Cationic gemini surfactants, twin tailed surfactants with spacers between the cationic head groups, have emerged as a fascinating area of study during the last decade. There has been little research conducted to determine counterion interactions with the head group. Chemical trapping results of 1-n-1 (n = 2-4) show that Br- is bound better than Cl- and that as the spacer length, n, gets longer, counterion binding decreases. We believe that this strong counterion binding can be explained by the “pincer” binding model in which an anion can simultaneously interact with both positive charges, and the flexibility of the spacer is related to the strength of interaction. By placing the two positive charges on a rigid five membered ring, 1,1,3,3-tetramethylimidazolidinium (1-Im-1), counterion binding should be reduced. Chemical trapping of 1-Im-1 2 Br- demonstrated that the rigid spacer has weaker counterion binding than the flexible 1-n-1 cations, however, 1-Im-1 2 Cl- displayed stronger counterion binding than any of the 1-n-1 cations. Crystal structures were obtained of 1-Im-1 2 X- (X- = Cl-, Br- and I-) in which one counterion sits above the ring in position to interact with the methyl groups of the imidazolidinium. FT-IR spectra are consistent with hydrogen bonding occurring between the methyl groups and the halide ion in a CH•••X- nature.
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