Mass-analyzed Ion Kinetic Energy Research Articles

Electron Impact Induced Fragmentation of Dibenzo Crown Ethers

Six structurally related crown ethers have been studied by electron impact (EI) mass spectrometry. From accurate mass measurements, metastabel ion and mass-analyzed ion kinetic energy (MIKE) spectra, plausible fragmentation pathways were deduced. All compounds showed very abundant molecular ions and some amount of the doubly charged molecular ions. The primary fragmentations always involved the catechol moiety and consequent loss of oxirane although the fragment(ation)s also showed a clear dependence on the functionalities X in one of the crown ether segments, —OCH2XCH2O— (compound/X: 1/—CH2—, 2/—C=C—, 3/trans—CH=CH—, 4/cis—CH=CH—, 5/—HC—CH—, and 6/—CH2CH2—).

Studies in organic mass spectrometry. Part 20: a hidden ortho effect in the electron ionisation mass spectra of some 2′-alkyl substituted 2-and 3-thiophenecarboxanilides

The electron-ionisation-induced amide-bond cleavage of some 2′-methyl- and 2′-ethyl-substituted 2- and 3-thiophenecarbox-anilides, which yields formally anilylium ions having relative intensities apparently in contrast with the Stevenson–Audier rule, has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and compared to that of the 3′- and 4′-isomers. It has been shown that, in the case of the 2′-methyl and 2′-ethyl derivatives, the amide-bond cleavage is anchimerically assisted through the hidden migration of a benzyl hydrogen to the nitrogen. Analysis of the MIKE and collision-induced decomposition (CID) MIKE spectra of model compounds indicates that this cryptic ortho effect produces a stable ortho quinoide or aminotropylium structure.

Formation of stable “Proton-bridged dimers” by decomposition of α,β-diether radical cations. Part II: Two populations with different energy content are evidenced in the case of 1,2- diethoxycyclohexane+•

The mass analyzed ion kinetic energy (MIKE) spectrum of the 1,2-diethoxycyclohexane radical cation 1 exhibits fragment ions generated from two molecular ion populations with different energy contents. An ethanol··H+··acetaldehyde proton-bridged dimer originates from molecular ions of low internal energy, whereas cyclohexene radical cation formation results from more energetic ions. This statement is based on the comparison of the kinetic energy released for each fragmentation and on the extent of H/D exchange preceding the formation of fragment ions from (C2D5O)2–cycloC6H10 (1a).

Electron impact mass spectra of δ-keto enol esters

Enol esters with a remote keto function in the δ-position produce upon electron impact (EI) (70 eV) molecular ions of very low abundance (< 1%) due to two fast main fragmentations. The first one involves loss of a carboxylic acid fragment from the α-carbon and subsequent cyclisation to a stable furan ion. A second, more abundant, fragmentation leads, by loss of ketene and an acyl moiety, to a protonated α,β-enone. The stability of this ion is governed by the substituents in the βand γ-positions, which thus determine the order in which a ketene or an acyl radical is lost. Mass-analysed ion kinetic energy (MIKE) spectra of the most abundant ions were also recorded. In one case, a perfect match was found between the collisional activation (CA) mass spectrum of the most abundant ion of one enol-ester under EI conditions and the CA mass spectrum of the corresponding protonated α,β-enone obtained under chemical ionisation (CI) conditions. The fragmentation mechanisms were also inferred from the spectra of two deuterium-labelled δ-keto enol esters.

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.

How to measure the kinetic energy distribution of the precursor ion main beam in mass-analyzed ion kinetic energy spectrometry experiments

Scans of the electrostatic analyzer (ESA) across the precursor ion beam in reverse-geometry (BE) mass spectrometers that are operated under double-focusing conditions do not measure the "energy resolution of the main beam": They only measure double-focusing resolution. The only way that ESA scans can measure the kinetic energy distribution of the main beam is to operate the instrument so that angular (directional) focusing is not achieved. Thus, the mass spectrometer is no longer double-focusing. Under double-focusing conditions, however, scans of the accelerating voltage while the magnetic field and ESA are held constant can be used to measure either the kinetic energy distribution of the main beam that enters the magnet or the energy-resolving power of the instrument. Scans at a constant ratio of B(2)/E can be used similarly. The energy-resolving power of any ESA is defined by its dispersion and the widths of the energy-resolving object and image slits that immediately precede and follow the ESA, respectively. The use of BE, EB, and triple-sector instruments to measure energy-resolving power and the kinetic energy distribution of the precursor ion main beam is compared and discussed.

Metastable ion study of organosilicon compounds VIII. Dimethoxydiphenylsilane

The spontaneous unimolecular dissociation reaction of the molecular ion of dimethoxydiphenylsilane ( 1) has been investigated by mass-analyzed ion kinetic energy spectroscopy, collision-induced dissociation, a D-labeling study and high resolution data. The results are compared with those of the corresponding carbon analogue, dimethoxydiphenylmethane ( 2). The fragmentation of the metastable 1 .+ is more complex than that of 2 .+. The latter eliminates the methoxy radical only, whereas the former eliminates methanol and benzene molecules, and C 7H 7 radical in addition to the formation of the molecular ion of biphenyl ( 3). The intensity of the [MC 6H 5] + ion at m/ z = 167 is much larger than that of [MOCH 3] + ion at m/ z = 213 in the normal mass spectrum of 1. On the contrary, in the case of 2, the intensity of [MC 6H 5] + ions is smaller than that of [MOCH 3] 3+ ions.

Characterization and differentiation of heterocyclic isomers. Tandem mass spectrometry and molecular orbital calculations on 3-methylisoxazolo- and 2-methyloxazolopyridines

Metastable mass-analyzed ion kinetic energy (MIKE) and collision-induced dissociation MIKE spectrometries have been applied to the study of all members of two classes of heteroaromatic isomers: 3-methylisoxazolo-and 2-methyloxazolopyridines. The study revealed that tandem mass spectrometry can characterize and differentiate the isomeric ion structures produced by these heterocycles. In particular, the MIKE spectra of both the molecular ions and abundant fragments formed by CO and CH3CN losses show characteristic differences that allow distinction among the isomers dependent on the position of the nitrogen atom in the pyridine ring, and distinction of isoxazole derivatives from oxazoles. The results indicate that the isomerization of the isoxazole moiety to oxazole-proposed for other analogous compounds-does not occur in these heterocyclic systems. The experimental work is supported by molecular orbital calculations both on neutral molecules and on molecular and fragment ions.

Fragmentation of organosulfur compounds upon electron impact: 2-mercaptoethanol and 1,2-ethanedithiol

The spontaneous unimolecular dissociations of 2-mercaptoethanol ( 1) and 1,2-ethanedithiol ( 2) have been investigated by mass-analyzed ion kinetic energy (MIKE) spectra, collision-induced dissociation (CID) spectra and deuterium labeling. Most of the fragmentation pathways of 1 and 2 are similar to those of ethylene glycol ( 3), but there are exceptions. The results are rationalized by thermochemical considerations. Protonated methanethiol (CH 3SH 2 +; m/ z 49) is produced from 1 and 2, and protonated methanol (CH 3OH 2 +; m/ z 33) is also generated from 1. The latter is a minor route in comparison with the case of 3. The m/z 49 ion contains both hydroxyl and mercapto hydrogens of 1 +. The former is contained in the methyl group, and one of the β-methylene hydrogens is bonded to the sulfur atom.

On the use of scans at a constant ratio of B/E for studying decompositions of peptide metal(II)-ion complexes formed by electrospray ionization

The use of sector mass spectrometers to study metastable ion decompositions of peptide metal-ion complexes formed by electrospray ionization is discussed. Products that are formed by charge-separation reactions are characterized by large kinetic energy release distributions. This causes scans at a constant B/E to give incorrect product ion abundances and possibly incorrect mass assignments. Two instrumental methods exist that can be used either to detect the ions or to estimate relative ion abundances: a floated collision cell or mass-analyzed ion kinetic energy spectrometry (MIKES) scans. The floated collision cell, by virtue of an altered B/E scan law, however, discriminates against important metastable ion reactions that occur outside the cell. MIKES scans provide a clearer estimate of product ions that arise by metastable ion charge-separation reactions. Problems with pseudotandem (first field-free region) experiments are also discussed.

Reinvestigation of the photodissociation kinetics of m-iodotoluene molecular ion

Photodissociation kinetics of the m-iodotoluene molecular ion has been investigated on a nanosecond time scale by mass-analyzed ion kinetic energy spectrometry. Photodissociation rate constants have been determined at molecular ion internal energies of 3.7–4.3 eV. The influence of the collisional relaxation of the molecular ion occurring in the ion source on the rate constant has been corrected, which was not considered in the previous photodissociation experiment. The kinetic energy release distributions (KERDs) determined from the ion kinetic energy profiles are composite due to the participation of two competing reaction channels, one leading to the formation of m-tolyl ion and the other to tropylium ion. By separating these bimodal KERDs, the branching ratios for the two channels have been obtained. From the overall rate constants and the branching ratios, rate constants for each channel have been determined on a nanosecond time scale. It has been found that the rate-energy data for each channel can be explained adequately by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The production of m-tolyl ion proceeds via a loose transition state while that of tropylium ion proceeds via a tight transition state. Refined experimental data together with the microsecond data reported by Lin and Dunbar have been found to be essential to arrive at the correct mechanistic pictures for these reactions

Radical cation adduct formation between dihydroxybenzenes and ammonia under CI conditions

AbstractUnder ammonia chemical ionization conditions, instead of the expected [M + H]+, and [M + NH4]+ and [M + N2H7]+ ions, hydroquinone gives rise to M+·, [M + NH3]+· and [M + 2NH3]+· ions, whereas resorcinol gives both series of ions. Catechol shows M+·, [M + NH3]+·, [M + NH4]+ and [M + N2H7]+ ions. The favourable ionization energy difference between ammonia and the dihydroxybenzenes leads to charge exchange and the resulting radical cations, being highly acidic, form clusters with the NH3. The effect of ion source parameters and mass‐analysed ion kinetic energy (MIKE) and collisional activation spectra confirm proton‐bound structures for [M + NH3]+· and [M + 2NH3]+ ions. Because of the proximity of the two OH groups, NH4+ attachment is favoured in catechol. The MIKE spectra of [M + H + Et2O]+ ions show that resorcinol has the highest proton affinity among the dihydroxybenzenes.

Fragmentation of the metastable molecular ion of methyl lactate: The formation of oxygen-protonated methanol [CH 3OH 2+ involving double hydrogen atom transfers

The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1(+·) decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ĊH3, CO, CH3ĊO, and ĊOOCH3, respectively. Double hydrogen atom transfer occurs in the third reaction.The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH3OH 2 (+) ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 (+·) are present in the resultant protonated methanols.

Determination of artefact peaks in mass‐analysed ion kinetic energy spectra using a new linked scan for ebe geometry magnetic sector instruments

AbstractThe possible presence of artefact peaks in mass‐analyzed ion kinetic energy (MIKE) spectra has been recognized when such scans are performed using a two‐sector instrument of reverse (BE) configuration. Since the presence of an artefact peak can be misleading when interpreting a spectrum, a new scan is proposed which allows these artefacts to be identified. This scan involves linking and scanning the two electrostatic sectors of an E1BE2 geometry instrument, while maintaining the magnet at the mass selected for the MIKE scan. This selected mass‐analyzed ion kinetic energy (SMIKE) scan at E1=E2 results in a spectrum containing only artefact peaks that can be present in the corresponding MIKE spectrum.

A study of protonated molecules of crown ethers containing two thiourea segments by collision‐induced dissociation mass analysed ion kinetic energy spectrometry

AbstractFast atom/ion bombardment mass spectrometry was used to investigate the structures of protonated molecules of macrocyclic nitrogen‐containing crown ethers with two polyoxyethylene chains linked by two thiourea groups. The collision‐induced dissociation (CID) of their MH+ ions, using the mass analysed ion kinetic energy (MIKE) spectrometry method reveals four clearly defined competitive fragmentations. The product‐ion intensities clearly show that, in the MH+ ions of asymmetric crown ethers, the proton is preferentially located in the region of the longer polyoxyethylene chain. The characteristics of CID MIKE spectra of isomeric crown ethers are discussed.

Mass spectrometry in stereochemical problems: the case of 2- hydroxy-4t-butylcyclopentylcarboxylic acids

The mass spectrometric behaviour of four diastereoisomeric 2-hydroxy-4-t-butylcyclopentyl carboxylic acids has been investigated by means of electron impact ionisation experiments, mass analysed ion kinetic energy spectroscopy and ion trap mass spectrometry. The comparison of the data obtained by these different techniques allowed the unambiguous differentiation of the four compounds.

Rearrangement by intermediate ion/neutral complexes during the McLafferty fragmentation of unsaturated ketones

The fragmentation of 1-phenylhex-5-ene-1-one 1 was studied by mass analyzed ion kinetic energy spectrometry of its deuterium and ''carbon labelled derivatives as a further example of interactions within ion/neutral complexes, The analysis of the MIKE spectra demonstrate two important reaction channels of metastable molecular ions, The first one is a McLafferty rearrangement by elimination of a 1,3-butadiene molecule, The second energetically even more favoured fragmentation corresponds to the loss of an ethyl radical. The MIKE spectra of labelled molecular ions, especially that of the (13)carbon labelled compound, indicate a skeletal rearrangement specifically of the butenyl moiety of the alkyl side chain by interchanging the terminal carbon atoms C-6 and C-5 with the inner C atoms C-3 and C-4 prior to the loss of the ethyl radical, This specific skeletal rearrangement is explained by an internal return within an intermediate ion/neutral complex f, consisting of the acetophenone enol radical cation and the neutral butadiene, by addition of the enol radical cation to the remote double bond of the diene, The second reaction channel is the irreversible cyclization of an intermediate distonic ion c leading to the formation of an ionized ethyl-alpha-dihydronaphthol d. This ion is stabilized by loss of an ethyl radical resulting in protonated alpha-naphthol e.