Competing Gas-Phase Substitution and Elimination Reactions of Gemini Surfactants with Anionic Counterions by Mass Spectrometry. Density Functional Theory Correlations with Their Bolaform Halide Salt Models

Abstract

Understanding ion specific effects on the solution properties of association colloids is a major unsolved problem, and we are studying the chemistry of gemini surfactants in the gas-phase by mass spectrometry and density functional theory (DFT) to probe ion specific effects in the absence of water. Products from gas-phase fragmentation chemistry of dication-monoanion pairs, M2+X(-), of C16H33(CH3)2N+-(CH2)(n-) +N(CH3)2C16H33.2X(-) gemini surfactants were determined by using sequential collision induced dissociation mass spectrometry. The spacer length "n" was systematically varied (n = 2, 3, 4, and 6) for each counterion investigated (X(-) = F(-), Br(-), Cl(-), I(-), NO3(-), CF3CO2(-), and PF6(-)). The M2+X(-) pairs fragment into monocationic products from competing E2 and S N2 pathways that are readily quantified by tandem MS. The dominant reaction pathway depends on dication and anion structure because it switches from E2 to S N2 with decreasing anion basicity and increasing spacer length. For spacer lengths n = 4 and 6, the major S N2 product shifts from attack at methylene to methyl on the quaternary ammonium group. DFT calculations of gemini headgroup model bolaform salts, CH3(CH3)2N+-(CH2)(n-)+N(CH3)2CH3.2X(-) (X(-) = F(-), Cl(-), Br(-), and I(-), n = 2-4), primarily of activation enthalpies, DeltaH, but also of free energies and entropies for the dication-monoanion pairs, M2+X(-), provide qualitative explanations for the MS structure-reactivity patterns. DeltaH values for S N2 reactions are independent of X(-) type and spacer length, while E2 reactions show a significant increase in DeltaH with decreasing anion basicity and a modest increase with spacer length. Comparisons with the DeltaH values of model CH3CH2(CH3)3N+X(-) halides show that the second charge on the dicationic ion pairs does not significantly affect DeltaH and that the change in distance between the nucleophile and leaving group in the ground and transition states structures in S N2 reactions is approximately constant indicating that DeltaH is governed primarily by electrostatic interactions.