Mass-analyzed Ion Kinetic Energy Research Articles

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.

Single, double and triple ionization of tetraphenyl iron(III) porphyrin chloride

Tetraphenyl iron(III) porphyrin chloride (FeTPPCl) cations are generated in the gas phase by electron impact ionization. The ionization is accompanied by extensive fragmentation as well as formation of doubly and triply charged ions. The most prominent fragments are analyzed and identified by fitting with calculated natural isotope patterns. Appearance energies of the most abundant singly and doubly charged product ions are determined. For the singly charged parent ions FeTPPCl +, CuTPP + and the fragment ion FeTPP + we obtain a value of 9.7 ± 0.5 eV which is about 3 eV higher than the value published for photo ionization of FeTPPCl. The appearance energy of the doubly charged ion FeTPP 2+ is obtained to be 18 eV. The additional loss of one or two phenyl groups requires between 10 and 14 eV more for singly and doubly charged ions. Also, the metastable decay of singly and doubly charged ions is investigated with the mass analyzed ion kinetic energy (MIKE) scan technique, performed on a three sector field mass spectrometer (BEE-geometry). In the mass spectrum and the MIKE scans a strongly reduced stability of the porphyrin ions is observed with increasing charge state.

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.

Study of Hexanitrohexaazaisowurtzitane Ion Dissociation Mechanisms Based on Mass-Analyzed Ion Kinetic Energy Spectrum

The ion dissociation pathways of hexanitrohexaazaisowurtzitane (HNIW) are studied in this paper. A mass spectrometer with mass-analyzed ion kinetic energy spectrum is used for studying ions produced by electron impacts onHNIW. Dissociation processes from various precursor ions to product ions are proposed. Fifteen fragments including the largest m/z 392 and the smallest m/z 67 and m/z 54 are discussed. Dissociation of HNIW precursor ions with NO 2 was found to produce product ions with removal of NO 2 or NO molecules through β-inducement or α-inducement effects. HNIW framework precursor ions produce ions at (m-27)/z through losing HCN and ions at (m-1)/z through losing H atoms. Possible structures of HNIW-NO 2 ions and framework ions are suggested.

Unimolecular Dissociations of C70+ and Its Noble Gas Endohedral Cations Ne@C70+ and Ar@C70+: Cage-Binding Energies for C2 Loss

The energetics and dynamics of unimolecular decompositions of C70+ and its noble gas endohedral cations, Ne@C70+ and Ar@C70+, have been studied using tandem mass spectrometry techniques. The high-resolution mass-analyzed ion kinetic energy (HR-MIKE) spectra for the unimolecular reactions of C70+, Ne@CC70+, and Ar@C70+ were recorded by scanning the electrostatic analyzer and using single-ion counting that was achieved by combination of an electron multiplier, amplifier/discriminator, and multichannel analyzer. These cations dissociate unimolecularly via loss of a C2 unit, and no endohedral atom is observed as fragment. The activation energies for C2 evaporation from Ne@C70+ and Ar@C70+ are lower than those for elimination of the endohedral noble gas atoms. The kinetic energy release distributions (KERDs) for the C2 evaporation have been measured and, by use of the finite heat bath theory (FHBT), the binding energies for the C2 emission have been deduced from the KERDs. The C2 evaporation energies increase in the order DeltaEvap(C70+) < DeltaEvap(Ne@C70+) < DeltaEvap(Ar@C70+), but no big difference in the cage binding was observed for C70+, Ne@C70+, and Ar@C70+, indicating incorporations of the Ne and Ar atoms into C70 contribute a little to the stability of C70 toward C2 loss, which is in good agreement with theoretical calculations but contrasts with the findings in their C60 analogues and in metallofullerenes that the decay energies of the filled fullerenes are much higher than those of the corresponding empty cages.

Investigation on Mass Spectral Fragmentation Mechanism of C2‐Symmetric Chiral Bis(oxazoline) and Bis(thiazoline)

AbstractThe electron impact mass spectra of ten new C2‐symmetric chiral bis(oxazoline) and bis(thiazoline) have been studied. Bis(thiazoline) and bis(oxazoline) possess the same fragmentation mechanism under EI conditions. An unusual fragmentation pathway has been found in the compounds studied. Due to the presence of phenyl group, compounds 6 and 10 undergo a new fragmentation pathway except for the common way as the other eight compounds. Mass analyzed ion kinetic energy spectra experiments and high resolution accurate mass measurement were conducted to confirm the proposed fragmentation pathways.

Energetics, kinetics and dynamics of decaying metastable ions studied with a high-resolution three-sector field mass spectrometer

Mass spectrometric analysis of metastable decay reactions is devoted to the measurements of the kinetic energy release distribution (KERD) for the decay of singly charged rare gas dimer ions and , the doubly charged acetylene parent ion and the singly and doubly charged SF6 fragment ions, like for example , and . The KERDs are obtained either from high-resolution mass analysed ion kinetic energy spectra or the measurement of ion beam profiles using a specially improved mass spectrometric system. Due to the high energy resolution measurements and theoretical studies based on ab initio calculations of potential energy curves it is possible to assign the reaction products of the rare gas dimer decays to electronic transitions in the excited parent ion. The and also the ions are investigated because of obscurities in the production of their fragment ions. The unusual shape of the ionization cross section indicates that at sufficiently high electron energies the fragmentation channel of doubly charged contributes significantly to the ion yield. Additional measurements of the corresponding appearance energies confirm the existence of this second production channel.

Coulomb explosion of [formula omitted] ions produced by electron impact ionization of toluene

Doubly charged C 7 H 7 2 + ions produced by electron impact ionization of toluene (C 7H 8) show metastable unimolecular dissociation into two singly charged fragments. A high-resolution double focusing mass spectrometer (reversed Nier–Johnson type BE geometry) was used to monitor these decomposition reactions. We have identified four competing charge separation channels. The mass-analyzed ion kinetic energy (MIKE) technique was used to determine mean values of the kinetic energy release (KER). We obtained values of the average KER for the charge separation reactions of C 7 H 7 2 + ranging from 2.5 to 3.5 eV.

Ne2+[II (1/2)u]: radiative decay and electronic predissociation

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

Synthesis and characterization of zinc bis(O-isopropylxanthate) as a single-source chemical vapor deposition precursor for ZnS

The utilization of single-source molecular precursors in chemical vapor deposition (CVD) experiments requires a deep knowledge of their chemico-physical properties, with particular regard to thermal stability and fragmentation pattern. This study describes the synthesis and characterization of zinc bis(O-isopropylxanthate), Zn(O-iPrXan)2, [O-iPrXan = (CH3)2CHOCS2], a single-source precursor for the CVD of zinc(II) sulfide thin films and nanorods. Several analytical methods yielding complementary information (extended X-ray absorption fine structure, Raman, FT-IR, UV–Vis optical absorption, 1H and 13C NMR, thermogravimetric analysis, differential scanning calorimetry as well as mass spectrometry techniques, i.e. electrospray and electron ionization, mass-analyzed ion kinetic energy) are adopted for a comprehensive investigation of purity, structure, thermal behavior and decomposition pathways of the molecule. The most significant results are discussed critically and the properties useful for CVD applications are highlighted. Copyright © 2005 John Wiley & Sons, Ltd.

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.

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.

Unimolecuclar Fragmentation and Isomerization of Gaseous Arsanyl Cations Y—C6H4As+X (X=F, Cl, Br, I). The Role of Non-Classical Nido Structures

The halogeno-(4-halogenophenyl)arsanyl cations Y–C6H4As+X (X=F, Cl, Br, I) are abundant in the 70 eV EI mass spectra of dihalogeno-(4-halogenophenyl)arsanes Y–C6H4AsX2. This has been used to study the unimolecular reactions of ions Y–C6H4As+X by techniques of tandem mass spectrometry with respect to a possible rearrangement to nido-isomers, ñ4-(XYC6H4)As+, in which the As+ is capping a dihalogenobenzene moiety, YC6H4AX. The mass analyzed ion kinetic energy (MIKE) spectra of Y–C6H4As+X, 1a+–3a+ and 5a+ (X, Y=F, Cl, Br, see Scheme 2), display broad and even dish-topped peaks for the loss of HX and HY, respectively, indicating a large kinetic energy release ( KER) during these reactions and the presence of a large reverse activation energy. The mean value of the KER, &lt; T&gt;, is specific for the type of the halogen substituent but independent of its original position and is identical to &lt; T&gt; found for the loss of HX or HY from metastable halogeno–phenylarsanyl cations, C6H5As−X or C6H4As+Y. These results give strong evidence for a positional change of X and Y in Y–C6H4As+X before fragmentation, plausibly via intermediate ñ4-(XYC6H4)As+. However, the relative intensities of the signals for loss of HX and HY are different in the MIKE spectra of isomeric arsanyl cations and the spectra of the isomers obtained by collision-induced decomposition show that the metastable arsanyl cations represent a mixture of isomers with the original arsanyl cation prevailing. Clearly, interchange of halogen substituents in Y–C6H4XAs+ is slow compared with fragmentation and the intermediate nido-isomer, ñ4-(XYC6H4)As+, is separated from the isomers of the classical structure of a halogeno-(halogenophenyl)arsanyl cation by substantial activation barriers. Therefore, in the case of 4a+ and 6a+ (X, Y=Cl, I), any rearrangement is obstructed by the facile loss of the weakly bonded I substituent. This reaction model was verified by theoretical calculations of relevant stationary points of the minimum energy reaction pathways ( MERP) of fragmentation and rearrangement at the level UBHLYP/6-311+g(2d,p)//UBHLYP/6-31+g(d). In particular, the calculations show that the nido isomers, ñ4-(XYC6H4)As+, are stable species but higher in energy than their “classical” isomers.

Mechanisms and dynamics of the metastable decay in Ar2+.

A detailed experimental as well as theoretical investigation of the properties of the metastable dissociation Ar2+ --> Ar+ + Ar is presented. The mass-analyzed ion kinetic energy (MIKE) scan technique has been performed using a three sector field mass spectrometer. The possible mechanisms of the metastability of Ar2+ have been examined and the observed decay process is assigned to the II(1/2)(u)-->I(1/2)(g) bound to continuum radiative transition, in agreement with earlier work. The calculation of the theoretical shape of the kinetic energy release distribution of fragment ions allowed us to construct the theoretical MIKE peak and compare it with the raw experimental data. The accuracy of various sets of potential energy curves for Ar2+ is discussed, as well as the way of production of the metastable Ar2+[II(1/2)(u)] electronic state by electron impact. Excellent agreement between the experimental data and theoretical model has been observed.

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.

Electron ionization mass spectrometry as a tool for the investigation of the ortho effect in fragmentation of some Schiff bases derived from amphetamine analogs

The electron ionization-induced fragmentation patterns of three forensically relevant Schiff bases, originating from the condensation between 2-, 3- and 4-methoxyamphetamine and the corresponding ketones, were studied. The proposed fragmentation routes and ion structures are supported by high-resolution data and B/E linked-scan and mass-analyzed ion kinetic energy spectra. The rationalization of the ortho effect, which is responsible for the formation of the [M-OCH3] fragment in the case of the imine bearing ortho-substituted methoxy group, is given.

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited.

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.