A method for the analysis of four widely used explosives based on reversed-phase liquid chromatography coupled to a quadrupole mass spectrometer is presented. A microflow rate particle beam interface was employed that offers simplified operation procedures and improved interfacing performance. A positive role played by the reduced size of the aerosol droplets generated by the microflow rate interface is outlined in this work. Greater vaporization efficiency and negligible thermal decomposition were observed for the selected compounds in the ion source of the mass spectrometer. Electron capture ionization allowed specific and sensitive determination of the analytes. Detection limits that ranged between 60 and 200 pg and had a signal-to-noise ratio of 5:1 were obtained in selected ion monitoring mode after column elution.

A first demonstration of the existence of the concerted SN2' mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA' acids (GA+ = C,H: (n = I , 2), iC,H:, and (CH,),F+) with trans-(1) and cis-2-buten-1-01 (2) as well as with I-buten-3-01 (3). Firm evidence in favor of the concerted SN2' pathway accompanying the classical SN2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA' attack on 1-3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3-butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive SN2' and SN2 pathways in the oxonium ions from 1 (SN2' (57f2%) and SN2 (43 2 YO)) and 3 (SN2' (54 + 2 YO) and SN2 (46+2%)), whereas, with 2, the SN2 (66 2 YO) reaction prevailed over the SN2' one (34 2 %). The role of intrinsic structural factors in determining the SN2'/SN2 branching in the selected oxonium ions is discussed.

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Our radiolytic study of the occurrence in the gas phase of concerted SN2' reactions on several open-chain allylic oxonium ions generated in the gas phase from the attack of gaseous GA+ acids (GA' = C , H i ( n = l , 2), iC,H:, and (CH,),F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3-methyl-2-cyclohexen-1-01 (1) and 1-methyl-2-cyclohexen-1-01 (2), with both MeOH and NMe, as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compound, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe,, only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the substitution and elimination products allows definition of the corresponding reaction patterns. As for open-chain oxonium ions, the nucleophilic attack on O-protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the 0-protonated 1 undergoes the concerted S,2 process with MeOH almost exclusively (> 99 %). With 0-protonated 2, however, the concerted SN2' pathway (84-95%) prevails over the classical S J one (6-17%). Concomitant [1,2] (E2) and [1,4] elimination (E 2') pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2'/E2:1.78-1.96 (1); 1.43-1.70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe,), and the leaving group (whether H,O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the SN2'/SN2 and E2'1 E 2 branchings in the selected oxonium ions is discussed and compared with related gas-phase data.

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