Ion–molecule reaction in the gas phase 3—nucleophilic substitution of 17ξ-R-5α,14β-androstane-14,17ξ-diols under [NH4]+ chemical ionization conditions1

Under Ammonia chemical Ionization conditions the source decompositions of [M + NH₄]⁺ ions formed from epimeric tertiary steroid alchols 14 OH_β, 17OH_α or 17 OH_β substituted at position 17 have been studied. They give rise to formation of [M + NH₄? H₂O]⁺ dentoed as [MH_sH]⁺, [M_sH? H₂O]⁺, [M_sH? NH₃]⁺ and [M_sH? NH₃? H₂O]⁺ ions. Stereochemical effects are observed in the ratios [M_sH? H₂O]⁺/[M_sH? NH₃]⁺. These effects are significant among metastable ions. In particular, only the [M_sH]⁺ ions produced from trans-diol isomers lose a water molecule. The favoured loss of water can be accounted for by an S_N2 mechanism in which the insertion of NH₃ gives [M_sH]⁺ with Walden inversion occurring during the ion-molecule reaction between [M + NH₄]⁺ + NH₃. The S_N1 and S_Ni pathways have been rejected.     

Chemical ionization and collisional activation spectra of α,β-unsaturated nitriles

A study of the chemical ionization (CI) and collisional activation (CA) spectra of a number of α, β-unsaturated nitriles has revealed that the even-electron ions such as [MH]⁺ and [MNH₄]⁺ produced under chemical ionization undergo decomposition by radical losses also. This results in the formation of M ⁺˙ ions from both [MH]⁺ and [MNH₄]⁺ ions. In the halogenated molecules losses of X˙ and HX compete with losses of H˙ and HCN. Elimination of X˙ from [MH]⁺ is highly favoured in the bromoderivative. The dinitriles undergo a substitution reaction in which one of the CN groups is replaced with a hydrogen radical and the resulting mononitrile is ionized leading to [M ? CN + 2H]⁺ under CI(CH₄) or [M ? CN + H + NH₄] and [M ? CN + H + N₂H₇]⁺ under CI(NH₃) conditions.     

Ammonium adduct ion in ammonia chemical ionization mass spectrometry: 2—Mechanism of elimination of neutrals from the ammonium adduct ion

The mechanism of elimination of ROH (R = H or CH₃) from the ammonium adduct ion, [M+NH₄]⁺, of 1-adamantanol and its methyl ether is examined by using linked-scan metastable ion spectra and by measuring the dependence of the peak intensity ratio [M+NH₄]⁺/[M+NH₄? ROH]⁺ on ammonia pressure. For 1-adamantanol both S_Ni and S_N1 reactions are suggested in metastable ion decomposition, while only the S_N1 mechanism is operative in the ion source. For 1-adamantanol methyl ether the S_N1 reaction predominates both in metastable ion decomposition and in the ion source reaction.     

Substitution reactions under NH3 chemical ionization conditions in cis-/trans-1,2-dihydroxybenzosuberans

The nucleophilic substitution reaction under NH₃ chemical ionization (CI) conditions in cis- and trans-1,2-dihydroxybenzosuberans (1–4) has been studied with the help of ND₃ CI and metastable data. The results indicate that in the parent diols 1 (cis) and 2 (trans), the substitution ion [M_sH]⁺, is produced mainly by the loss of H₂O from the [MNH₄]⁺ ion (S_Ni reaction) while in their 7-methoxy derivatives 3 and 4, the ion-molecule reaction between [M? OH]⁺ and NH₃ seems to be the major pathway for the formation of [M_sH]⁺. The substitution ion from 1 and 2 and the [MH]⁺ ion from trans-1-amino-2-hydroxybenzosuberan give similar collision-induced dissociation mass-analysed ion kinetic energy spectra. Interestingly, their diacetates do not undergo the substitution reaction.     

Study of selected ions in the ammonia desorption chemical ionization mass spectra of peracetylated gentiobiose and two isotopically labelled peracetylated gentiobioses

The ammonia desorption chemical ionization (NH₃-DCI) mass spectra of peracetylated gentiobiose (1) and two isotopically labelled gentiobioses (2 and 3) were examined. Compound 2 is labelled with trideuteroacetyl groups in the non-reducing moiety and 3 with trideuteroacetyl groups in the reducing moiety. It is shown that the [M + NH₄ – 42]⁺ ion is not formed direct from [M + NH₄]⁺ by loss of ketene but appears to be formed by way of a nucleophilic acyl substitution reaction resulting in a neutral species which complexes with NH₄⁺. The disaccharides undergo cleavage at either side of the glycosidic oxygen joining the two sugar residues, a process which is accompanied by addition of H or CH₃CO to afford neutral species which complex with NH₄⁺. The structures of the ions resulting from H transfer have been inferred by comparison of their mass-analysed ion kinetic energy (MIKE) spectra with MIKE spectra of the [M + NH₄]⁺ ions of compounds of established structure. A ring fragmentation reaction of 1, 2 and 3 is reported.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Collision-induced dissociation study of cyclization of α-diazo-ω-arylsulphonylaminoalkan-2-ones

The collision-induced dissociation mass-analysed ion kinetic energy (CID MIKE) spectra (electron impact and chemical ionization) of five α-diazo-ω-arylsulphonylaminoalkan-2-ones and corresponding N-arylsulphonylazetidin-3-ones and N-arylsulphonylpyrrolidin-3-ones were studied. The [M ? N₂]⁺˙ and [MH ? N₂]⁺ ions of two types of the diazo ketones provide CID MIKE spectra similar to those of the corresponding M⁺˙ and MH⁺ of the heterocyclic compounds, i.e. a cyclization analogous to that in solution takes place. For the other three types of diazo compounds the Wolff rearrangement prevails in both the gas and liquid phases. The effect of the substituents on the cyclization process was studied. The data obtained permit the results of acid-catalysed cyclization of similar diazo ketones to be predicted on the basis of their CID MIKE spectra. Chemical ionization provides a closer similarity with reactions in solution than electron impact ionization, which can be rationalized by the protonation of the diazo ketone molecule being the driving force of the cyclization reaction either in solution or in the ion source of a mass spectrometer.     

Ammonia in the ion source. II. Clustering with aromatic nitro compounds

Under positive ion chemical ionization conditions with ammonla at relatively low pressure, aromatic nitro compounds do not form [M + H]⁺ ions but often form ionic clusters [M + NH₄]⁺ and [M + N₂H₇]⁺. Nitrobenzene forms a cluster [2M + NH₄]⁺ and aniline, formed by nucleophilic substitution, leads to a cluster [anilinium ion + nitrobenzene]⁺. The dinitrobenzenes form [M + NH₄]⁺ clusters and show evidence of nitroaniline formation and clustering. 1,3,5-Trinitrobenzene gives little indication of clustering or of substitution. The six isomers of trinitrotoluene appear to be stabilized by the methyl group and form clusters up to [M + N₃H₁₀]⁺. Nucleophilic substitution leads to dinitrotoluidines, which also form clusters with ammonium ions.     

Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products

The mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p-XC₆H₄) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO₂NHR', RSO₂SR, RSSR and NH₂R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO₂C₆H₄NH₂, RSOC₆H₄NH₂ and RSC₆H₄NH₂ (in the case of arylsulfinyl arylamides of the type of X = H) The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]⁺, [M ? R]⁺, [M ? R + H]⁺, [M ? SO]⁺, [RS]⁺, [NHR']⁺, [NHR' + H]⁺, [RSO]⁺, [RSO + H]⁺, [R]⁺, [R + H]⁺, [R']⁺ and [M ? OH]⁺, and that the thermal decomposition products give the following ions: [RSO₂SR]⁺, [RSSR]⁺, [M ? O]⁺, [M + O]⁺ and [RSOC₆H₄NH₂]⁺.     

Mass-analysed ion kinetic energy and collisionally induced dissociation mass spectra of clusters of acetylated xylo-oligosaecharides with protonated ammonia and methylannine

Per-O-acetylated methyl glycosides of D -xylan-type di- and trisaccharides were studied by mass-analysed ion kinetic energy (MIKE) and collisionally induced dissociation (CID) mass Spectrometry using protonated ammonia and methylamine, respectively, as reaction gases in chemical ionization (CI). The oligosaccharides form abundant cluster ions, [M + NH₄]⁺ or [M + CH₃NH₃]⁺, and the main fragmentation of these ions in the MIKE and CID spectra is the cleavage of interglycosidic linkages. Thus, CI (NH₃) or CI (CH₃NH₂) spectra in combination with the MIKE or CID spectra allow the molecular masses, the masses of monosaccharide units and the branching point in oligosaccharides to be established. In the case of disaccharides, it is possible to distinguish the (1 → 2) linkage from the other types of linkages.     

Collisional activation spectra of the adduct ions of 1,2,3-triarylpropenones under ammonia chemical ionization conditions

Under ammonia chemical ionization (CI) conditions triarylpropenones undergo hydrogen radical-induced olefinic bond reduction on metal surfaces, resulting in [M + 2H + NH₄]⁺ ions corresponding to the ammonium adduct of the saturated ketone. The decomposition of the adduct ions, [MNH₄]⁺ and [M + 2H + NH₄]⁺, was studied by collision-induced dissociation mass-analysed ion kinetic energy (CID-MIKE) spectroscopy in a reverse geometry instrument. From the CID-MIKE spectra of the [MNH₄]⁺, [M + 2H + NH₄]⁺, [MND₄]⁺ and [M + 2D + ND₄]⁺ ions it is clear that the fragmentation of the adduct ions involves loss of NH₃ followed by various cyclization reactions resulting in stable condensed ring systems. Elimination of ArH and ArCHO subsequent to the loss of NH₃ and formation of aroyl ion are characteristic decomposition pathways of the [MNH₄]⁺ ions, whereas elimination of ArCH₃ and formation of [ArCH₂]⁺ are characteristic of the [M + 2H + NH₄]⁺ ions of these propenones.     

Unusual neutral and radical losses in 2′-substituted N-phenylanthranilic acids on electron impact

Unusual expulsions of [H₂O + CO₂] from the M⁺˙ of N-(o-carboxyphenyl)anthranilic acid, [H₂O + CH₂O] from the M⁺˙ of N-(o-methoxyphenyl)anthranilic acid and [H₂O + ˙NO₂] from the M⁺˙ of N-(o-nitrophenyl) anthranilic acid were observed under electron impact conditions. These processes are stepwise in the corresponding para-substituted N-phenylanthranilic acids. The proposed fragmentation pathways and their mechanisms are supported by B/E linked-scan spectra, collision-activated decomposition (CAD)–mass-analysed ion kinetic energy (MIKE) spectra, high-resolution data, deuterium labelling and chemical substitution.     

Ion–molecule reactions in the gas phase. XVIII.—nucleophilic substitution of diastereomeric norborneols,norbornyl acetates and benzoates under ammonia chemical ionization

The comparative behaviour of the endo- and exo-norborneols and diastereomeric derivatives (acetates and benzoates) towards the NH₃/NH₄⁺ system was investigated. It appears that the proton affinity (PA) of the substrate relative to Pa(NH₃) strongly influences competition between the protonation and nucleophilic substitution processes yielding the MH⁺ and [M + NH₄ ? H₂O]⁺ ions, respectively. Tandem mass spectrometry was used to compare collision-activated dissociation spectra of [M + NH₄ ? H₂O]⁺ with those of analogous endo- and exo-norbornylamines protonated in the source. This demonstrates that an S_Nimechanism occurs specifically for the isomeric norborneols; in contrast, for acetates and benzoates, stereospecific S_Ni and S_N2 pathways take place for exo and endo derivatives, respectively. This particular behaviour is explained by considering the steric effect induced by the endo-H at C(6). In addition, the competitive decompositions of [M + NH₄ – H₂O]⁺ into NH₄⁺ and [C₇H₁₁]⁺ daughter ions are consistent with the formation of a proton-bound complex intermediate. The observed stereochemical effects for these dauther ions are rationalized by means of arguments based on the estimated heats of formation of the transition states, which is lower for the exo-norbonyl protonated amine, consistent with anchimeric assistance, rather than a stepwise pathway which is proposed for the endoisomer.     

Distinction between 17-epimeric hydroxy steroids of the 3,17-dioxygenated androstane series by chemical ionization

The distinction between 17-epimeric 3,17-dioxygenated hydroxyandrostanes has been made by comparison of both their methane or ammonia positive and OH^? negative chemical ionization (CI) mass spectra. In the methane or ammonia positive CI, the 17α-configuration in the eight stereoisomeric 5ξ-androstane-3ξ,17ξ-diols can be determined by the relative abundances of the ion [MH? 2 H₂O]⁺. In the ammonia CI spectra, the ion [M+NH₄? H₂O]⁺ possesses only a low abundance, but a comparison of the relative rates of the loss of water v. the loss of ammonia from [M.NH₄]⁺ in the second field-free region allows a clear distinction to be made between the 17α- and 17β-series. In the OH^? negative CI mass spectra, the 5ξ-androstane-3-one-17ξ-ols produce an intense ion [M? H? H₂O]^? in the 17α-series only.     

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.     

Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Oxidation reactions in triterpenes under chemical ionization conditions

From a collisional activation spectral study it has been found that certain triterpene alcohols with an ursane or oleanane skeleton undergo oxidation to the corresponding ketones under chemical ionization (NH₃) conditions giving rise to abundant [M + NH₄ ? 2]⁺ ions. Mass-analysed ion kinetic energy and B²/E scan results indicate that both [M + NH₄]⁺ and [M + N₂H₇ ? 2]⁺ ions contribute to the formation of the [M + NH₄ ? 2]⁺ ion.     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

Thermospray mass spectrometry of N-methylcarbamates

The thermospray mass spectrometry (TSP/MS) of five N-methylcarbamates is presented. This is the first time that ions other than [M + H]⁺ and [M + NH₄]⁺ have been reported using positive TSP/MS. Protonation of ROCONHCH₃ yields the [CH₃NH₂CO]^+· ion, with formation of the ion–molecule adduct [ROCONHCH₃ · CH₃NH₂CO]^+· through elimination of CO^+· from [CH₃NH₂CO]^+·, and the adduct [M + 75]^+·, [ROCONHCH₃ · OCONH₂CH₃]^+·, is also obtained.     

Ammonia Chemical Ionization Mass Spectra of Sydnones

The mass spectra of twenty-nine sydnones were measured by using NII₃ as a reagent gas. The [M + H + NH₃]⁺ ions are the base peaks for the entire series. Very few fragment ions were observed except for the derivatives of 4-hydroxymethylenesydnones (XXI-XXIX). Only the relatively stable sydnonyl-methylene cations are formed. With consistent formation of [M + 18]⁺ ion as the base peaks with simple spectra, the NH₃ CI-MS can be an excellent method for the molecular weight determination.