Ion Kinetic Energy Spectrometry Research Articles

Reactivity of the Ethenium Cation (C2H5+) with Ethyne (C2H2): A Combined Experimental and Theoretical Study.

The gas-phase reaction between the ethyl cation (C2H5+) and ethyne (C2H2) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C2H5Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C2H5+, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C4H7]+ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C3H3++CH4, with BR = 0.76±0.05 and (b) C4H5++H2, with BR = 0.22±0.02. A third channel giving C2H3+ in association with C2H4 is shown to emerge at both high internal excitation of C2H5+ and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH3CH2CHCH+, from which barrierless and exothermic pathways exist for the production of both cyclic c-C3H3+ and linear H2CCCH+ isomers of the main product channel. For the minor C4H5+ product, two isomers are energetically accessible: the three-member cyclic isomer c-C3H2(CH3)+ and the higher energy linear structure CH2CHCCH2+, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).

Probing ultracold chemistry using ion spectrometry.

Rapid progress in atomic, molecular, and optical (AMO) physics techniques enabled the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. In particular, both the external and internal quantum degrees of freedom of the reactant atoms and molecules are controlled, allowing studies that explored the role of the long-range potential in ultracold reactions. The kinetics of these reactions have typically been determined using the loss of reactants as proxies. To extend such studies into the short-range, we developed an experimental apparatus that combines the production of quantum-state-selected ultracold KRb molecules with ion mass and kinetic energy spectrometry, and directly observed KRb + KRb reaction intermediates and products [M.-G. Hu and Y. Liu, et al., Science, 2019, 366, 1111]. Here, we present the apparatus in detail. For future studies that aim for detecting the quantum states of the reaction products, we demonstrate a photodissociation based scheme to calibrate the ion kinetic energy spectrometer at low energies.

Mass spectrometry evidence for self-rigidification of π-conjugated oligomers containing 3,4-ethylenedioxythiophene groups using RRKM theory and internal energy calibration.

The self-rigidification of ionized π-conjugated systems based on two combinations of thiophene (T) and 3,4-Ethylenedioxythiophene (E) is investigated using mass-analyzed ion kinetic energy spectrometry (MIKES) of ions produced from electron impact ionization at 70 eV. The m/z 446 radical cations of the two isomers ETTE and TEET lead to detect m/z 418 and 390 daughter ions. The MIKE spectra differ only by the intensities of these fragment ions. As the m/z 418 daughter ion is produced through a same retro-Diels Alder reaction whatever the fragmenting isomer, the difference in daughter ion intensities is interpreted in term of unimolecular dissociation rate constants ( k( Eint)) ratios. Considering that the transition state (TS) of such reaction is attributed to a quinoid form, equivalent vibration modes are assumed for the TS of both dissociating ETTE and TEET radical cations. As a result, by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the difference in daughter ion intensities is interpreted by considering that the fragmenting ion is more or less ordered in its ground state than at the transition state, resulting from the influence of the number of the S…O interactions in the planarization of the TEET ion toward the ETTE charged species. The comparison of this behavior in MIKES experiments is supported by the modeling of ion behavior in mass spectrometer and the calibration in internal energy of the radical cations produced in an EI source.

Gas-Phase Ion Chemistry and Organic Chemistry: The Story of a Hybrid Six-Sector Mass Spectrometer—The “AutoSpec 6F”

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.

Site-specific hydrogen exchange and hydrogen transfer processes preceding the fragmentation of long-lived radical cations of ethyl dihydrocinnamate and related arylalkanoates

An electron ionisation study on the fragmentation of metastable molecular radical cations of ethyl 3-phenylpropanoate (ethyl dihydrocinnamate) and related arylalkanoic acid esters was performed by mass-analysed ion kinetic energy (MIKE) spectrometry. Six deuterium-labelled isotopomers of ethyl dihydrocinnamate were synthesised and studied by MIKE spectrometry. The fragmentation leading to ions C7H7O+ (m/z 107) involving migration of the alkoxycarbonyl group was also observed in the 70-eV mass spectra of related alkyl dihydrocinnamates, but it was found to be a high-energy process that does not compete at low energies in metastable molecular ions. Instead, the metastable ions of ethyl dihydrocinnamate undergo competing losses of carbon monoxide, ethanol and the combined loss of these neutral fragments, giving ionised styrene, C8H8•+ (m/z 104). A highly specific H/D interchange involving the four hydrogen atoms at the benzylic α- and ortho-positions was found to precede the losses of ethanol and [ethanol+CO]. This represents another striking case of complete 4H – scrambling that enables the molecular ion to fully equilibrate the interchanging hydrogen atoms prior to fragmentation. A mechanism rationalising these observations and extending previously suggested mechanisms is proposed involving a series of distonic ions and the intermediacy of an ion/neutral complex. The metastable ions of the related esters exhibit in part similar fragmentation behaviour, but the McLafferty reaction turns out to be more favourable with higher alkyl dihydrocinnamates. For example, n-propyl 3-phenylpropanoate and isopropyl 3-phenylpropanoate react through highly distinct fragmentation channels.

ChemInform Abstract: Electron Impact Induced Synthesis of Benzo(2,1-b:3,4-b′)dithiophene.

The photochemical behaviour of cis and trans 1,2-bis(2-bromo-3-thienyl)ethene has been separately studied with respect to cyclization to benzo[2,1-b:3,4-b′]dithiophene (BDT). The gc/ms analysis of the mother liquors of the two isomers photoreaction shows the opposite direction of isomerization followed by production of intractable materials resulting by the radical breaking of one of the two C-Br bonds on the thiophene moiety. On the contrary, in EI mass spectrometry both the isomers give rise to fragment ions at m/z 190 studied and compared to BDT with the aid of mass analysed ion kinetic energy spectrometry and collisional spectroscopy.

Mass Spectral Fragmentation of ( S, S)- N, N′-Bis [1-(hydroxymethyl) alkyl] anthracene/Naphthalene-1, 8-dicarboxamides under Electron Impact Ionization Conditions

The mass spectrometric fragmentation of ( S, S)- N, N′-bis[1-(hydroxymethyl) alkyl] anthracene/naphthalene-1, 8-dicarboxamides was investigated with the aid of mass-analyzed ion kinetic energy spectrometry and the elemental compositions of important fragment ions were determined by accurate mass measurement under electron impact ionization conditions. All the compounds could eliminate formaldehyde. The [M-CH 2O] ions could also eliminate imine, aziridine, aziridinone, 2-amonoalkan-1-ol, water, and other fragments. Several cyclizations were observed under electron impact ionization.

Mass Spectrometric Fragmentation of 1, 8-Bis[4-substituted-(oxazolin-2-yl)] anthracenes: Ring-contraction Rearrangement of Oxazoline Ring under Electron-Impact Ionization Conditions

The mass spectrometric fragmentation of 1, 8-bis[4-substituted(oxazolin-2-yl)] anthracenes (AnBOXes) was investigated by using mass-analyzed ion kinetic energy spectrometry under electron-impact ionization conditions. All the compounds could undergo a ring contraction by the loss of an RCH == CH 2 molecule to form 1-oxazirinyl-8-(oxazolin-2-yl) anthracene ions, which could further lose an R radical, followed by the loss of oxazoline. These ions could also eliminate an R radical and subsequently undergo ring-contraction rearrangements to lose a CHCH 2O fragment and an epoxide molecule, respectively. The ions that are formed could further lose CHCH 2O and CH 2 ==CH fragments. Several ring-contraction rearrangements of the oxazoline ring were observed under electron-impact ionization conditions.

Comparative Studies on Mass Spectrometric Fragmentation of Linear Chiral Secondary Alcohols ( R)-1-(4-Alkylphenyl) and ( R)-1-(4-Alkoxyphenyl/Alkylthiophenyl) Alcohols

Mass spectrometric behaviour of (R)-1-(4-alkylphenyl) alcohols, 1-(4-alkoxylphenyl) alcohols, and 1-(4-alkylthiophenyl) alcohols were studied with the aid of mass-analyzed ion kinetic energy spectrometry under electron impact ionization. All the title compounds show a tendency to eliminate a water molecule to form alkene ions and undergo an a-cleavage to produce protonated aldehyde ions by the loss of alkyl radicals. Except these two common fragment ions, they also show some different fragmentations due to with or without oxy/thioether-linkage.

Kinetic Energy Release in the Loss of Water from n-Propanol Cation

Unimolecular dissociations of ionized aliphatic alcohols have been investigated extensively. Interestingly, competition between loss of hydrogen atom and water is quite different for lower aliphatic alcohol cations with different chain lengths. Loss of H dominates in the dissociations of methanol and ethanol cations, while loss of H2O does in those of n-propanol and n-butanol cations. Theoretical studies agree that a H atom of the methyl group migrates to the O atom of the hydroxyl group prior to loss of H2O from these alcohol ions. In a recent computational study, the energy barriers for the H migration in the H2O loss were compared. The barrier decreases as the aliphatic chain increases. Very recently, we proposed a potential energy surface (PES) for the loss of H2O and H from n-propanol ion by density functional theory (DFT) molecular orbital calculations. Initially CH3CH2CH2OH +• isomerizes to CH2CH2CH2OH2 + via a five centered transition state (1,4-H shift). This is the rat-determining step in the H2O loss. CH2CH2CH2OH2 + undergoes further isomerizations to ion-dipole complexes, c-C3H6 •••H2O prior to loss of H2O. Based on the obtained PES, the experimental results of the metastable dissociation such as the branching ratio and the isotopic effect on the H loss could be well interpreted by the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The kinetic energy release (KER), however, has not been investigated in detail, which provides valuable information on the exit channel in dissociation. In this work, the KER distribution (KERD) in the H2O loss from metastable npropanol cation has been investigated using mass-analyzed ion kinetic energy (MIKE) spectrometry. The nature of the dissociation is discussed by comparing the experimental result with statistical phase space theory (PST) calculations.

Dissociation Mechanism of HNIW Ions Investigated by Chemical Ionization and Electron Impact Mass Spectroscopy

AbstractChemical Ionization (CI) with Collision‐Induced Dissociation (CID) spectroscopy and Electron Impacting (EI) with metastable Mass analyzed Ion Kinetic Energy (MIKE) spectroscopy have been applied to study ionic dissociations of Hexanitrohexaazaisowurtzitane (HNIW). Similarities and differences between EI/MIKE and CI/CID mass spectra of HNIW were analyzed. In EI mass spectra, the ions [HNIW−n NO2]+ (n=2–5), such as the ion at m/z 347, were less frequent (1–2% relative abundance), but in CI mass spectra, these ions were very abundant. For some ions of large molar mass from HNIW, their dissociations pathways from parent ions to daughter ions were built according to CID and MIKE spectra. Molecular ions of HNIW with a protonated nitro group at five‐member ring seem more stable than at six‐member ring. The HNIW ions losing five of six nitro groups are very stable based on CID spectra, which agrees with some research results for thermal decomposition of HNIW in literature.

Tunneling in the Loss of Hydrogen Chloride from Isopropyl Chloride Cation

The unimolecular dissociation of isopropyl chloride cation has been investigated using mass-analyzed ion kinetic energy spectrometry. The C3H6*+ ion was the only product ion in the metastable dissociation. The kinetic energy release distribution for the HCl loss was determined. Ab initio molecular orbital calculations were performed at the MP2/6-311++G(d,p) level together with single point energy calculations at the QCSID(T)/6-311++G(2d,2p) level. The calculations show that the molecular ion rearranges to an ion-dipole complex prior to loss of HCl via a transition state containing a four-membered ring. The rearrangement involves H atom transfer. On the basis of the potential energy surface obtained for the loss of HCl and Cl*, the rate constants were calculated by transition-state statistical theories with considering tunneling effect. From the calculated result, it is proposed that the observed HCl loss would occur via tunneling through the barrier for isomerization to the ion-dipole complex, CH3CHCH2*+...HCl.

Kinetic Energy Release of C70+ and Its Endohedral Cation N@C70+: Activation Energy for N Extrusion

Unimolecular decomposition of C70(+) and its endohedral cation N@C70(+) were studied by high-resolution mass-analyzed ion kinetic energy (MIKE) spectrometry. Information on the energetics and dynamics of these reactions was extracted. C70(+) dissociates unimolecularly by loss of a C2 unit, whereas N@C70(+) expels the endohedral N atom. Kinetic energy release distributions (KERDs) in these reactions were measured. By use of finite heat bath theory (FHBT), the binding energy for C2 emission from C70(+) and the activation energy for N elimination from N@C70(+) were deduced from KERDs in the light of a recent finding that fragmentation of fullerene cations proceeds via a very loose transition state. The activation energy measured for N extrusion from N@C70(+) was found to be lower than that for C2 evaporation, higher than the value from its neutral molecule N@C70 obtained on the basis of thermal stability measurements, and coincident with the theoretical value. The results provide confirmation that the proposed extrusion mechanism in which the N atom escapes from the cage via formation of an aza-bridged intermediate is correct.

Mass Spectrometric Fragmentation of 2a,4-Disubstituted 2,2a,3,4-Tetrahydro-2-phenyl-1H-azeto[2,1-d]- [1,5]benzothiazepin-1-ones Under Electron Impact Ionization Conditions and Comparison with their 2-Heteroatom Substituted Analogues

The mass spectrometric behavior of seven 2a,4-disubstituted 2,2a,3,4-tetrahydro-2-phenyl-1H-azeto[2,1-d][1,5]benzothiazepin-1-ones has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate a phenylketene molecule, an ethyl or benzyl radical, or a phenylketene molecule plus an SH radical. The mass spectrometric behavior of the title compounds was compared to those of 2-heteroatom-substituted analogues.

Methyl Migration in the Metastably Decomposing Acetylphenyl Ions, CH3COC6H4+ (m/z 119), Followed by the Loss of CO

The metastable ion dissociations of o-, m-, and p-acetylphenyl ions, CH3COC6H4+ (m/z 119), generated upon electron ionization from o-, m-, and p-diacetylbenzenes (CH3COC6H4COCH3, Mw : 162, 1-3), have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry in conjunction with thermochemistry. All these m/z 119 ions, which are generated by the consecutive losses of CH3 and CO from 1+·-3+·, dissociate to only the ion at m/z 91 by the loss of CO. These suggest that in the CH3COC6H4+ ions, the CH3 migration from the acetyl group to the benzene ring occurs prior to the loss of CO. The amounts of the kinetic energy release (KER) accompanying with the dissociation of the m/z 119 ions from compounds 1-3 to m/z 91 ion were larger than those of the corresponding, respective methylbenzoyl ions. In the m-, and p-CH3COC6H4+ ions, the charge migrates from the incipient charge site to the ortho position via the so-called hydrogen “ring-walk.” Then CH3 migrates from the acetyl group to the charge site at the ortho position in the benzene ring prior to the loss of CO. A part of the m/z 91 ions from o-, m-, and p-acetylphenyl ions may be generated via the migration of CH3 to the incipient charge site, followed by the loss of CO.

Ｎａｎｏ‐ＥＳＩプローブの開発と高分解能測定法およびＭＩＫＥスペクトル法を用いた有機イオンの分解機構に関する研究

A new technique is described for accurate mass measurement by using multiple sprayers nano electrospray ionization mass spectrometry (nano-ESI/MS). A nano-scale high-performance liquid chromatography (nano-LC) was combined with a magnetic sector instrument. A metal-coated glass capillary was used for the introduction of the reference compound solution, and a metal-coated fused silica capillary was used to connect the nano-LC column to ionization chamber. By shifting the position of the each sprayers to the sampling orifice, the spectra were obtained for the sample compounds eluted from the column and the reference one. Several standard compounds were examined by using this technique and satisfactory accurate masses were obtained. It was not problems for the differences between the ionization efficiency of the sample and the reference compounds. Studies on the unimolecular metastable ion decomposition of several organic compounds are also described by using high-resolution mass spectrometry, MIKE (mass-analyzed ion kinetic energy) spectrometry and D-labeling technique. The m/z 45 ions generated from 2-methoxyethylamine (CH3OCH2CH2NH2) consist of two isobaric ions, methoxymethyl ion, CH3OCH2+, and C2H7N+. The m/z 69 ions generated from methyl trifluoroacetoacetate (CF3COCH2COOCH3) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (CF3COCH2COCF3) also consist of two isobaric ions, trifluoromethyl ion, CF3+, and protonated carbon suboxide, OCCH+CO.

Formation of C 6H nSi + ( n = 5–8) from phenylsilane: an experimental and theoretical study

The metastable ion decomposition (MID) spectra of C 6H n Si + ( n = 6–8) generated by electron ionization of phenylsilane were obtained using mass-analyzed ion kinetic energy spectrometry (MIKES). H and H 2 losses were the main channels in the MID of C 6H 8Si + ( 1). The dominant product ion in the MIDs of C 6H 7Si + ( 2) and C 6H 6Si + ( 3) was C 6H 5Si + ( 4) formed by H 2 and H losses, respectively. Density functional theory calculations were performed to investigate isomerizations and dissociations of 1– 3 at the B3LYP/6-311++G(d,p) level. RRKM model calculations were performed to understand their dissociation kinetics. These theoretical results predict that the consecutive dissociation 1 → 3 (+ H 2) → 4 + H dominates at low energies, while 1 → 2 (+ H ) → 4 + H 2 at high energies. It has been found that several isomers of 1– 3 including ion–molecule complexes, C 6H 6·SiH x + ( x = 0–2), play important roles in the dissociation of 1. Reaction mechanisms are proposed for the formation of several isomers of 1– 4 from the phenylsilane molecular ion.

Isomerization and dissociation of the acetonitrile molecular cation

The unimolecular dissociation of the acetonitrile molecular cation (CH 3CN +) has been investigated using mass-analyzed ion kinetic energy spectrometry. Kinetic energy release distribution was obtained for the H loss. Density functional theory calculations have been performed to investigate isomerization and dissociation of CH 3CN +. The potential energy surface (PES) for the H loss has been constructed from the calculations at the UB3LYP/6-311++G(3df,3pd) level. The RRKM model calculations based on the obtained PES predict that CH 3CN + interconverts to CH 2CNH + and CH 2NCH + followed by dissociation to a cyclic C 2H 2N + via a cyclic C 2H 3N + intermediate near the threshold. The present and previous experimental data for the dissociations of C 2H 3N + isomers are interpreted with the theoretical results.

Mass spectral fragmentation of 2a,4-disubstituted 2,2a,3,4-tetrahydro-2-phenoxy-1H-azeto[2,1-d][1,5]benzothiazepin-1-ones under electron impact ionization conditions.

The mass spectrometric fragmentation of 2a,4-disubstituted 2,2a,3,4-tetrahydro-2-phenoxy-1H-azeto[2,1-d][1,5]benzothiazepin-1-ones has been investigated using the aid of mass-analyzed ion kinetic energy spectrometry. All compounds tend to eliminate a phenoxy, phenol, and phenoxyketene, respectively, from molecular ions. [M(+)-PhO] ions could also form [M(+)-PhOH] ions, which could further lose alkenes. [M(+)-PhOCH=C=O] ions could further yield 2-arylbenzothiazole ions and other fragment ions.

Unimolecular Reactions of Diethyl Malonate Cation in Gas-phase

Unimolecular reactions of diethyl malonate cation (CH3CH2OC(=O)CH2C(=O)OCH2CH3+· ; 1+·, Mw : 160) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemistry. In the metastable time window, the molecular ions 1+· lead not only to the formation of the major fragment ions m/z 133 ([M-C2H3]+) through a McLafferty + 1 rearrangement, but also to two minor fragment ions m/z 132 ([M-C2H4]+·) through a McLafferty rearrangement (or [M-CO]+·) and m/z 88 ([M-C3H4O2]+·). These dissociations are rationalized by the idea of ion-neutral complex. The m/z 88 ions are generated via at least two different routes. The m/z 133 ([M-C2H3]+) ions decompose into the ions at m/z 115, 106, and 105 by the losses of H2O, C2H3, and C2H4, respectively. A large part of the first fragment ions would be in the keto form and it decomposes into the m/z 71 and 43 ions by the losses of CO2 and C3H4O2 (or C2O3), and a small part would be in the enol form and decomposes into the m/z 87 ions by the loss of C2H4, not CO. The m/z 43 ions are isobaric, C2H3O+ and C3H7+.