Structure and fragmentation of [C6H13O]+ ions formed by chemical ionization

The structure and fragmentation of eight [C₆H₁₃O] ⁺ ions formed by protonation of C₆H₁₂O carbonyl compounds in the gas phase have been investigated using isotopic labeling and metastable ion studies to investigate the fragmentation reactions and collisional dissociation studies to probe ion structures. Protonated 3-methyl-2-pentanone and protonated 2-methyl-3-pentanone readily-interconvert by pinacolic-retro-pinacolic rearrangements; the remaining six ions represent stable ion structures, although in many cases fragmentation is preceded by pinacolic-type rearrangements. Unimolecular (metastable ion) fragmentation of the [C₆H₁₃O] ⁺ species occurs by elimination of H₂O, C₃H₆, C₄H₈ and C₂H₄O. The last three elimination reactions appear to occur through the intermediacy of a proton-bound complex of a carbonyl compound and an olefin, with the proton residing with the species of higher proton affinity on decomposition of the complex.     

Mass spectra of halogenated esters 5—Chloromethyl esters of aliphatic C2?C12 n-carboxylic acids and their monochlorinated derivatives

A study has been made of the mass spectral fragmentation upon electron impact of aliphatic C₂? C₁₂ chloromethyl esters and all their 66 monochlorinated derivatives. The fragmentation pathways of the parent chloromethyl esters were elucidated with the aid of the 1st FFR metastable ions. A McLafferty rearrangement gives the base peak in the C₆? C₁₁ parent esters and in almost all the 4-chloro and ω-chloro isomers. The subsequent loss of HCl gives a very characteristic peak of the chloromethyl esters and their (3-ω)-chloro derivatives at m/z 72, [C₃H₄O₂]⁺. The 2-chloro isomers have the corresponding chlorine-containing fragment ion at m/z 106/108. The mass spectra of 2-, 3-, 4-, 5- and ω-chloro isomers give the characteristic fragment ions, the mass spectra of the other isomers being very similar.     

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 spectrometry of heterocyclic compounds—II: Electron-impact induced fragmentation of 3,5-diphenyl-1,2,4-oxadiazole

The electron-impact induced fragmentation of 3,5-diphenyl-1,2,4-oxadiazole has been investigated by labelling experiments, defocused metastable ion detections and high resolution mass measurements. The main fragmentation process suggests heterocyclic cleavage at the 1 to 5 and 3 to 4 bonds confirming our previous interpretation. The structure of the major fragment ion [C₇H₅NO]⁺· has been interpreted as being represented by the isomeric benzonitrile oxide and phenylisocyanate structures, the latter isomerising irreversibly from the former. The benzonitrile oxide structure is consistent with [C₇H₅NO]⁺· formation by cleavage of the 1 to 5 and 3 to 4 bonds.     

Investigation of azoles and azines. 76. Mass spectra of 5- and 6-substituted uracils

Peaks of molecular ions that generally have the maximum intensity are observed in the mass spectra of most of the investigated 5- and 6-substituted uracils and 5-substituted orotic acids and their deutero analogs and methylated derivatives. The principal pathway of the fragmentation of the molecular ions is retrodiene fragmentation with the formation of [O=C₍₄₎C₍₅₎R⁵C₍₆₎R₍₆₎N₍₁₎R¹]⁺ (F₁) ions. The stabilities of the latter depend on the nature and position of the substituents attached to the C₍₅₎ and C₍₆₎ atoms. The fragmentation of the F₁ ions can be realized via four principal pathways (B-E) with the detachment of N-CR⁶ (B), O=C=CR⁵ (C), CO (D), and R⁶ (E) fragments. The most general pathway for the fragmentation of 5-substituted uracils is pathway C, whereas the most general pathway for 6-substituted uracils is pathway E. In the spectra of 5-substituted orotic acids the intensities of the molecular-ion peaks are high (100%) only in the case of electron-donor R⁵ and decrease sharply with an increase in the electron-acceptor strength of the substituent. The principal pathways of fragmentation of the molecular ions are decarboxylation (F) and retrodiene fragmentation (A), the contribution of which is appreciably smaller. The M-CO₂ ions formed after decarboxylation undergo fragmentation via a scheme similar to that observed for 5-substituted uracils.See [1] for Communication 75.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 520–531, April, 1990.     

The mass spectrometric [M ? C2H4O]+ process in cyclic ketones. I—the nature of the C2H4O losss in bicyclo[3.3.1]nonan-2-one and 3-one

Although the loss of a C₂H₄O molecule from the molecular ions of the isomeric bicyclo[3.3.1]nonan-2- and 3-ones gives rise to the base peak at m/e 94 in both spectra, deuterium labelling results show that the processes leading to this ion are markedly different in the two cases. While the 2-one follows essentially (>80%) the same [M ? C₂H₄O]+ fragmentation pathway described for other 2-alkylcyclohexanones, the 3-one exhibits a more complex decomposition pathway in which one α-hydrogen is transferred away from the departing C₂ unit and three hydrogen atoms are subsequently rearranged to the eliminated C₂H₄O molecule. Similar competing fragmentation schemes have been invoked to explain deuterium labelling results in cycloheptanone, 2-methylcycloheptanone and cyclooctanone.     

Mass spectrometry of π-complexes of transition metals : XII. Ferrocenyl and cymantrenyl derivatives of chalcones

The main processes of the electron-impact induced fragmentation of the ferrocenyl and cymantrenyl analogues of chalcones RCOCH=CHR′ (R, R′ = C₅H₄FeC₅H₅, C₅H₄Mn(CO)₃, or aryl) involve a cleavage of the metalligand bond. A “chalcone” mode of fragmentation is weakly pronounced, due to the fact that the positive charge is localized preferably on the metal atom.     

193-nm Photodissociation of ions from saturated and unsaturated aliphatic molecules

To determine the analytical utility of photodissociation as a general fragmentation technique for tandem mass spectrometry of organic ions, the ability to fragment those ions considered least likely to absorb photons efficiently was investigated. To this end, the ability to photodissociate ions of aliphatic compounds by using 193-nm photons has been studied. Three fragment ions, the C₄H ₉ ⁺ ion from n-hexane, the C₄H ₇ ⁺ ion from 2-hexene, and C₄H ₅ ⁺ from 2-hexyne, have been photodissociated. The fragmentation efficiencies for all three ions studied were between 25 and 45%. The photofragment ion spectrum for each precursor ion studied is made up of characteristic fragments. These spectra demonstrate the ability to photodissociate aliphatic ions that originate from both saturated and unsaturated molecules. This provides substantial hope that virtually all organic ions will be able to be photodissociated by using 193-nm photons.     

Degenerate skeletal rearrangement and energy partitioning in the fragmentation of the norbornyl cation

The electron-impact-induced fragmentation of a ¹³C labelled exo-2-norbornyl chloride has been studied. When the norbornyl cation, [C₇H₁₁]⁺, [M – Cl], dissociates by elimination of C₂H₄, carbon atoms are randomly lost showing that extensive skeletal rearrangement (in addition to the complete hydrogen atom scrambling reported earlier) has taken place in the norbornyl cation prior to dissociation. The metastable ion peak associated with the above fragmentation consists of superimposed gaussian and flat-top components; it is proposed that these correspond to the formation of isomeric [C₅H₇]₊ daughter ions whose heats of formation differ by ～0.4 eV.     

Mass spectral fragmentation pattern of 5-methyl-4-[(phenylamino)methylene]-2,4-dihydro-3h-pyrazol-3-one and its 2-methyl and 2-phenyl derivatives

The mass spectral fragmentation patterns of 5-methyl-4-[(phenylamino)methylene]-2,4-dihydro-3H-pyrazol-3-one and its 2-methyl and 2-phenyl derivatives have been elucidated. The principal initial fragmentation route involves rupture of the exocyclic CH-NH bond. Minor routes involve loss of H, OH and C₆H₅ from the molecular ion and rupture of the pyrazolone ring.     

Structural characterization and identification of flavonoid aglycones in three Glycyrrhiza species by liquid chromatography with photodiode array detection and quadrupole time‐of‐flight mass spectrometry

Flavonoids, including flavones, isoflavones, flavanones, chalcones, and isoflavans, have long been recognized as the main active ingredients in licorice. A method combining liquid chromatography with photodiode array detection and quadrupole time‐of‐flight mass spectrometry was developed to characterize components in three Glycyrrhiza species, and to expound the characteristic fragmentation behaviors in the positive ion mode. Based on the fragmentation patterns of reference compounds, a total of 39 compounds, including 37 flavonoid aglycones and two coumestans, were identified or tentatively identified. Besides, some common features, such as H₂O, CO, and CH₂O₂ losses, together with retro‐Diels–Alder fragmentation, were observed in these compounds. Furthermore, diagnostic fragmentations of C‐ring cleavages and UV absorption on the skeleton groups were observed to structurally characterize flavonoid aglycones. In addition, typical losses of different substituent groups were detected: Neutral losses of 56 (C₄H₈) and 68 Da (C₅H₈) were yielded from a prenyl chain; neutral losses of 42 (C₃H₆), 54 (C₄H₆), and 70 Da (C₄H₆O) were generated by a pyran ring. Particularly, neutral losses of 18 (H₂O), 16 (CH₄), 112 (C₈H₁₆), and 98 Da (C₇H₁₄) predicted a hydroxyl, a methoxyl, double prenyl chains, and a prenyl chain with a pyran ring, respectively.     

Unimolecular fragmentation reactions of protonated and methyl-cationated n-octanones and N-nonanones. Sequential fragmentation reactions

Protonated and methyl-cationated n-octanonts and n-nonanones have been prepared by chemical ionization methods and the unimolecular fragmentation reactions occurring on the metastable ion time-scale have been exam ined. The [R₁R₂COR₃]⁺ (R₃ = H or CH₃) species fragment by elimination of R₃OH and by elimination of neutral alkenes. The elimination of (R₁ — H), where R₁ is the larger alkyl group of the original ketone, is particularly important. In addition, alkenyl ions are observed corresponding, nominally, to elimination of C₃H₇OR₃ from the ionized octanones and to elimination of C₃H₇OR₃ and C₄H₉OR₃ from the nonanones. These ions are shown to arise largely, if not completely, by sequential elimination of R₃OH plus an olefin (C₃H₆ or C₄H₈) from [R₁R₂COR₃]⁺. A comparison is made of the unimolecular fragmentation reactions occurring in the second field-free region and in the radio-frequency-only quadrupole of a hybrid BEQQ mass spectrometer.     

The mass spectra of isomeric hydrocarbons—II: The C5H8S isomers,spiropentane, cyclopentene, 1,3-pentadiene and isoprene; The mechanisms and energetics of their fragmentations

The mass spectra of spiropentane, cyclopentene, 1,3-pentadiene (piperylene), isoprene and some deuterium labelled analogues have been studied. The heats of formation of the molecular ions and the major daughter ions [C₅H₇]⁺ and [C₄H₅]⁺ have been estimated. Fragmentation of these molecules mostly proceeds via common intermediates in which hydrogen atoms have lost their positional identity. Only spiropentane displays any specific fragmentation behaviour related to its ground state geometry.     

Mass spectra of halogenated esters 6—Methyl esters of some trihalogenated propanoic and butanoic acids

The mass spectral fragmentation of trihalogenated methyl esters, formed in the reactions of monochlorinated methyl propenoates and 2-butenoates with Cl₂, BrCl and Br₂, have been investigated. In most cases α-cleavage gives the base peak, [COOCH₃]⁺, the peaks originating from the subsequent losses of one or two halogen atoms also being abundant. The primary loss of a halogen atom is more prominent in the C₄ derivatives, Br˙ and Cl˙ being preferentially lost from the 2- and 3-positions, respectively. The McLafferty rearrangement yields in one case the base peak; the 2-halo compounds could in general be distinguished by that fragmentation. Typical for all 2-bromo-substituted methyl butanoates studied is the base peak, [C₃H₃]⁺, at m/z 39, and for some 3-halo compounds the peaks at m/z 95, [C₂H₄ClO₂]⁺ and 139, [C₂H₄BrO₂]⁺.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.     

The high-resolution mass spectra of aliphatic aldehydes

The mass spectra of the C₃ to C₉ n-alkanals and a number of branched aldehydes have been obtained at a resolution sufficient to resolve the O? CH₄ doublets. From the resolved spectra, a study of metastable transitions, and the spectrum of one deuterium-labelled alkanal, (n-hexanal-2,2-d₂) the major fragmentation reactions have been elucidated. Of particular interest are the γ-cleavage reaction, leading to [C₃H₅O]+ in the n-alkanals, which proceeds both by a simple cleavage and by cleavage preceded by hydrogen interchange, and the loss of C₂H₄, which involves loss of the C₂ and, probably C₃, carbons.     

Mass spectrometric fragmentation of saturated six membered heterocyclic systems containing two chalcogen family atoms

The mass spectra of six-membered saturated heterocycles containing oxygen, sulphur, selenium and tellurium in the 1,4-positions have been measured. The differing fragmentation modes have been characterized using high resolution, low voltage and metastable ion scan techniques. The important decomposition reactions of the molecular ions involve elimination of C₂H₄ and CH₂X (X is a chalcogen atom) and formation of [C₂H₄X]⁺ and C₂H₅⁺. The propensities of these reactions vary systematically as a function of the ability of the chalcogen to stabilize a positive charge.     

Photoionization and Dissociation Study of 2-Methyl-2-propen-1-ol: Experimental and Theoretical Insights

The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultravioletsynchrotron radiation in the photon energy region of 8.0-15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C₄H₈O⁺、C₄H₇O⁺、C₃H₅O⁺、C₄H₇⁺、C₄H₆⁺、C₄H₅⁺、C₂H₄O⁺、C₂H₃O⁺、C₃H₆⁺、C₃H₅⁺、C₃H₃⁺、CH₃O⁺、CHO⁺ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d,p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO.     

Mass spectra of some sesquiterpene lactones of the eudesmane series with a C1-OH group

Summary The fragmentation of some C₁-OH eudesmanolides has been studied with the aid of an isotopic label and high-resolution mass spectrometry. It has been shown that the most characteristic fragmentation is that with the cleavage of the C₁-C₁₀ and C₄-C₅ bonds. The sesquiterpene lactone arabsin has also been assigned to the C₁-OH eudesmanolides and this has been confirmed by its conversion into anhydroarabsin — an ,-unsaturated ketone.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 34–40, January–February, 1979.     

Peculiarities of dissociative ionization of alkylcyclopentadienyl nitrosyl π-complexes of nickel

The decomposition of alkylcyclopentadienyl nitrosyl -complexes of nickel, (C₅H₄R)(NO)Ni (R=H, Et,i-Pr, CH₂Ph), under the action of electron impact has been studied. The nature of the nitrosyl ligand has been shown to be the factor determining the main fragmentation pathway which involves the abstraction of an NO molecule. The effect of the nature of the ligand on the ability of the molecular ion (C₅H₄R)LNi⁺ (L=C₅H₄R, C₅H₅, C₃H₅, NO) to rearrange with hydrogen atom migration from one ligand to another has been considered. The structure of the alkyl group R determines a competing fragmentation pathway involving cleavage of the -C-C bond with respect to the cyclopentadienyl ring in the substituent.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1985–1988, November, 1993.