Ion Kinetic Energy Spectrometry Research Articles

The gas-phase reactions of the methoxymethyl cation with aldehydes and ketones

The results of both mass-analysed ion kinetic energy spectroscopy and Fourier transform ion cyclotron resonance experiments involving reactions of the methoxymethyl cation CH3OCH2+ with a variety of aldehydes and ketones are reported. With ketones, the reaction yields a covalent complex whose dissociation either gives back the CH3OCH2+ cation or leads to a methyl cation transfer to the ketone. Except for CH2O, a third pathway is open with aldehydes. A hydride transfer to CH3OCH2+ yields a RCO+ acylium product ion. The branching ratio of these three pathways strongly depends on the structure of the aldehyde. Ab initio calculations confirm that the results can be explained by the interconversion of covalent structures and electrostatic complexes on the potential energy surface.

Unimolecular Decompositions of 2,2,2-Trifluoroethyl Acetate, 2,2,2-Trifluoroethyl Trifluoroacetate, and S-Ethyl Trifluorothioacetate upon Electron Impact.

The unimolecular decompositions of the metastable ions produced from 2,2,2-trifluoroethyl acetate, CH3COOCH2CF3, 1, (Mw: 142), 2,2,2-trifluoroethyl trifluoroacetate, CF3COOCH2CF3, 2, (Mw: 196), and S-ethyl trifluorothioacetate, CF3COSCH2CH3, 3, (Mw: 158) upon electron ionization have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. The metastable ions 1+ decompose exclusively to give the ion at m/z 122 by losing HF via an 1,2-elimination.In the normal mass spectrum of 2, no molecular ion is observed, while the ion corresponding to the loss of CF3 gives rise to an intense peak at m/z 127 (about 90% of the base peak) and the ion corresponding to the loss of F gives the peak at m/z 177 with moderate intensity (about 20%). Both ions consist of two isomeric ions. Some interesting rearrangement processes, such as CF3 or OCH2CF3 migration followed by the loss of a CO molecule, occur during the metastable fragmentations of these ions.The molecular ion of 3, which is a base peak, decomposes into the ions at m/z 89 and 61 by losing CF3 and CF3CO, respectively, in the metastable time region. The relative abundance of ions in the MIKE spectrum are rationalized with the thermochemical data estimated by the PM 3 method.

Stability of secondary structural elements in a solvent-free environment. II: The β-pleated sheets

The stability of single β-strands and multistrand β-pleated sheets as elements of secondary structure is examined in the absence of intermolecular interactions. Such experimental conditions (e.g., complete removal of solvent molecules and counterions) are achieved by placing the peptide ions in the gas phase. The metastable multiply- charged peptide ions produced by electrospray ionization undergo unimolecular dissociation. Intercharge repulsion within the precursor ions gives rise to the elevated kinetic energy of fragment ions, which is measured using Mass-analyzed Ion Kinetic Energy (MIKE) spectrometry. Intercharge distances calculated based on these measurements are compared to the numbers derived from molecular mechanics calculations with charge site assignments based on relative proton affinities. Evidence is presented suggesting that single β-strands form collapsed structures in the absence of solvents, while multistrand β-pleated sheets are likely to retain “native-like” secondary structures under the same conditions. These results indicate that intramolecular hydrogen bonds are the major factor determining the three-dimensional arrangements of polypeptides in the gas phase, compensating both long- and short-range electrostatic repulsions. This is in good agreement with our earlier findings (Proteins 27:165–170, 1997) concerning stability of helical conformation of melittin in the absence of solvent. Proteins Suppl. 2:22–27, 1998. © 1998 Wiley-Liss, Inc.

Mass spectrometric study on methyl 5-methyl-2-oxo-3-[2-(4-R-phenyl)-1-ethyl]cyclopentanecarboxylates and methyl 2-oxo-5-(4-R-phenyl)-3-propylcyclopentanecarboxylates

The mechanisms of mass spectrometric fragmentation of eight new compounds, including methyl 5-methyl-2-oxo-3-[2-(4-R-phenyl)-1-ethyl]cyclopentanecarboxylates (Group I, R = substituent) and methyl 2-oxo-5-(4-R-phenyl)-3-propylcyclopentanecarboxylates (Group II, R = substituent), were studied using mass analyzed ion kinetic energy spectrometry. The chemical compositions of these compounds and some of their important fragment ions were verified by high-resolution MS data. There were two types of H-rearrangement as regards the route of fission. In one type, the γ-H migration was influenced by both the electron effect and the steric hindrance of substituents, whereas on the other α-H migration losing methanol was due to the α-position active hydrogen located between two carbonyls, and this was verified by the method of deuterium labelling. © 1997 John Wiley & Sons, Ltd.

Mass Spectrometry of the Organic Hydrate 2-Bromo-3,3-dihydroxy-1-(2-thienyl)-4,4,4-trifluoro-1-butanone

The ionized molecular species obtained by electron impact and chemical ionization of the organic hydrate 2-bromo-3,3-dihydroxy-(2-thienyl)-4,4,4-trifluoro-1-butanone were analysed using the mass-analysed ion kinetic energy spectrometry method. The structure of the side chain and the position of the C-atom of the gem-diol group in the compound were established from the characteristic product ions formed from the metastable and collisionally-activated M+• and MH+ ions. © 1997 John Wiley & Sons, Ltd.

Mass Spectrometry of the Organic Hydrate 2-Bromo-3,3-dihydroxy-1-(2-thienyl)-4,4,4-trifluoro-1-butanone

The ionized molecular species obtained by electron impact and chemical ionization of the organic hydrate 2-bromo-3,3-dihydroxy-(2-thienyl)-4,4,4-trifluoro-1-butanone were analysed using the mass-analysed ion kinetic energy spectrometry method. The structure of the side chain and the position of the C-atom of the gem-diol group in the compound were established from the characteristic product ions formed from the metastable and collisionally-activated M+• and MH+ ions. © 1997 John Wiley & Sons, Ltd.

Metastable and Collision-activated Decomposition of Protonated Molecules of Isomeric N7- and N9-Substituted Purines in the Gas Phase

The structures and reactivities of protonated metastable and collision-activated molecules of isomeric N7- and N9-substituted purines were investigated using fast atom/ion bombardment ionization and mass analysed ion kinetic energy spectrometry methods. Calculated proton affinities (PAs) of isomeric purine derivatives and the intensities of the observed metastable-ion peaks revealed a direct correlation of PA with the reactivity of protonated molecules. The reactivity of the N9-substituted purines possessing N3-site specific proton affinities, i.e. N3–H+, relates to the strength of hydrogen bonding of N3–H+ to oxygen atoms(s) of the N9-substituent, e.g. CH2—O—CH2CH2—O—C(CH3) = O or CH2—OCH2CH2—O—H. Various intramolecular (‘intra-ion’) hydrogen-bond(s) in the protonated molecules of isomeric purine derivates were revealed using proton affinities and metastable-ion peak intensities. © 1997 John Wiley & Sons, Ltd.

Stability of secondary structural elements in a solvent-free environment: the α helix

The stability of the alpha helix as an element of secondary structure is examined in the absence of solvation, in the gas phase. Mass-analyzed ion kinetic energy (MIKE) spectrometry was applied to measure intercharge repulsion and intercharge distance in multiply protonated melittin, a polypeptide known to possess a stable helical structure in a number of different environments. The experimental results, interpreted in combination with molecular mechanics calculations, suggest that triply charged melittin retains its secondary structure in the gas phase. The stability if the alpha-helical conformation of the polypeptide in the absence of solvent molecules reflects the fact that a network of intrinsic helical hydrogen bonds is energetically more favorable than unfolded conformations.

Stability of secondary structural elements in a solvent‐free environment: the α helix

The stability of the α helix as an element of secondary structure is examined in the absence of solvation, in the gas phase. Mass-analyzed ion kinetic energy (MIKE) spectrometry was applied to measure intercharge repulsion and intercharge distance in multiply protonated melittin, a polypeptide known to possess a stable helical structure in a number of different environments. The experimental results, interpreted in combination with molecular mechanics calculations, suggest that triply charged melittin retains its secondary structure in the gas phase. The stability if the α-helical conformation of the polypeptide in the absence of solvent molecules reflects the fact that a network of intrinsic helical hydrogen bonds is energetically more favorable than unfolded conformations. © 1997 Wiley-Liss, Inc.

Account: Impulsive excitation in collision-induced dissociation reactions of polyatomic cations

Our understanding of the mechanisms of energy transfer and subsequent dissociations in the collision of ions with a neutral [collision-induced dissociation, (CID)] has been significantly advanced by recent dynamics studies using the crossed-beam tandem mass spectrometry technique. In this account two classifications of impulsive excitation, both involving significant angular deflection of product ions from the direction of travel of the parent ion, are described in microscopic detail. Recent experimental examples of these classes of impulsive mechanisms are described, along with their dynamics “signatures”. Connections are drawn between these models, beam experiments and conventional studies of CID reactions by means of mass-analyzed ion kinetic energy spectroscopy (MIKES).

Proton locations in doubly charged peptides and association with specific fragmentation pathways

Mass-analyzed ion kinetic energy (MIKE) spectrometry has been employed to measure intercharge distances in the doubly protonated octapeptide PPGFSPFR. The interpretation of the experimental observations, aided by macromolecular mechanics calculations, suggests that two isomeric forms of the doubly charged ions are produced by both FAB and electrospray, which differ from each other by the location of one of the protons. Different fragmentation pathways are shown to occur in each of these isomers. This is interpreted following the mechanism of Adams et al., J. Am. Soc. Mass Spectrom., 7 (1996) 30.

Sequential methyl elimination from ionized dimethyl[2.n]paracyclophane-enes (n = 3, 4, 5, 6): interaction of ionized aromatic rings in strained cyclophane conformations?

The fragmentations of the molecular ions of a series of dimethyl[2.n]paracyclophane-enes (n = 3, 4, 5, 6) have been studied by mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labeling, The characteristic fragmentation sequence of the molecular ions of all dimethyl[2.n]paracyclophane-enes is the sequential elimination of three methyl radicals, The first methyl radical originates predominantly from the methyl substituent of the ethylene bridge and this process is accompanied by a large kinetic energy release (KER) indicative of a preceding rearrangement, The kinetic energy release distribution (KERD) observed is independent of the size of the polymethylene bridge of the dimethyl[2.n]paracyclophane-enes and even of the source of the methyl radical, Abundant loss of a second methyl radical is observed from metastable [M - CH3](+) ions in spite of a violation of the ''even-electron-rule''. Again, the methyl radical lost is chiefly a methyl substituent at the ethylene bridge, although methyl loss from the inner methylene groups of the polymethylene bridge occurs also, A small KER is observed for this fragmentation, which is again independent of the source of the methyl radical, These observations are rationalized by a rearrangement of the molecular ions by bond formation between the aromatic rings and subsequent hydrogen migrations either before or after a benzylic cleavage of the saturated bridge.

Mass spectrometry of tert-butylnaphthalenes–a comparison with the unimolecular fragmentation of tert-butylbenzene

The unimolecular fragmentation of ionized 1- and 2-tert-butylnaphthalenes 1 and 2 have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry and specifically deuterated and 13 C labeled derivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3. Metastable molecular ions of 1, 2 and 3 lose exclusively a methyl radical yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2propyl cations 3a. Further fragmentations of 1a and 2a include, as the main processes, the elimination of C 2H4 and of CH3 • . The loss of C2H4 is associated with a large kinetic energy release giving rise to a broad flat-topped peak in the MIKE spectra (as in the case of 3a) while loss of CH3 gives rise to a Gaussian shaped signal. The latter reaction is not observed for metastable 2-phenyl-2-propyl cations 3a. The fragmentations of labeled derivatives of 1 and 2 show that H/D exchange in 1a and 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C 2H4 and of CH3 • , is much more extensive than in 3a. This H/D exchange decreases for metastable 1a and 2a decomposing in the second field-free region, and the interchange of the C atoms within the C 3H6 is less than for 3a. These observations are explained by H migrations between the side chain and the naphthalene rings of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2-methyl shift known for the reactions of 3a. The differences observed for the reactions of 1a and 2a and of 3a are explained by the more basic aryl group of 1a and 2a. These fragmentation mechanisms are corroborated by an AM1 calculation of the minimum energy reaction pathways (MERP).

Measurements of the kinetic energy released in decompositions of proton-bound dimers of primary amines

The kinetic energy release (KER) associated with the decomposition of protonated dimers into the protonated and neutral monomer has been measured for a homologous series of primary monoamines, primary diamines, monobasic α-amino acids and dibasic α-amino acids by linked scans (B 2/E) on a two-sector instrument and by mass-analyzed ion kinetic energy spectroscopy (MIKES) on a four-sector instrument. The T 0.5 values were between 16.1 meV and 26.1 meV for the linked scans and between 10.3 meV and 17.7 meV for the MIKE spectra. For the protonated dimers of monoamines and monobasic amino acids, the KER of the decomposition increases with the number of degrees of freedom of the ion. For the protonated dimers of diamines and dibasic amino acids, the KER of the decomposition reaction is independent of the number of degrees of freedom. This difference must reflect a difference in the potential energy surfaces across which the decomposition of the two types of protonated dimers takes place. There seems to be a tendency of KER for decomposition of the proton-bound dimers of primary amines to fall with the proton affinity (PA) of the neutral within an isomeric series. This indicates a correlation between the PA of the amine and the critical energy for decomposition of the protonated dimer. For the protonated dimers of monoamines, the results are in agreement with the assumption that the reaction occurs without a barrier for the reverse reaction.

Mass Spectrometric Study of Some Zirconocene and Hafnocene Dialcoholates Containing a Norbornane Moiety

The electron ionization mass spectrometric behaviour of a new series of hafnocene and zirconocene dialcoholates is discussed. Deprotonized norborneol and 3-methyl-2-norbornanemethanol were used as lateral ligands. These data are also compared to those of mass-analysed ion kinetic energy spectrometry. Fragmentation patterns of the complexes in question give relevant information on their gas-phase behaviour and on an influence of the ligand's structure on decomposition pathways as well.

Mechanisms of single-electron capture by the dichlorocarbene dication

The single-electron capture (SEC) by dichlorocarbene dications with eight different atomic and molecular target gases, CCl 2 (2+) + G → CCl 2 (+) + G(+), has been studied by product ion spectroscopy and ion kinetic energy spectroscopy. The experimental data have been interpreted in the framework of a theoretical model mat describes the charge exchange process. Exothermic charge exchange is handled within the Landau-Zener model, whereas endothermic charge exchange is described by the Demkov model. The calculated data reproduce qualitatively the essential features of the experimental results: (1) the appearance of a reaction window centered at an exothermicity in the 4-4.5-eV range, (2) the lower SEC cross sections for endothermic charge exchange, (3) the wider internal energy distributions obtained for CCl 2 (+) in the endothermic regime than in the exothermic one, which results in larger dissociation yields, (4) the excitation of molecular targets that accompany their ionization in the SEC process, and (5) the kinetic energy released on the CCl(+) + Cl fragments in dissociative SEC.

Excitation Mechanism in the Collision-Induced Dissociation of Methane Molecular Ion at Kiloelectronvolt Translational Energy

Collision-induced hydrogen loss from methane molecular ion has been studied at kiloelectronvolt laboratory translational energy by using mass-analyzed ion kinetic energy spectrometry (MIKES). The kinetic energy release (KER) distribution evaluated from the MIKE profile is bimodal: a small KER component arising from vibrational excitation and a large KER component from electronic excitation. CH4•+ has been generated by electron ionization and by charge exchange ionization to vary its internal energy content. It has been found that the collision-induced dissociation (CID) via electronic excitation becomes more efficient at higher incident energy, while the efficiency of the vibrational excitation is hardly affected by the incident energy. Most importantly, it has been found that the electronic excitation is the main mechanism in CID at kiloelectronvolt incident energy. Vibrational excitation also occurs but supplies only a few vibrational quanta and hence is important for the least endoergic process only.

The importance of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II formed by electrospray ionization: Experimental considerations and structural implications

The occurrence of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II is demonstrated by the use of mass-analyzed ion kinetic energy spectrometry (MIKES) and kinetic energy release distributions (KERDs). Linked scans at a constant B/E severely discriminate against product ions formed by charge-separation reactions. Although the products are significantly more abundant in MIKES experiments, instrumental discrimination still makes quantitation of relative product ion abundances highly inaccurate. The most probable KERs (T m. p.) and the average KERs (T ave.) of the reactions are determined from the KERDs, and these values are compared to the KERs determined from the peak widths at half-height (T 0. 5). The measurement of T 0. 5 is a poor approximation to T m. p. and T ave.. The T m. p. is used to calculate a most probable intercharge distance, which is compared to results from molecular dynamics calculations. The results provide evidence with regard to the mechanisms of fragmentation of multiply charged ions and the location of the charge site in relation to the decomposition reactions.

Photodissociation Dynamics of Nitrobenzene Molecular Ion on a Nanosecond Time Scale

Photodissociation dynamics of nitrobenzene molecular ion has been investigated on a nanosecond time scale by mass-analyzed ion kinetic energy spectrometry. Photodissociation has been found to occur through various reaction channels. Three dissociation channels to C6H5O+, C6H5+, and NO+ have been observed together with a consecutive dissociation to C5H5+ via C6H5O+. The photodissociation rate constant of the molecular ion and the kinetic energy release distributions in each channel have been determined. The rate constant of the second step of the consecutive reaction has been determined in real time also. It has been found that the direct bond cleavage to C6H5+ occurs in competition with the rearrangement processes to C6H5O+ and NO+ on a nanosecond time scale. Statistical theories have been used to gain insight into the dynamics of the reactions.

Photodissociation Dynamics of n-Butylbenzene Molecular Ion

Photodissociation dynamics of n-butylbenzene molecular ion has been investigated on a nanosecond time scale. The rate constants for production of C7H8•+ and C7H7+, their branching ratios, and the kinetic energy release distributions have been determined by the photodissociation method using mass-analyzed ion kinetic energy spectrometry. The branching ratios have been found to be in excellent agreement with the previously established results. All the experimental data could be explained with statistical theories such as Rice−Ramsperger−Kassel−Marcus (RRKM) and phase space theories. RRKM fittings for these reactions have been improved. The present result supports the previous suggestion that the dissociation to C7H8•+ occurs via a stepwise McLafferty rearrangement.