Mass-analyzed Ion Kinetic Energy Research Articles

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.

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined.

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).

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+.

Investigation of 4-(nitrophenylamino)pent-3-en-2-ones and 4-(nitrobenzylamino)pent-3-en-2-ones by electron ionization mass spectrometry. Observation of characteristic ortho effects.

The compounds 4-(phenylamino)pent-3-en-2-ones (1-3) and 4-(benzylamino)pent-3-en-2-ones (4-6) substituted with a nitro group on the aromatic ring were studied by electron ionisation mass spectrometry (EIMS). It was deduced that the compounds 1-3 are converted into the tautomeric 4-(arylimino)pentan-2-one during the EI process. Mass spectrometric decompositions of ortho-substituted derivatives (1 and 4) were found to be different from those observed for meta- and para-isomers. The fragmentation pathways are discussed on the basis of data from linked B/E and B(2)/E scans, mass-analysed ion kinetic energy (MIKE) spectra, accurate mass measurements and isotope labelling.

Fragmentations of some dinitrile anions

The anions formed by deprotonation of heptane-, octane- and nonanedinitriles have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry with and without collisional activation. In addition, high level quantum chemical calculations have been used to model relevant parts of the potential energy surfaces. These models are in good agreement with the experimental findings. The lowest energy fragmentations are due to bond scissions initiated by intramolecular hydrogen rearrangements. The calculations show that formation of the enamine anion which is an intermediate in the Thorpe–Ziegler reaction occurs via a high barrier.

On the chemistry following methoxy migration in the metastably decomposing (M − COOCH 3) + ions ( m/ z 135) from dimethyl phthalate, isophthalate and terephthalate

The metastable ion dissociations of the (M − COOCH 3) + ions ( m/ z 135) generated upon electron ionization from dimethyl phthalate ( 1), isophthalate ( 2) and terephthalate ( 3), have been studied by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. These ions all show as primary fragmentation channels the losses of methyl and carbon monoxide to give the ions m/ z 120 and 107, respectively. The latter ions decompose further by the losses of another molecule of carbon monoxide or a molecule of formaldehyde to generate the ions at m/ z 79 and 77, respectively. An additional dissociation channel is observed for the (M − COOCH 3) + ions from 2 + and 3 +, the loss of methanol to give the ions m/ z 103. The dissociation mechanism of the (M − COOCH 3) + ions from 1 + is shown to be identical to that of the (M − CH 3) + ions from ionized 2-methoxyacetophenone. The latter ions have the 2-methoxybenzoyl cation structure demonstrating a methoxy migration in the (M − COOCH 3) + ions from 1 + precedes their metastable ion dissociations. Part of the (M − COOCH 3) + ions from 2 + and 3 + has also rearranged to the 2-methoxybenzoyl cation structure prior to dissociation, but the significantly enhanced loss of methyl and the loss of methanol from these ions occur from the unrearranged structures as indicated by comparison with the dissociation behavior of the (M − CH 3) + ions from ionized 3- and 4-methoxyacetophenones.

Mass spectral fragmentation of (S,S)-2-substituted 4,4-diphenyl-3,1-oxazabicyclo[3.3.0]octanes: ring contraction of pyrrolidine and 1,3-oxazolidine in mass spectrometry.

The mass spectral behaviour of (S,S)-2-substituted 4,4-diphenyl-3,1-oxazabicyclo[3.3.0]octanes has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under fast atom bombardment (FAB) and electron impact (EI) ionization conditions. Under FAB ionization, all compounds show a tendency to form protonated aldehyde or benzophenone ions and to form protonated 1-azabicyclo[3.1.0]hexane ions, which can further lose an ethylene or cyclopropane from the pyrrolidine ring to produce protonated 1-azabicyclo[1.1.0]butane ions and 3H-azirine ions, respectively. Under EI ionization, a similar fragmentation to that under FAB ionization was observed. The title compounds also show a tendency to yield oxirane ions and oxirenium ions by loss of pyrrolidine and pyrrolidine plus H. Ring contractions of 1,3-oxazolidine by loss of an aldehyde or ketone and of pyrrolidine by loss of an ethylene or cyclopropane were observed under both FAB and EI ionization conditions.

Anisotropic photodissociation of vinyl chloride molecular cation in the ground and first excited electronic states

Photodissociation of the vinyl chloride molecular ion generated by charge exchange or electron impact ionization has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES). The MIKE spectra for the Cl and HCl losses from the vinyl chloride ion have been measured at 357, 488.0, and 514.5 nm using 0 and 90° laser polarization angles. The anisotropy parameters and kinetic energy release distributions have been determined. The vinyl chloride ion in the ground state did not undergo photodissociation in the visible spectral region while the same ion generated by more energetic means displayed fairly strong photodissociation. Presence of the vinyl chloride ion in the very long-lived first excited electronic state reported previously is compatible with this observation. In addition, isotropic dissociation to C 2H 2 + occurring in the ground electronic state was observed. Results from the photoelectron spectroscopy and quantum chemical calculations at the TDDFT/UB3LYP/6-311++G ∗∗ level were used to identify the reaction pathways involved.

The Characterization and Metastable Decomposition of Isobaric Ions from Several Compounds

The characterization and metastable decomposition of isobaric ions from several compounds are described by using high-resolution mass spectrometry (HR-MS) and mass-analyzed ion kinetic energy (MIKE) spectrometry. The m/z 45 ions from 2-methoxyethylamine, CH3OCH2CH2NH2 (1) consists of two isobaric ions, methoxymethyl ion, CH3OCH2+, and C2H7N+ ion. The former decomposes into the m/z 29 ion by losing CH4 in metastable time window. Although the latter decomposes into the m/z 30 and 18 ions, its structure could not be confirmed in this study. The m/z 69 ions generated from methyl trifluoroacetoacetate, CF3COCH2COOCH3 (2) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, CF3COCH2COCF3 (3) also consists of two isobaric ions, trifluoromethyl ion, CF3+, and protonated carbon suboxide, OCCHCO+. The former does not decompose into any ions in the metastable time window. On the other hand, the latter decomposes into m/z 41 ion by losing CO.

MIKEスペクトル法および熱化学的方法による有機イオンの分解と構造に関する研究

It is summarized that the fragmentation and structure of several organic ions, including organosilicon and organofluorine ions, have been investigated by using ion kinetic energy (IKE) spectrometry, mass-analyzed ion kinetic energy (MIKE) spectrometry, D-labeling technique and thermochemistry. Further, several new methods, such as collision energy-dependent collision-induced dissociation (CID) spectra are described.

Unimolecular Gas-phase Reactions of Organosilicon Compounds. Part XV. Acetyltrimethylsilane, CH<sub>3</sub>COSi(CH<sub>3</sub>)<sub>3</sub>

Unimolecular metastable ion decompositions of acetyltrimethyl silane, CH 3 COSi(CH 3 ) 3 (MW: 116, (1)) generated by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometryand D-labeling. In the metastable time window, the molecular ions 1 + decompose exclusively into the ions at m/z 101, [M-CH 3 ] + . A considerable extent of H/D exchange occurs prior to this [M-CH 3 ] + formation. However, the H/D exchange is hardly observed for the [M-CH 3 ] + formation in the ion source. The generated m/z 101 ions from 1 + consist of two isomeric ions. A trimethylsilyl ion, (CH 3 ) 3 Si + , at m/z 73 is generated by at least three different routes.

Studies on Unimolecular Metastable Decomposition of Several Organic Compounds

The unimolecular metastable decomposition mechanism of several organic compounds, which have small molecular weights, was investigated in this study by means of mass-analyzed ion kinetic energy (MIKE) spectrometry, D-labeling technique and molecular orbital (MO) calculations. Accurate mass determination is also described by using a multiple sprayers Nano-ESI arrangement on a double-focusing magnetic sector instrument in this thesis. For example, ‘Unimolecuar HCl Loss from the Molecular Ions of Chlorophenols, a “Ring-walk” Mechanism for a Chlorine Atom’ is described in this abstract. The loss of hydrogen chloride (HCl) is one of the main fragmentation processes in the molecular of o, m-, and p-chlorophenols in the metastable time window. By investigation of D-labeled compounds, it could be established that the eliminated HCl molecule contains only the hydroxyl hydrogen atom, not other benzene ring hydrogen atoms. This process is rationalized by the so-called “ring-walk” mechanism for a chlorine atom.

Unimolecular Gas-Phase Reactions of Diethyl Phthalate, Isophthalate, and Terephthalate upon Electron Ionization

Unimolecular gas-phase reactions of diethyl phthalate (1), isophthalate (2), and terephthalate (3), upon electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labelling. The metastable molecular ions (1)+ decompose to give exclusively the ions m/z 176 ([M – CH3CH2OH]+) and not the ions by the loss of CH3CH2O as proposed earlier in the literature. The metastable molecular ions (2)+ and (3)+ fragment differently from (1)+ and lead not only to the formation of the major fragment ions m/z 194 ([M − CH2CH2]+) via a McLafferty rearrangement but also to minor fragment ions m/z 193 ([M – CH2CH3]+).Yet, molecular ions decomposing in the ion source all show as primary fragmentation channel the loss of CH3CH2O to give the ions at m/z 177, which further dissociate to give the ions at m/z 149 through the loss of C2H4 or CO, indicating the resulting ions are +COC6H4COOH and +C6H4COOCH2CH3. The +COC6H4COOH ions decompose into the m/z 121, 93, and 65 ions by the consecutive losses of three carbon monoxide molecules, respectively. Prior to the second CO loss, a migration of the OH group to the benzene ring occurs. During the metastable fragmentation of the +C6H4COOCH2CH3 ions no ethoxy migration occurs, in contrast to the methoxy migration taking place in the metastable decomposition of the lower homologue +C6H4COOCH3 ions.

Unimolecular Gas-Phase Reactions of Ionized Organo-Silicon Compounds. Part XVI. (CH3)3SiOCH2CH3 and (CH3)3SiOCH2CF3

The unimolecular metastable decompositions of ethoxytrimethylsilane ((CH3)3SiOCH2CH3; 1, MW: 118) and its fluorine analogue, 2,2,2-trifluoroethoxytrimethylsilane ((CH3)3SiOCH2CF3, 2; MW: 172) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. Both molecular ions are formed in low abundance. The fragmentation of 1+ · is different from that of 2+ · , except for the loss of CH3 from the corresponding molecular ions. The MIKE spectra of the [M - CH3]+ ions from 1+ · and 2+ · gave five and three fragment ions, respectively. Two of the five reactions for 1 leading to the formation of the ion at m/z 101, (CH3)2Si+OCH = CH2, and m/z 61, CH3Si+HOH, concern new mechanistic pathways, which were not described in previous reports. Skeletal rearrangement by among others F and CF3 migrations, occurs during the fragmentation of the [M - CH3]+ ions from 2+ · . The m/z 61 ions from 2+ · are both CH2OCF+ (or CH2FCO+ etc.) and CH3Si+HOH.

Reactions between Glycolic Acid and Cu+in the Gas Phase. An Experimental and Theoretical Study

The gas-phase reactions between glycolic acid and Cu+ have been investigated by means of mass spectrometry techniques. The mass-analyzed ion kinetic energy (MIKE) spectrum reveals that the adduct ions [HOCH2COOH−Cu+] decompose spontaneously by losing CH2O2, CO, and H2O. A minor fragmentation corresponding to the loss of H2 is also observed. The structures and bonding characteristics of the different complexes involved in the glycolic acid−Cu+ potential energy surface (PES) have been theoretically studied by density functional theory (DFT) calculations carried out at the B3LYP/6-311+G(2df,2p)//B3LYP/6-31G* level. The attachment of Cu+ to glycolic acid gives rise to two energetically degenerated structures: a chelated form in which Cu+ interacts with the carbonyl oxygen and the hydroxyl oxygen atoms and another one in which the attachment of Cu+ takes place exclusively at the carbonyl oxygen atom. The estimated glycolic acid−Cu+ binding energy is 222 kJ/mol. Different mechanisms for the spontaneous fragmentations observed experimentally are proposed. The main conclusion is that the loss of 46 u should correspond dominantly to the loss of formic acid and in a lower proportion to the loss of CO + H2O.

Fragmentation Mechanism of Trans‐α‐Aryl‐β‐enamino Esters

AbstractElectron impact‐induced fragmentation mechanisms of trans‐α‐aryl‐β‐enamino esters were investigated using mass‐analyzed ion kinetic energy (MIKE) spectrometry and high resolution accurate mass data. It was found that the main characteristic fragmentations of compounds studied were: an odd electron ion M+ ‐ EtOH was formed by losing a neutral molecule of ethanol; and the skeletal rearrangements took place; and the ring opening reaction happened after losing a carbon monoxide; and the typical McLafferty rearrangement underwent in ester group. The cyclization reaction caused by losing neutral molecule of TsNH2 due to the ortho‐effects of substituted group of aromatic ring was also observed.

Loss of methane and ethene from long-lived gaseous xylenium ions (protonated xylene) after “composite” scrambling

The unimolecular fragmentation of long-lived gaseous xylenium ions, CH 3C 6H 5 +CH 3, has been studied in detail using 13 C -labeling, in addition to deuterium labeling, in combination with mass-analyzed ion kinetic energy (MIKE) spectrometry. Metastable xylenium ions generated from the EI-induced loss of COOR (R=H, CH 3) from 1,4-dimethyl-1,4-dihydrobenzoic acid or its methyl ester or by protonation of para-xylene under CI(CH 4) conditions eliminate H 2, CH 4 and, most remarkably, C 2H 4. The kinetic energy release characteristics of these fragmentation processes are reported. 2 H -labeling reveals that the loss of methane occurs by protonolytic cleavage of the C αC ipso bonds, excluding intra-complex benzylic hydride abstraction by CH 3 + ions. 13 C -labeling confirms the composite scrambling behavior preceding both methane and ethene losses. CH 4 loss of a major fraction (92%) of xylenium ions occurs without C scrambling but with some concomitant hydrogen exchange (H α/H ring) via nonclassical tolylmethonium ions, whereas a minor fraction (8%) undergoes complete C and H scrambling. C 2H 4 loss is preceded by different sequences of reversible ring expansion and ring contraction reactions involving methyldihydrotropylium ions (protonated methylcycloheptatriene) which, by irreversible ring contraction, eventually form ethylbenzenium ions (protonated ethylbenzene). A major fraction (66–75%) of the ions exhibits “specific” behavior, with the one of the methyl carbons being incorporated specifically into the ethene fragment and the other only after randomization with the carbons of the tolyl unit. A minor fraction (34–25%) of the ions undergoes complete C and H scrambling prior to ethene loss, involving both methyl groups. 1,2-CH 3 shifts are invoked to occur in the xylenium and/or dihydrotropylium ions.

Unimolecular gas-phase reactions of methyl and ethyl trifluoroacetoacetates upon electron ionization

The unimolecular metastable decompositions of methyl and ethyl trifluoroacetoacetates, CF 3COCH 2COOCH 3 (MW: 170 ( 1)) and CF 3COCH 2COOCH 2CH 3 (MW: 184 ( 2)) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. In the metastable time window, the molecular ions 1 + decompose to give exclusively the ions at m/ z 101 [ M−CF 3] +. However, the metastably decomposing ions 2 + lead not only to the formation of the major fragment ion m/ z 115 [ M−CF 3] +, but also to three minor fragment ions m/ z 164 [ M−HF] +, m/ z 156 [ M−C 2H 4] + and m/ z 87. A large part of the metastably decomposing ions 1 + and 2 + has the enol form. The loss of CO 2 from the ions m/ z 101 and m/ z 115 occurs through migration of the methyl and ethyl groups, respectively. The source-generated m/ z 69 ions from 1 + and 2 + are most abundant and consist of both CF 3 + and OCCHCO +. The latter ion, a protonated carbon suboxide, is generated by at least three and four different fragmentation routes from 1 + and 2 +, respectively. The m/ z 43 ion, C 2H 3O +, from 2 + is formed by at least two different routes.

Electron impact mass spectral fragmentation of chiral bisbenzoxazole, bisbenzothiazole and biscarbamide derivatives of 2,2-dimethyl-1,3-dioxolane.

The mass spectrometric fragmentation behaviour of five pairs of (R,R)- and (S,S)-4,5-bis(benzoxazol-2-yl)-2,2-dimethyl-1,3-dioxolane derivatives, one pair of (R,R)- and (S,S)-4,5-bis(benzothiazol-2-yl)-2,2-dimethyl-1,3-dioxolanes, and three pairs of (R,R)- and (S,S)-N,N'-bis(2-hydroxyaryl)-2,2-dimethyl-1,3-dioxolane-4,5-dicarbamides, all important compounds for asymmetric catalysis (P. Jiao et al., Tetrahedron Asymmetry 2001; 12: 3081), has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization conditions. The spectral observations have been rationalized in terms of fragment ion structures and fragmentation mechanisms that will provide an aid to spectral interpretation for new compounds of this type.