement of the molecular ion is discussed in terms of specific ion-molecule proton transfer reactions and fragment ion products.     

Tautomeric equilibria of 2‐ and 4‐thiouracil in gas phase and in solvent: A density functional study

The relative stabilities of the five favored tautomers of 2‐ and 4‐thiouracil in gas phase and in water solution were determined by density functional theory employing the Becke, Lee, Yang, and Parr (B3LYP) exchange–correlation potential and the three 6‐31G(d,p), 6‐311++G(d,p), and triple‐zeta valence (TZVP) basis sets. Zero‐point vibrational corrections were also computed. Bulk solvent effects were studied in the framework of the self‐consistent reaction field approach by the polarizable continuum model. All calculations indicate that the most stable tautomer for both species, in the gas phase as well as in solution, has the oxo‐thione form, in full agreement with the previous ab initio and experimental studies. The tautomeric stability orders obtained in the aqueous solution are sensibly different from that in the gas phase. At B3LYP/6‐311++G(d,p) level in the gas phase, the following orders of stability for 2‐ and 4‐thiouracil tautomers were observed, respectively: S2U1>S2U2>S2U4>S2U5>S2U3 and S4U1>S4U2>S4U3>S4U4>S4U5. The corresponding trends in the aqueous phase are S2U1>S2U3>S2U2>S2U5>S2U4 and S4U1>S4U2>S4U3>S4U5>S4U4. On the basis of the computed energy differences we can hypothesize that only the oxo‐thione forms of 2‐ and 4‐thiouracil should exist in the gas phase and in water solution. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 44–52, 2001     

Stable adduct formation during the gas phase reactions between the trimethylgermanium cation and organic nucleophiles. An ion cyclotron resonance study

The trimethylgermanium cation (Me₃Ge⁺) reacts with aliphatic organic neutrals (M) which contain N, O or S, to form adducts [M + Me₃Ge⁺] in the gas phase. These adducts are more stable than their analogues formed from the trimethylsilyl cation, and thus Me₃Ge⁺ is a useful analytical reagent for chemical ionization mass spectrometry.     

The long-lived molecular ion of propionitrile: An ion cyclotron resonance study

Evidence has been reported that primary loss of H and of HCN from the molecular ions of propionitrile, isobutyronitrile and butyronitrile in the mass spectrometer is preferentially preceded by hydrogen migration from C-2 to C-1. Ion cyclotron double resonance spectra of proton (or deuteron-) transfer products derived from propionitrile-2-d2 and -3-d3 and a series of bases provide evidence that such migration occurs also in long-lived propionitrile molecular ions.     

The mechanism of borohydride reductions. Reactions of gas phase borohydride ion with carbonyl groups by ion cyclotron resonance spectroscopy.

The reaction between BH₄^? and CH₂O has been investigated in the gas phase using ion cyclotron resonance spectroscopy. No hydride transfer from BH₄^? to the carbonyl group is observed, however a novel reaction between enolate ions and diborane has been observed.     

Tautomeric equilibria of 2(4)-monooxopyrimidines in the gas phase,in low-temperature matrices and in solution

IR absorption spectra, including the NH, OH and CO stretching regions, have been recorded for 4-oxo-6-methyl- and 2-oxo-4,6-dimethyl pyrimidines and several related derivatives, in the gas phase, in low-temperature inert matrices, and in several liquid solvents.All the 4-oxopyrimidines in the gas phase, and 4-oxo-6-methylpyrimidine in low-temperature matrices, exhibit comparable populations of the keto and enol forms. By contrast the 2-oxopyrimidines are predominantly in the enol forms. Both classes of com pounds are predominantly in the keto form in liquid solvent systems. The tautomeric equilibrium constant (K_T) in the vapour phase for 4-oxo-2,6-dimethylpyrimidine is about 2, and for the other 4-oxopyrimidines is about 1. For 4-oxo-6-methylpyrimidine, the equilibrium constant in inert matrices varies slightly with the activity of the matrix gas, with the keto tautomer favoured in the more active matrix. From the temperature-dependence of K_T the free energy difference between the two tautomeric forms of 4-oxo-6-methylpyrimidine in the vapour phase has been calculated. Heats of vaporization have also been calculated for the various compounds and related to their abilities to associate by hydrogen bonding in the condensed phase.The UV absorption spectra of some of the foregoing have also been recorded in the gas phase, but these were of only limited value in studies of tautomeric equilibria, as connpared to the IR spectra.     

Structural determination of [C7H7O]+ ions in the gas phase by ion cyclotron resonance spectrometry

The [C₇H₇O]⁺ ions from a series of compounds have been studied using ion cyclotron resonance spectrometry. The techniques employed in this gas phase ion structure determination of [C₇H₇O]⁺ were photodissociation, ion-molecule reactions, and collisionally activated dissociation using a Fourier transform mass spectrometer (FTMS-CAD). In addition to the low energy FTMS-CAD results, high energy CAD data obtained with a sector mass spectrometer is also provided. Evidence was found for five unique [C₇H₇O]⁺ structures, including the hydroxybenzyl ion, the hydroxytropylium ion, the protonated benzaldehyde ion, the methylaryloxy ion and the phenyl methylene ether ion. Ion-molecule reactions, invovling both proton transfer and methylene transfer, provided the most unambiguous results and yielded qualitative and quantitative evidence for the five structures. However, a combined approach using the three techniques was necessary to identify all of the structures. The tropylium form of [C₇H₇O]⁺ was found to absorb strongly at 305 nm, while the protonated benzaldehyde ion was found to have a strong absorption band at 305 nm and a weak band at 370 nm. The proton affinity of 2,4,6-cycloheptatrienone was determined to be 918±8 kJ mol^?1, which is considerably lower than a previously reported value. In addition, deprotonation reactions of the methylaryloxy ion yielded a proton affinity of 871±14 kJ mol^?1 for 4-methylenecyclohexa-2,6-diene-1-one.     

A Fourier-transform ion cyclotron resonance study of the 3,5-didehydrophenyl cation

The 3,5-didehydrophenyl cation has been generated in good purity via sustained off-resonance irradiation for collision-activated dissociation of 3,5-dinitrobenzoyl chloride in a Fourier-transform ion cyclotron resonance mass spectrometer. Differences in the ion-molecule reactivity of this species from that of its cyclic and acyclic isomers allowed isomeric distinction to be achieved. This study represents the first definitive identification of this fundamentally interesting, doubly aromatic ion. However, the formation of the 3,5-didehydrophenyl cation was found to be the exception rather than the rule, with most 1,3,5-substituted benzenes yielding mainly acyclic C6H3+ isomers under electron ionization conditions. This mixed ion population was attributed to isomerization of fragmentation intermediates rather than any intrinsic instability of the 3,5-didehydrophenyl cation.     

Fourier transform ion cyclotron resonance spectroscopy

An ion cyclotron resonance (ICR) absorption spectrum has been obtained by exciting an ICR spectral segment with a fixed-frequency electric field pulse, followed by broad-band detection, digitization of the (time-domain) transient response, and digital Fourier transformation to produce the (frequency-domain) absorption spectrum. For a given signal-to-noise ratio and resolution, the FT-ICR method generates a spectrum in a time which is two orders of magnitude shorter than that required in conventional slow-sweep ICR detection. In the present example, a signal-to-noise ratio of 8:1 and a mass resolution of about 0.005 amu for CH₄⁺ (from CH₄ at a pressure of 8 X 10^?7 torr) have been achieved, using a single data acquisition period of 25.6 msec.     

Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps

We present a numerical method for computation of electrostatic (trapping) and time-varying (excitation) electric fields and the resulting ion trajectory and detected time-domain-induced voltage signal in a rectangular (or cubic) ion cyclotron resonance (ICR) ion trap. The electric potential is calculated by use of the superposition principle and relaxation method with a large number of grid points (e.g., 100 × 100 × 100 for a cubic trap). Complex ICR experiments and spectra may now be simulated with high accuracy. Ion trajectories may be obtained for any combination of trapping and excitation modes, including quadrupolar or cubic trapping in static or dynamic mode; and dipolar, quadrupolar, or parametric excitation with single-frequency, frequency-sweep (chirp), or stored waveform inverse Fourier transform waveforms. The resulting ion trajectory may be represented either as its three dimensional spatial path or as two-dimensional plots of x-, y-, or z-position, velocity, or kinetic energy versus time in the absence or presence of excitation. Induced current is calculated by use of the reciprocity principle, and simulated ICR mass spectra are generated by Fourier transform of the corresponding time-domain voltage signal.     

The resolution obtained from low energy ion scattering using an ion cyclotron resonance spectrometer

The resolving power of the low energy ion scattering technique with FT-ICR spectrometry is discussed. Resolution improvement due to simultaneous registration of scattered ions with different masses is shown.     

Proton bound homodimers and heterodimers of amides and amines in the gas phase. Equilibrium studies by Fourier transform ion cyclotron resonance spectrometry

By injection of the proton bound homodimer [DMF.H+.DMF] of N,N-dimethylformamide (DMF) generated in an external ion source into a mixture of DMF and a second base within the cell of a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer the equilibria between [DMF.H+.DMF] and the other possible proton bound dimers [DMF.H+.base] and [base.H+.base] have been studied for 13 different bases. Strongly polar bases like aliphatic amides and dimethyl sulfoxide (DMSO) exchange both DMF in [DMF.H+.DMF] by a two step process, while the almost non-polar amines exchange only one DMF. If the base is a primary or secondary amine, the proton bound heterodimer [DMF.H+.amine] reacts further by the addition of one DMF to create a proton bound trimer [(DMF)2.H+.amine]. The affinity deltaG(DMFH+) of the bases towards protonated DMF relative to neutral DMF depends linearly on the difference deltaGB of the gas phase basicity of DMF and the other base, but different correlation lines are obtained for polar and non-polar ligands (deltaGDMFH+ = 0.44GB(base)-375 [kJ/mol] (r = 0.97) and deltaGDMFH+ = 0.46GB(base)-397 [kJ/mol] (r = 0.99), respectively). This different behavior is explained by a different character of the proton bridge in the heterodimers containing only polar ligands and those incorporating a non-polar ligand besides DMF. The former dimers contain a more or less symmetric proton bridge while the latter can be viewed as a protonated base solvated by DMF. The available data have been used to calculate the molecular pair gas phase basicity of DMF and the 13 bases used and to estimate the dissociation energies of the bonds of the proton bridge in various proton bound heterodimers.     

Frequency-sweep fourier transform ion cyclotron resonance spectroscopy

A single ion cyclotron resonance (ICR) absorption spectrum showing both CH⁺₃ and CH⁺₄ signals has been obtained by exciting both ion cyclotron resonances with a frequency-swept rf irradiation, followed by broad-band detection, digitization of the (time-domain) response, and finally discrete Fourier transformation to produce the (frequency-domain) spectrum. Pulsed-excitation Fourier transform ICR has demonstrated the use of broad-band detection in rapid generation of ICR spectra by Fourier transform methods; this paper demonstrates that frequency-sweep excitation can provide the broad-band irradiation required to excite ion cyclotron resonances throughout any desired mass range. It will thus be possible to obtain an ICR absorption spectrum of given mass range, signal-to-noise ratio, and resolution in an observation period which is two orders of magnitude shorter than that needed to obtain the same spectrum by conventional slow-sweep detection.     

Study on the reactions of pyridine with fluorine-containing compounds in the gas phase by electron impact/Fourier transfer ion cyclotron resonance mass spectrometry

The gas-phase ion-molecule reactions of pyridine and fluorine-containing compounds have been studied by the electron impact/Fourier transfer ion cyclotron resonance mass spectrometry (EI/FTICRMS). Comparing the reaction of the standard sample perfluorotributylamine (PFTBA), in the case of 3-oxa-5-iodo-octafluoropentylsulfonyl azide (ICF₂CF₂OCF₂CF₂SO₂N₃), a series of addition-elimination products and loss 28 from the expected adducts were obtained. A possible reaction mechanism was discussed, which was consistent with the results obtained in the condensed phase. These results demonstrated that EI/FTICRMS is a complementary technique for study of the organic reactions.     

Initial implementation of an electrodynamic ion funnel with fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has become a widely used method to study biopolymers. The method, in combination with an electrospray ionization (ESI) source has demonstrated the highest resolution and accuracy yet achieved for characterization of biomolecules and their noncovalent complexes. The most common design for the ESI interface includes a heated capillary inlet followed by a skimmer having a small orifice to limit gas conductance between a higher pressure (1 to 5 torr) source region and the lower pressure ion guide. The ion losses in the capillary-skimmer interface are large (estimated to be more than 90%) and thus reduce achievable sensitivity. In this work, we report on the initial implementation of a newly developed electrodynamic ion funnel in a 3.5 tesla ESI-FTICR mass spectrometer. The initial results show dramatically improved ion transmission as compared to the conventional capillary-skimmer arrangement. An estimated detection limit of 30 zeptomoles (approximately 18,000 molecules) has been achieved for the analysis of the proteins with molecular weights ranging from 8 to 20 kDa.