Electron impact fragmentations of 1- and 3-aryl-3-buten-1-ols

The electron impact fragmentation of 1- and 3-aryl-3-buten-1-ols show several distinguishing fragmentations. α-Cleavage predominates in the fragmentation of the 1-aryl-3-buten-1-ols to such an extent that molecular ions of only low intensity are observed. The ion resulting from α-cleavage fragments readily with the loss of the ring substituent to the phenyl ion. An intense molecular ion is observed in the 3-aryl series and a loss of 70 u is a major fragmentation in this series. Based on deuterium labeling studies, this unique fragmentation was explained by a hydroxylic hydrogen migration to the ring accompanied by the loss of allene and formaldehyde. Other major fragmentations observed in the 3-aryl series are: a McLafferty-type rearrangement (loss of formaldehyde), loss of 33 u (water + methyl radical), and the loss of 43 u (C₂H₃O and C₃H₇). The proposed mechanisms have been substantiated by deuterium labeling and high resolution mass spectrometry. Substituent effects play a major role in the 3-aryl series, but are insignificant in the 1-aryl series.     

Fragmentation mechanism for the loss of XCY (X and Y = O,OR, S) from 2-phenyl-1,3,4-oxadiazole-5-one and related sulfur-containing compounds

The fragmentation mechanism for loss of X?C?Y (X and Y = O or S) from 2-phenyl-1-3-4-oxadiazole-5-one and related sulfur-containing compounds begins with the breaking of the C? N bond of the heterocyclic ring. Then a X?C?Y molecule is ejected with the initial formation of a non-rearranged ion. The major part of ΔH_R⁰ is associated with the stabilization of this neutral fragment. The initial fragment ion is further rearranged before it decomposes.     

Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

Mass spectral fragmentation pattern of 3-(2′-hydroxyethyl)quinolin-2(1H)-one and its derivatives

Fragmentation patterns resulting from electron impact ionization of 3-(2′-hydroxyethyl)quinolin-2(1H)-one, three of its monosubstituted derivatives and four of its disubstituted derivatives were studied. The molecular ion of quinolinone-2-etbanol undergoes initial fragmentation with the loss of OH^·, H₂O, CO, ^·CHO, CH₂O, CH₂OH^·, CH₂?CHOH and HCNO species. The [M – CHO]⁺ ion is tentatively suggested to have been formed by the expulsion of H^· from the [M – CO]^+· ion and the [M - CHO]⁺ peak may be considered as diagnostic of a 2-quinolone-3-ethanol.     

Electron-impact induced skeletal re-arrangement of 2-methoxy-3-methylpyrazine and 2-methylthio-3-methylpyrazine

The mass spectra of 2-methoxy-3-methylpyrazine (I), 2-methoxy-6-methylpyrazine (II), 2-methylthio-3-methylpyrazine (III) and 2-methylthio-6-methylpyrazine (IV), are given and the major fragmentation pathways discussed. The novel loss of H₂O from the molecular ion of I and the corresponding loss of H₂S from the molecular ion of III indicate that a skeletal rearrangement takes place in the molecular ion preceding the expulsion of H₂O and H₂S. Proposed mechanisms for this behavior are discussed with evidence being drawn from accurate mass measurement, metastable ions, and deuterium and carbon-13 labeling of the methoxy group. The absence of ions in the spectra of II and IV corresponding to the loss of H₂O and H₂S from these molecular ions clearly indicates that the position of the methyl group with respect to the methoxy group, or the methylthio group is in-timately involved in this mechanism.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.     

The mass spectra of some α-lactams

The electron-impact induced fragmentation of eight aziridinones has been studied by conventional as well as by high resolution mass spectrometry. All α-lactams exhibit a molecular ion. The major primary step, in the fragmentation, is the ejection of carbon monoxide from the molecular ion. Ions of the general formula R₁? NC and R₂R₃C?O were found in the mass spectra of all α-lactams investigated. A skeletal rearrangement to rationalize these ions is proposed. The fragmentation of the molecular ion is affected by the N-substituent. Exact mass measurement and specific deuterium labeling indicate the absence of McLafferty rearrangement from either the N- or C-substituent.     

Electron impact induced fragmentation of dibenzo-18-crown-6 polyether and its nitro derivatives

The mass spectra of dibenzo-18-crown-6 polyether and three of its nitro derivatives have been determined. The fragmentation pathways of all the compounds suggested that ring contraction was taking place. The molecular ion of the polyether dissociated by three competing processes, mainly through loss of C₂H₄O units. The molecular ions of the three derivatives dissociated with ring contraction, as well as through losses of O, No and NO₂.     

SN-2-artige massenspektrometrische Fragmentierungen bei substituierten N-Alkylpiperidinen. 10. Mitteilung über das massenspektrometrische Verhalten von Stickstoffverbindungen [1]

An S_N2-type fragmentation was observed on mass spectrometric analysis of N-(ω-X-alkyl)-2-alkylpiperidines (X = NRCOR′,  CONH₂,  COOR,  NH₂). The molecular ion after loss of the 2 alkyl substituent on the piperidine ring may eject the neutral piperideine on attack of the substituent X on the α-carbon of the N-substituted alkyl-chain. Through this process a cyclic fragment ion is formed. The influence of its ring size and of the substituent X on the stability of this fragment was investigated. The S_N2 reaction is favoured in the case of production of five- and six-membered rings.     

Die massenspektrometrische Fragmentierung von NiII-Chelaten mit O-, N-, S-, Se-Donoratomen

Mass Spectrometric Fragmentation of Ni^II Chelates with O-, N-, S-, Se-Donor Atoms The mass spectra of 6Ni^II chelates having the same substituents but different donor atoms (sequence of ligand atoms ? C(X)? NH? C(Y)? ; X, Y = O, S, Se, NH; see formula (1)) are discussed. The fragmentation pattern of the (thio)benzoyl(thio, seleno)ureas are characterized by the abstraction of S or Se atoms. The appearence of small neutral molecules seems to be typical for the fragmentation of all complexes. The mass spectrum of the Ni^II chelate having a NiS₂Se₂ coordination sphere shows no molecular ion peak. The contents of molecular ions in the total ion stream correlates with the stability of the complexes.     

Mass spectra of new heterocycles: XV. Fragmentation of 1-substituted 3-alkoxy-2-(propargylsulfanyl)- and 3-alkoxy-2-(allenylsulfanyl)-1<Emphasis Type="Italic">H</Emphasis>-pyrroles under electron impact

The electron impact mass spectra of 1-R-substituted 3-alkoxy-2-(propargylsulfanyl)- and 3-alkoxy-2-(allenylsulfanyl)-1H-pyrroles (R = Me, i-Pr, s-Bu, Ph) have been studied for the first time. These compounds give rise to stable molecular ions whose primary fragmentation follows three competing pathways: cleavage of the C–O bonds with expulsion of alkyl radical, cleavage of the C–S bonds with formation of [M–C₃H₃]⁺ ions, and cleavage of the C–N bonds with synchronous hydrogen transfer to give odd-electron [M–C_nH_2n]+ · ion. The main fragmentation pathway of 2-(propargylsulfanyl) derivatives is cleavage of the C–S bond with formation of [M–C₃H₃]⁺ ion.     

Electron-impact studies. XCIX—negative ion mass spectrometry of the o-NO2?C6H4?X?C6H5 systems. Aryl hydrogen scrambling and proximity effects

We report the first example of aryl hydrogen scrambling occurring in a molecular anion prior to or accompanying fragmentation, i.e. for the reaction [M]^?· → [M ? H₂NO₂.]^? from o-NO₂? C₆H₄? X? C₆H₅ (X = O or S). Proximity effects occur in these spectra when X = CO, NH, O, or S, and certain of these have been substantiated by ²H and ¹⁸O labelling.     

Why Are B ions stable species in peptide spectra?

Protonated amino acids and derivatives RCH(NH₂)C(+O)X · H⁺ (X = OH, NH₂, OCH₃) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH ₂ ⁺ . By contrast, protonated dipeptide derivatives H₂NCH(R)C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B₂ ions by elimination of HX. These B₂ ions fragment on the metastable ion time scale by elimination of CO with substantial kinetic energy release (T _1/2 = 0.3–0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH₃C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B ions by loss of HX. These B ions also fragment unimolecularly by loss of CO with T _1/2 values of ～ 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fragments by way of activation to a reacting configuration that is higher in energy than the products, and some of the fragmentation exothermicity of the final step is partitioned into kinetic energy of the separating fragments. We conclude that the stable configuration is a protonated oxazolone, which is formed by interaction of the developing charge (as HX is lost) with the N-terminus carbonyl group and that the reacting configuration is the acyclic acylium ion. This conclusion is supported by the similar fragmentation behavior of protonated 2-phenyl-5-oxazolone and the B ion derived by loss of H-Gly-OH from protonated C₆H₅C(+O)-Gly-Gly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC(+O)NHCH₂CO⁺, show that the lowest energy structure is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in energy than the fragmentation products, HC(+O)N⁺H = CH₂ + CO, which is consistent with the kinetic energy releases measured.     

Metastable ion and induced stable ion fragmentation of isomeric 5-methyl-3-tolyl-1,2,4-oxadiazoles

The electron ionization fragmentation patterns of 5-methyl-3-(o-, m- and p-tolyl)-1,2,4-oxadiazoles (1a—c) have been examined by metastable ion and high resolution mass spectrometry. The o-tolyl isomer loses CO and C₂H₂O from the metastable molecular ion whereas the m- and p-tolyl isomers lose only CH₃CN thus indicating a strong ortho effect in directing the fragmentation in 1a. Slight differences between o-, m- and p-tolyl isomers in the collisional activation fragmentation of stable [C₇H₆N]⁺ ions suggest that structural differences exist even after a series of extensive rearrangements of the molecular ions. Metastable ion kinetic energy (MIKE) and collisional activation (CA) spectra were very helpful in providing valuable information about many fragments.     

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S? S bond, we show, by using [CH₃SSX] (X=CH₃, NH₂, OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH₄, NH₃, H₂O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H₂O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH₃SSX]^? (X=OH, F) occupies a σ*(S? X) MO, which favors the cleavage of the S? X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H₂O and HF are rather low. Only when the substituents are less electronegative (X=H, CH₃, NH₂) the extra electron is located in a σ*(S? S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S? S bond dissociation process also depends strongly on the nature of the substituent. For X=H or CH₃ the process is strictly adiabatic, while for X=NH₂ it proceeds through a conical intersection ( CI ) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a σ*(S? S) antibonding orbital, two fragments with the proper charge localization.     

Mechanism of CO loss for protonated 1-indanone and isomeric [C9H9O]+ ions

In order to establish the mechanism of CO loss occurring during metastable decomposition of protonated 1-indanone, fragmentations of monocyclic [C₉H₉O]⁺ isomers have been studied. These ions of known structure were prepared by CI protonation and fragmentation of the corresponding acids chlorides. It is demonstrated that the wide component of the [MH? CO]⁺ metastable peak induced by protonated 1-indanone fragmentation is the result of fragmentation of the [C₆H₅CH₂CH₂CO]⁺ isomer ion.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

Mass spectra of organoarsenic compounds—II: Electron-impact-induced fragmentation of 2-substituted-1,2,2-dioxarsenanes

The electron-impact-induced fragmentation of 2-substituted 1,3,2-dioxarsenanes has been studied. The main fragmentation modes have been determined with the use of high resolution mass measurements and by application of the metastable defocusing technique. The predominant fragmentation for the 2-alkyl-1,3,2-dioxarsenanes proceeds via the loss of the 2-substituent from the molecular ion. In the case of 2-phenyl-1,3,2-dioxarsenanes elimination of the phenyl group competes with the formation of a C₆H₅ As ion as well as loss of aldehyde from the molecular ions.     

Mass spectra of some 1-substituted-4-acetoacetyl-3-methylpyrazol-5-ols

The fragmentation of 1-phenyl-, l-(2′-pyridyl)- and 1-(4′-methyl-2′-quinolyl)-4-acetoacetyI-3-methyIpyrazol-5-ols (compounds 1, 2 and 3, respectively) on electron impact has been studied and the major processes interpreted. The common feature in the mass spectra of these compounds is the loss of ketene, acetonyl radical, acetone and two molecules of ketene from the molecular ion. Whereas the ion generated after the last process, which corresponds to 1-substituted-3-methyIpyrazol-5-ols, loses methyl cyanide in the case of 1, similar ions in the case of 2 and 3 lose ?₂HO moiety, necessitating an intramolecular hydrogen transfer followed by ring fission and subsequent loss of methyl cyanide. All these and other related processes have been substantiated with the help of accurate mass measurements of the fragment ions and B/E linked-scan spectra.     

Losses of isobaric neutral species from molecular ions of isomeric nitroanisoles

Alternative losses of the isobaric neutral species CH₂O and NO˙ have been assessed for molecular ions of isomeric nitroanisoles fragmenting in the ion source and the first field free region of a double focusing mass spectrometer. Mass analyses of the primary fragment ions indicate that specific loss of CH₂O occurs from molecular ions of 2-nitroanisole, while specific loss of NO˙ occurs from molecular ions of 3-nitroanisole. Although the peak due to [M? NO]⁺ ions is negligible in the mass spectrum of 2-nitroanisole, evidence is presented to show that they are transient intermediates in the consecutive fragmentation for loss of the elements of CNO₂ from the molecular ions.