Organic mass spectrometry—X. Electron impact fragmentations of alkylthio group in 2-alkylthio-5-aminothiazolo[5,4-d] pyrimidines

In order to discuss hydrogen transfer in the skeletal fragmentation of thioethers on electron impact, mass spectra of a series of 2-n-alkylthio-5-aminothiazolo [5,4-d]pyrimidines have been determined. To aid the interpretation of the hydrogen migration, deuterium-labeled compounds which are substituted with deuterium in each position of 2-n-butylthio-5-aminothiazolo-pyrimidines were studied. By correlation of the spectra obtained from such labeled compounds, the initial hydrogen migration in the fragmentation to produce [M ? SH], [MS ? CH₃] and m/e 184 ions is concluded to be as follows: migration of the α-hydrogen atom to the sulfur induces formation of the [M ? SH] ion; migration of the β-hydrogen atom to the sulfur or nitrogen atom by a McLafferty rearrangement induces formation of the m/e 184 ion; and migration of γ-hydrogen atom to the sulfur induces formation of the [M ? SCH₃] ion.     

The ortho effect in the mass spectra of ortho/para isomers of bisphenol a derivatives and related compounds

New examples of the ortho effect in bisphenol A derivatives including interaction of the hydrogen of the ortho-hydroxy group with the neighbouring aromatic ring have been observed. The characteristic ions [M ? PhOH]^+middot; (m/z = 134) and [M ? CH₃ ? PhOH]⁺ (m/z = 119) were shown to form through the hydrogen transfer from hydroxy and isopropyl groups, respectively. The spectra of cyclic derivatives having ortho-hydroxy functions show [M ? 43]⁺, [M ? C₈H₉O]⁺, m/z = 147, m/z = 135 and [M ? C₉H₁₀O]⁺ ions. The proposed mechanims of the corresponding transformations were supported by mass spectra of deuterated analogues, methyl and trimethyl silyl ethers.     

The fragmentation of ortho-methoxyphenyl-2-propanone oximes and related compounds–further examples of an ortho-effect

An [M ? 31]⁺ ion was a prominent fragment in the mass spectra of three ortho-methoxy-phenyl-2-propanone oximes and is shown to be due to the expulsion of a methoxyl radical from the molecular ion as a result of an ortho-effect. In contrast, an [M ? 31]⁺ ion was absent from the spectra of a structurally related ketone and a hydroxylamine, and was not observed in the spectra of meta- and para-methoxyphenyl-2-propanone oximes.     

Differentiation of anomeric C-glycosides by mass spectrometry using fast atom bombardment,mass-analysed ion kinetic energy and collision-activated dissociation

Positive-ion fast atom bombardment mass spectrometry appears to be a useful method for the differentiation of anomeric C-glycosides. The mass-analysed ion kinetic energy (MIKE) and collision-activated dissociation (CAD) MIKE spectra of selected positive ions can be used as fingerprints of the α- or β-anomers. The main fragmentation routes and particularly the formation of the [M ? H]⁺ ion and the [M + H ? PhCH₂OH]⁺ ion were traced for each anomer.     

The Loss of a hydroxyl group from the molecular ions of alkylnitrobenzenes

It is confirmed that the loss of HO˙ from the molecular ion of o-nitrotoluene involves exclusively a hydrogen from the methyl group. However, in higher homologues hydrogen atoms from non-benzylic sites are also implicated. With such compounds this fragmentation mode is shown not only by the ortho but, to a lesser extent, by the meta and para isomers as well. The proportion of the total ion current borne by the [M – 17]⁺ ion follows the order ortho > meta > para, which is attributed to substituent migration around the ring with a hydroxyl radical only being lost when the groups are on adjacent ring atoms. Other ions present in the spectra point to interaction between substituents to form a new heterocyclic ring.     

Ion chemistry of phthalamic acids. III. The origin of [M ? 1]+ ions under electron impact

Electron impact mass spectra and collisional activation/mass-analysed ion kinetic energy spectra of some phthalamic acids and their deuterium labelled analogues suggested that the genesis of [M ? 1]⁺ ions is due to the loss of an aromatic hydrogen ortho to the amidic group, as for aromatic amides and thioamides.     

Electron impact mass spectrometry of some 2,11-diaza[3.3]cyclophane derivatives

The most significant mass spectral features of thirteen title compounds are discussed with the aid of high-resolution mass measurements and metastable peak analysis. The decomposition patterns of the compounds investigated are strongly affected by N-substitution and by methyl substituents ortho to the bridging chains (ortho effects). A unique feature connected with symmetrical macrocycles, bearing at least two ortho methyl substituents on each phenyl ring, is the presence in their spectra of diagnostically important peaks, corresponding to [M ? RNH₂]⁺˙ and [M ? 2RNH₂]⁺˙ (R = Ts, H, CH₃). These daughter ions are proposed to be associated with the formation of cage compounds (multibridged cyclophanes), generated by an intramolecular [4 + 4] cycloaddition reaction of unstable linear bis-(o-xylylene) precursors.     

Electron impact mass spectra of some dioximes of o-diacyl benzenes

The electron impact mass spectra of the new synthesized dioximes of o-diacyl benzenes (2) are reported. In addition to the molecular ion, characteristic peaks appear at values corresponding to the [M ? OH] ⁺, [M ? NOH]⁺ and [M ? NHOH]⁺ ions. No initial dehydration of the molecular ion has been observed.     

Unexpected mass spectral skeletal rearrangements in aromatic thioamides

Interesting skeletal rearrangements, resulting in the formation of unexpected fragments, have been noticed in the mass spectra of aromatic thioamides of cyclic amines such as piperidine, morpholine and pyrrolidine. Suitable mechanisms, based on mass analysed ion kinetic energy spectra, high voltage scan spectra and high resolution data, have been proposed for the formation of [M? SH]⁺ ions and the fragment at m/z (103+R) in the mass spectra of these compounds. The mass spectra of the thioamides of non-cyclic amines and the thioamides of aliphatic acids contain peaks corresponding to a four-centred skeletal rearrangement followed by the elimination of either the thioalkoxy or the thiophenoxy radical from the molecular ions.     

Cyclic ions in the mass spectra of trimethylsilyl derivatives of substituted o-dihydroxybenzenes

The mass spectra of trimethylsilyl (TMS) ethers/methyl esters of phenolic acids containing o-dihydroxybenzene groups have base peaks at [M?119]⁺ instead of the usual [M?15]⁺ and [M?31]⁺ that are characteristic of TMS/methyl esters of monohydroxyphenolic acids. These ions, formed by the loss of 31+88 u from the parent ion, possess a cyclic moiety as proven by substitution of deuterium atoms for hydrogen atoms in the TMS groups of the methyl esters of 3,4,5-trihydroxybenzoic (gallic), 3,4-dihydroxybenzoic (protocatechuic) and β-(3,4-dihydroxyphenyl)propenoic (caffeic) acids. Although these cyclic ions are the base peaks in TMS-derivatized o-dihydroxyphenolic acid esters, similar ions represent intense peaks but not necessarily the base peak in other derivatized compounds such as 1,2-dihydroxybenzene, 1,2-dihydroxy-3-methyl- and 1,2-dihydroxy-4-methyl-benzenes and flavan-3-ols that possess o-dihydroxybenzene groups. Compounds possession m- or p-dihydroxybenzene groups do not form these cyclic ions; therefore, this procedure for derivatization and interpretation of mass spectra is valuable for the identification of compounds containing o-dihydroxybenzene groups in complex mixtures of isomeric compounds.     

Electron impact induced hydrogen migrations in organic molecules: IV—novel hydrogen transfers in alkyl o-methoxybenzoates

Simultaneous hydrogen transfers—one from the methoxy group and the other from the alkyl group—to both the oxygen atoms of the ester function result in the formation of a common ion at m/z 152 in the alkyl o-methoxybenzoates on electron impact. Expulsion of the formyl radical from this ion leads to a fragment resembling the protonated benzoic acid. Another novel feature in these compounds is the loss of H₂O from the [M? R]⁺ ion which arises through an ortho effect during a secondary fragmentation process.     

Gas-phase nazarov cyclization of protonated 2-methoxy and 2-hydroxychalcone: An example of intramolecular proton-transport catalysis

Upon CA, ESI generated [M + H]⁺ ions of chalcone (benzalacetophenone) and 3-phenyl-indanone both undergo losses of H₂O, CO, and the elements of benzene. CA of the [M + H]⁺ ions of 2-methoxy and 2-hydroxychalcone, however, prompts instead a dominant loss of ketene. In addition, CA of the [M + H]⁺ ions of 2-methoxy-β-methylchalcone produces an analogous loss of methylketene instead. Furthermore, the [M + D]⁺ ion of 2-methoxychalcone upon CA eliminates only unlabeled ketene, and the resultant product, the [M + D − ketene]⁺ ion, yields only the benzyl-d ₁ cation upon CA. We propose that the 2-methoxy and 2-hydroxy (ortho) substituents facilitate a Nazarov cyclization to the corresponding protonated 3-aryl-indanones by mediating a critical proton transfer. The resultant protonated indanones then undergo a second proton transport catalysis facilitated by the same ortho substituents producing intermediates that eliminate ketene to yield 2-methoxy- or 2-hydroxyphenyl-phenyl-methylcarbocations, respectively. The basicity of the ortho substituent is important; for example, replacement of the ortho function with a chloro substituent does not provide an efficient catalyst for the proton transports. The Nazarov cyclization must compete with an alternate cyclization, driven by the protonated carbonyl group of the chalcone that results in losses of H₂O and CO. The assisted proton transfer mediated by the ortho substituent shifts the competition in favor of the Nazarov cyclization. The proposed mechanisms for cyclization and fragmentation are supported by high-mass resolving power data, tandem mass spectra, deuterium labeling, and molecular orbital calculations.     

Bifunctional interaction and its effect on the esterification of dicarboxylic acids in chemical ionization mass spectrometry

The chemical ionization mass spectra of different dicarboxylic acids, including saturated and unsaturated aliphatic, aromatic, hydroxyl and amino-substituted dicarboxylic acids, have been studied using pure methanol as the reagent gas. Biomolecular monoesterification and diesterification product ions [M+15]⁺ and [M+29]⁺, and adduct ion [M+33]⁺, were observed, in addition to the protonated molecule [MH]⁺ and unimolecular water elimination product ions. The formation of a protonated molecule with bridged intramolecular hydrogen bond, and its effect on the esterification of dicarboxylic acids is discussed. Geometric isomers, such as maleic and fumaric acid, and ortho and meta isomers of phthalic acids can be distinguished from each other by methanol chemical ionization mass spectra. When ethanol was used as the reagent gas, similar mass spectra of some dicarboxylic acids were obtained.     

The origin of the [M – 1]+ ion in the mass spectra of N,N-dialkylbenzamides. A reinvestigation of the mechanism

A reinvestigation of the mechanism of formation of the [M – 1]⁺ ion in a series of N,N-dialkylbenzamides suggests that previous mechanisms put forward to account for the formation of the [M – 1]⁺ ion are deficient. A new mechanism is proposed which accounts for the data observed previously, as well as our results for a series of N,N-dialkyl-2-chlorobenzamides, 4-substituted N,N-dimethylbenzamides and some related compounds. For the N,N-dialkyl-2-chlorobenzamides, comparison of the abundances of the [M – 1]⁺ ion with the [M – 35]⁺ ion suggests that a concurrent reaction is occurring, besides loss of the ortho aromatic hydrogen atom. A study of substituent effects on the intensity ratio [M – 1]⁺/[M]⁺ shows an upward concave plot of this against σ⁺, suggesting that two competing mechanisms occur for the formation of the [M – 1]⁺ ion.     

Analysis of tetrabenzyl N-giucosidic,N-mannosidic and N-galactosidic Isomers using fast atom bombardment/mass-analysed ion kinetic energy spectrometry

A series of new synthetic tetrabenzyl N-glucosidic, N-mannosidic and N-galactosidic isomers were investigated by fast atom bombardment (FAB)/mass-analysed ion kinetic energy (MIKE) spectrometry. The [M + H]⁺ ions were obtained with high abundance in the FAB spectra when using 3-nitrobenzyl alcohol as the matrix. The FAB/MIKE spectra provide characteristic daughter ions fragmented from selected molecular parent ions, allowing these isomers to be differentiated. In addition, an interesting rearrangement was found from the MIKE spectra, indicating that the benzyl (Bzl) group on the sugar ring is rearranged on to the N atom of the base (R) group to form [R + Bzl + H]⁺ and [R+ 2Bzl]⁺ ions.     

o-nitroaniline derivatives. IV—The mass spectra of o-nitroanils

The principal feature of the mass spectra of o-nitroanils, ArCH?NC₆H₄NO₂(o-), is an intense peak corresponding to the [ArCO]⁺ ion; this implies oxygen transfer from the nitro group to the azomethine carbon during the fragmentation process. In this series of anils, loss of OH from the molecular ion is not apparently an important fragmentation pathway, in contrast to the fragmentation of o-nitrobenzylideneanilines. Benzylideneaniline derivatives with an o-nitro substituent in both rings have mass spectra which indicate interaction of both nitro groups with the ? CH?N? group, but in this series of spectra the [M—17]⁺ ion is again of low intensity.     

Charge localization and aromatic stabilization in large ring ions: Mass spectra of ms-porphyrins

The major mass spectrometric fragments of ms-tetraphenylporphin and ms-tetra(p-chloro)phenylporphin are [M ? H]⁺˙ and [M ? Cl]⁺˙, respectively. Metal derivatives of these compounds give a modified characteristic fragmentation pattern with peak groups ending in the ions [M ? 4H]⁺˙, [M ? ? ? 5H]⁺˙ and [M ? 2? ? 2H]⁺˙ for the metallo ms-tetraphenylporphins, and [M ? ?Cl ? 2Cl ? 3H]⁺˙ and [M ? 2?Cl ? Cl ? H]⁺˙ for Mgms-tetra(p-chloro)phenylporphin. Deuterated metal derivatives indicate random hydrogen loss from both phenyl and pyrrole carbons. However, metal substituents do not significantly modify the fragmentation pattern in the case of ms-tetra(p-methoxy)phenylporphin. These patterns can be explained in terms of aromatic stabilization of the fragmentation products, coupled with charge localization on the π system in the free base, on the metal atom in the metallo derivatives and on the methoxy function in the p-methoxyphenyl derivative.     

The gas enhanced negative ion mass spectra of polychloroanisoles

Methane or a methane–oxygen mixture was used as an enhancement gas to obtain negative ion mass spectra of polychloroanisoles. Dichloroanisoles did not react with oxygen but the more highly chlorinated anisoles did. Compounds with hydrogen ortho to the methoxy group had [M? 1]^? ions, while others gave . The fragment arose through loss of an ortho chlorine and amethyl hydrogen. The loss of HCl followed by oxygen displacement of a remaining ortho or para chlorine produced [M? 55]^? ions; the para position was the preferred site of displacement. Another ion-molecule reaction with oxygen leads to [M? CH₂Cl]^?. The fragmentations resemble those of chlorinated aromatics such as the polychlorodibenzodioxins.     

Mass spectometry of aralkyl compounds with a functional group—VI1: Mass spectra of 1-phenylethyanol-1, 2-phenylethanol-1 and 1-phenylpropanol-2

Mass spectra of 1-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain, suggest that the [M? CH₃]⁺ ion is represented partly by an α-hydroxybenzyl fragment. Moreover, the molecular ion loses successively—after scrambling of all hydrogen atoms, except those of CH₃? a hydrogen atom and C₆H₆, generation the CH₃CO⁺ ion. Diffuse peaks, found in the spectra of of 2-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain and in the phenyl ring, show that the molecular ion loses C₂H₄O, possibly via a four-center mechanism, after an exchange of aromatic and hydroxylic hydrogens. Mass spectra of 1-phenylpropanol-2 and its analogues, specifically, deuterated in the aliphatic chain, demonstrate that in the molecular ion exclusively the hydroxyl hydrogen atom is transferred to one of the ortho-positions of the phenyl ring via a McLafferty rearrangement, generating the [M ? C₂H₄O]⁺ ion. Furtherore, an eight-membered ring structure is proposed for the [M ? CH₃]⁺ ion to explain the loss of H₂O and C₂H₂O from this ion after an extensive scrambling of hydrogen atoms.     

Ortho effects in organic molecules on electron impact. 14—Concerted and stepwise ejections of SO2 and N2 from N-arylidene 2-nitrobenzenesulphenamides

Unexpected ortho interaction of the nitro group has been noticed during the mass spectral fragmentations of N-arylidene 2-nitrobenzenesulphenamides, where the molecular ions expel SO₂ and N₂ both in concerted and stepwise processes. Loss of a hydrogen or the substituent from this fragment leads to a very abundant ion in all the compounds studied. Based on chemical evidence and linked-scan studies, a 1,2-phenylenetropylium cation structure has been postulated for the [M–SO₂–N₂–H/substituent]⁺ ion.