Closed-Shell Ion Pairs:  Cation and Aggregate Dynamics of Tetraalkylammonium Salts in an Ion-Pairing Solvent

Abstract

Tetrabutylammonium ion (1) forms tight ion pairs with small anions (Cl-, BH4 -) in CDCl3 solution. These ion pairs aggregate as a response to increasing solution concentration with little temperature dependence. Maximum aggregate size is approximately four ion pairs, as measured by comparing self-diffusion coefficients of the aggregates with that of an internal nonaggregating standard of the same shape and nominal size, tetrabutylsilane (2). The magnitudes of steady state interionic 1H{1H} NOEs observed between 1 and the BH4 - anion in CDCl3 as a function of temperature in solutions of fixed concentration are well fit to the standard theoretical expression by assuming a single aggregate size that is independent of temperature. A simplified model-free analysis was applied to steady state 15N{1H} NOE and 15N T 1 measured at several magnetic field strengths, using 15N-labeled 1 to obtain estimates for reorientational correlation times for the ion aggregates. A similar analysis of 13C{1H} NOE and 13C T 1 gives local effective correlation times for C−H bond vectors of the 1-CH2 carbon of 1 and order parameters relating the local motion to overall cation motion. Comparison of these correlation times with those obtained from analysis of 29Si{1H} NOE, 13C{1H} NOE, and 13C T 1 for silane 2 provides an estimate of aggregate size which is independent of that obtained by diffusion, with good agreement between the different approaches.

Substituent effects on cation–π interactions: A quantitative study

“…There are clearly two different processes taking place. The literature indicates that the process that takes place at low concentrations is dissociation of the ion pair, and the process at high concentrations is dimerization or aggregation of the ion pair (19,20). The experimental data fit well to isotherms based on this model giving K ip ϭ 7.3 Ϯ 0.5 ϫ 10…”

Rigid Calix[4]arene as a Building Block for the Synthesis of New Quaternary Ammonium Cation Receptors

“…In chloroform, however, these cations are present as tightly-bound ion pairs, [18] and thus in order to clarify the role of the anion on the extent of complexation, their halides (chloride or iodide, depending on solubility) and tosylates (Ts) were studied. To gain further insight into the bind- [4]arene 10 [8a] and methylene-bridged double calix [4]arene 11, [19] flexible analogues of rigid 1 and 2, respectively ing mode of the flat N-methylpyridinium iodide, the recognition of N-methyl-2-picolinium (NM2Pic) and N-methyl-4-picolinium (NM4Pic) iodides was also studied (see Figure 1a).…”

An Effective Fluorescent Sensor for Choline-Containing Phospholipids

“…In addition, the binding of 3 a to BTABr should restrict the translational freedom of the bromide ion about the BTA cation, leading to an entropic cost not present during DPPC complexation. [20] These factors could explain why the binding free energy for DPPC is 1.2 kcal mol À1 higher than that for BTABr. We have designed a fluorosensor, in which unique squaric acid diamides serve as the binding substructure, for signaling the presence of selected choline phospholipids.…”