Intracluster reactions in negatively charged aggregates of diquaternary amines – Gemini surfactants with bromide and formate counterions

Abstract

Three series of the gemini surfactants (diquaternary amines) anionic clusters with formate or bromide anions were studied in the gas phase by ER-MS on triple quadrupole mass spectrometer. The gemini series were: alkanediyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium), alkanediyl-α,ω-bis-(N-hydroxyethyl-N-methyl-N-dodecyl ammonium) and oligo(oxa)ethyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium), where the alkyl spacer size was from 2 to 12 methyl groups, and the oxaethyl range was from 2 to 8 units. The clusters were formed by one dication and three anions with formulas: [MBr3]−, [MBr2HCOO]−, [MBr(HCOO)2]− and [M(HCOO)3]−. Collisions induced internal reactions: nucleophilic substitutions SN2 at nitrogen α-carbons, eliminations E2, and hydrogen transfer from hydroxy group to the anion. By quantitative determination of the amounts of fragments in each dissociation channel and by plotting the ratios in function of the spacer lengths it was found that the clusters with short and long spacers formed two distinct reaction patterns. By comparisons to the cationic cluster reactions it was found that crowding inside the anionic cluster caused by the extra anions makes them to reacts through pathways they avoided in cationic clusters. Mixed anion clusters were determined to produce the same sets of fragments as homogeneous clusters in the amount which roughly corresponds to the increment of a given anion; on these bases, it was decided that anions are mobile in the cluster prior to dissociation reaction. When ethyloxy group in the mixed cluster was one of the nitrogen substituents a slight preference was toward formate anion products was observed, which was explained by formation of hydrogen bonding.