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.     

Etude en Basse Tension de l'Androstane Dione-3,17—Application a la Localisation du Deuterium

The fragmentation mechanisms of (5α) 3,17-androstanedione have been studied in order to determine whether the deuterium is located at C-4 or C-6. This is possible at low voltage where a peak at m/e 216, arising from a A-ring cleavage allows the determination of isotope distribution between C-4 and C-6.     

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.     

Formation and characterization of gaseous adducts of carbon dioxide to magnesium, (CO2)MgX- (X=OH, Cl, Br)

A good fix: the structure and chemical reactivity of a reduced form of CO(2) bonded to magnesium, XMg(η(2)-O(2)C)(-), is reported. Upon reaction with water it loses CO, while it adds CH(3) upon reaction with alkyl halides, thereby signifying nucleophilicity of the carbon atom in XMg(η(2)-O(2)C)(-) in S(N)2 reactions.     

Gas-phase ionization/desolvation processes and their effect on protein charge state distribution under matrix-assisted laser desorption/ionization conditions

The charge state distribution of proteins was studied as a function of experimental conditions, to improve the understanding of the matrix-assisted laser desorption/ionization (MALDI) mechanisms. The relative abundances of the multiply-charged ions appear to be a function of the matrix chosen, the laser fluence and the matrix-to-analyte molar ratio. A correlation is found between the matrix proton affinity and the yield of singly- versus multiply-charged ions. These results are in good agreement with a model in which gas-phase intracluster reactions play a significant role in analyte ion formation. A new model for endothermic desolvation processes in ultraviolet/MALDI is presented and discussed. It is based upon the existence of highly-charged precursor clusters and, complementary to the ion survivor model of Karas et al., assumes that two energy-dependent processes exist: (i) a soft desolvation involving consecutive losses of neutral matrix molecules, leading to a multiply-charged analyte and (ii) hard desolvation leading to a low charge state analyte, by consecutive losses of charged matrix molecules. These desolvations pathways are discussed in terms of kinetically limited processes. The efficiency of the two competitive desolvation processes seems related to the internal energy carried away by clusters during ablation.