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 T1 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 T1 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 T1 for silane 2 provides an estimate of aggregate size which is independent of that obtained by diffusion, with good agreement between the different approaches.