The electron impact-induced fragmentation of 2-arylthiazoles

The mass spectrometric fragmentation of 2-aryl thiazoles centres, like that of simple thiazoles, around the cleavage of the 1,2 and 3,4 bonds of the thiazole ring. One characteristic species does arise from the fission of the 1,5 and 2,3 bonds, viz. a thiobenzoyl ion. The fragmentations of some substituents at the 4 and 5 positions of the thiazole ring, prior to ring cleavage, are also described.     

A study of the electron impact-induced fragmentation of 1,6-diaminohexane

The mass spectrometric behaviour of 1,6-diaminohexane has been studied with the help of deuterium labelling in every position of the molecule. Extensive intramolecular exchange reactions between the hydrogen atoms of the amino groups and those of the hydrocarbon chain are responsible for the apparent complexity of the fragmentation mode.     

Hydrogen rearrangements in cyclic phosphates during the electron impact-induced fragmentation

The fragmentation upon electron impact of a cyclic phosphate, 5,5-dimethyl-2-oxo-2-phenoxy-1,3,2-dioxaphosphorinane, was investigated. Using its deuterated analogue, it was shown that no randomization of hydrogen atoms among methyl, methylene and phenyl groups occurred within the molecular ion. The majority of the fragmentations involved single, double or triple hydrogen transfers. The origin of hydrogen(s) transferred in the formation of C-5 ions was also estimated. The existence both specific and nonspecific hydrogen rearrangement mechanisms was also revealed. The specific hydrogen rearrangement involves a methyl hydrogen transferring to the P?O group and this process seemed to be the initial step in the formation of many of the ions.     

Intramolecular hydrogen exchanges during the electron impact-induced fragmentation of complex alicyclic amines

The three-step mechanism of formation of the important fragment ion of mass 84 from 3-dimethylaminosteroids under electron impact is accompanied by intramolecular hydrogen exchanges, as shown by the mass spectra of deuterium labelled derivatives. The hydrogen exchanges are shown to occur in the ring A opened molecular ion, i.e. after the initial C-3–-C-4 bond rupture, and to compete with the final C-1–-C-10 bond rupture. (The intensity of hydrogen-deuterium exchange reactions depends upon the activation energy of the final rupture.) The electron impact-induced-fragmentation of large alicyclic molecules bearing a strong charge stabilizing functional group, such as amines, appears to be the result of a series of displacements of the radical over different sites of the molecular ion through hydrogen (or carbon) rearrangements, with some of the radical sites being in the appropriate position to induce a final rupture.     

Electron impact-induced fragmentation of some aminonitropyridines

The fragmentation behaviour of 18 vicinally substituted aminonitropyridines was studied under electron impact conditions. Tbe decomposition patterns were found to be strongly dependent on the position of substituents. The formation of the aza analogue of carbazole seems to affect considerably the fragmentation of 3-nitro-2-phenylaminopyridine.     

The mechanism of CO loss in the electron impact-induced fragmentation of benzoin and its methyl ether

The electron impact-induced fragmentation of benzoin methyl ether gives rise to the formation of an m/e 91 fragment ion which is not present in the mass spectrum of benzoin. Deuterium and ¹³C labelling, as well as low energy experiments, revealed that this ion is formed from [M – benzoyl]⁺ by transfer of both the ether methyl group and the hydrogen atom from the α-position to the adjacent phenyl ring followed by loss of CO and hydrogen.     

The electron impact-induced cyclization of o-carboxy- and o-carboxamidocyclopropylbenzenes

The isomerization of M^{+ ˙} of o-carboxy- and o-carboxamidocyclopropylbenzenes into five- and six-membered heterocycles due to the ortho effect has been demonstrated by means of electron impact mass spectra, high-resolution mass spectrometry data and metastable ion spectra. The criteria for the quantitative estimation of the degree of cyclopropane cyclization into these isomeric cycles were formulated. The nature and quantity of heterocyclic ions proved to be in good accord with the results for cyclization of the same cyclopropanes in the strong acid solutions.     

Electron impact-induced fragmentation of 2,1-benzisothiazoline 2,2-dioxide

The electron impact-induced fragmentation patterns of 2,1-benzisothiazoline 2,2-dioxide nitro derivatives were studied. The rationalizations proposed for the fragmentations are supported by accurate mass measurements, daughter ion (mass analysed ion kinetic energy and B/E linked-scan), parent ion, and constant neutral loss spectra in metastable and collision-induced dissociation modes and deuterium labelling.     

Electron impact-induced fragmentation of 15N-labelled 1,2,4-triazoles

The electron impact-induced fragmentation patterns of 3,5-diphenyl-lH-1,2,4-[4-¹⁵N]triazole, 4-ethyl-3,5-diphenyl-4H-1,2,4-[4-¹⁵N]triazole, l-ethyl-3,5-diphenyl-lH-1,2,4-[4-¹⁵N]triazole and the corresponding unlabelled compounds were established from exact mass measurements and from metastable ion evidence. Evidence for two fragmentation mechanisms was established for the formation of the m/z 104 peak in the spectrum of the 4-ethyl compound. Only the 1-alkylated triazole exhibited an intense peak at m/z 131. Major peaks at m/z 221 (222) and 118 are characteristic of the investigated triazoles.     

Electron impact-induced fragmentation of 2,3-dihydro-1,5-benzothiazepin-4(5H)-ones

The electron impact-induced fragmentations of 2,3-dihydro-1,5-benzothiazepin-4(5H)-ones and some related compounds were investigated. On the basis of low- and high-resolution measurements, metastable ion studies by means of mass-analysed ion kinetic energy spectroscopy and collision-induced dissociation experiments, the main fragmentation pathways were established. The effect of methyl and phenyl substituents at the C(2) and C(3) positions of the heterocyclic ring on the fragmentations was also studied. The (1,3) ring splitting was investigated in some detail, using semi-empirical molecular orbital calculations.     

The fragmentation of 2-aminooxazoles under electron impact

The 70 eV electron impact mass spectra of six 2-aminooxazoles are discussed in detail with the aid of exact mass measurements, metastable ion detection in the defocused mode and labelling experiments. The fragmentation is markedly influenced by strong electron donating substituents at the 2-position. Some 2-acetylamino and 2-diacetylamino derivatives have also been considered, which lose ketene from [M]⁺? giving the corresponding aminooxazole molecular ions. The mass spectral behaviour is peculiar and the most important differences between the fragmentation pathways of these compounds and those reported in the literature for oxazole (alkyl and aryl) derivatives are outlined. The mass spectrum of 4-methyl-5-phenylimidazol-2-one is compared with that of the isomeric 4-methyl-5-phenyl-2-aminooxazole.     

The fragmentation of isotetronic acids under electron impact

The mass spectral fragmentation of eleven α-hydroxybutenolide compounds (isotetronic acids) are reported. The main features are carboxyl and consecutive carbon monoxide expulsions. Masses were determined using high resolution techniques, metastable transitions were detected by defocusing experiments, and pathways were further documented by isotopic labelling experiments.     

The fragmentation behaviour of some spirobenzthiazoles under electron impact

The electron impact induced fragmentations of ten spiro-benzthiazoles have been studied. The principal decomposition reactions have been elucidated by metastable studies, exact mass measurements, and by deuterium exchange of the imino hydrogen. The primary fragmentation step is an α-cleavage reaction, which leads to ring opening of the ring fused to the benzthiazole moiety of the molecule. The subsequent decompositions resemble those of cyclohexylamines. They follow a general pattern, the details of which are discussed with respect to the nature and position of the heteroatom in the saturated ring.     

The electron impact fragmentation of 3- and 4-styrylpyridazine

The electron impact mass spectrometric fragmentation of trans-3- and trans-4-styrylpyridazine is reported in detail, including a comparison with other aza-stilbenes. With regard to a distinction between the two isomeric styrylpyridazines, the intensity ratio of the M⁺ and [M-1]⁺ ions, the general degree of fragmentation and the elimination pathways of nitrogen proved to be most characteristic.     

The electron impact induced fragmentation of 1,3-diphenyl-2-pyrazoline

The electron impact induced fragmentation of 1,3-diphenyl-2-pyrazoline was studied by nitrogen-15, carbon-13 and deuterium labelling, substitution in the para position of one of the phenyl rings by fluoro and methoxy groups, and ion kinetic energy spectroscopy, as well as accurate mass measurements. In the fragmentation pathway observed all the ions in the mass spectrum of 1,3-diphenyl-2-pyrazoline (except m/e 103) originate from the molecular ion, which in all compounds is the base peak. The formation of prominent fragment ions is discussed.     

The fragmentation of isotetronic acid O-methyl ethers under electron impact

The mass spectra of a series of methyl ethers of isotetronic acids have been examined and the modes of fragmentation rationalized on the basis of two general schemes.     

Barrierless electron transfer bond fragmentation reactions

The ultrafast N-O bond fragmentation in a series of N-methoxypyridyl radicals, formed by one-electron reduction of the corresponding N-methoxypyridiniums, has been investigated as potentially barrierless electron-transfer-initiated chemical reactions. A model for the reaction involving the electronic and geometric factors that control the shape of the potential energy surface for the reaction is described. On the basis of this model, molecular structural features appropriate for ultrafast reactivity are proposed. Femtosecond kinetic measurements on these reactions are consistent with a kinetic definition of an essentially barrierless reaction, i.e., that the lifetime of the radical is a few vibrational periods of the fragmenting bond, for the p-methoxy-N-methoxypyridyl radical.     

Charge remote fragmentation in electron capture and electron transfer dissociation

Secondary fragmentations of three synthetic peptides (human αA crystallin peptide 1-11, the deamidated form of human βB2 crystallin peptide 4-14, and amyloid β peptide 25-35) were studied in both electron capture dissociation (ECD) and electron-transfer dissociation (ETD) mode. In ECD, in addition to c and z· ion formations, charge remote fragmentations (CRF) of z· ions were abundant, resulting in internal fragment formation or partial/entire side-chain losses from amino acids, sometimes several residues away from the backbone cleavage site, and to some extent multiple side-chain losses. The internal fragments were observed in peptides with basic residues located in the middle of the sequences, which was different from most tryptic peptides with basic residues located at the C-terminus. These secondary cleavages were initiated by hydrogen abstraction at the α-, β-, or γ-position of the amino acid side chain. In comparison, ETD generates fewer CRF fragments than ECD. This secondary cleavage study will facilitate ECD/ETD spectra interpretation, and help de novo sequencing and database searching.