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

Tetrabutylammonium ion (1) forms tight ion pairs with small anions (Cl -, BH 4 -) in CDCl 3 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 1 H{ 1 H} NOEs observed between 1 and the BH 4 -anion in CDCl 3 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 15 N{ 1 H} NOE and 15 N T 1 measured at several magnetic field strengths, using 15 N-labeled 1 to obtain estimates for reorientational correlation times for the ion aggregates. A similar analysis of 13 C{ 1 H} NOE and 13 C T 1 gives local effective correlation times for C-H bond vectors of the 1-CH 2 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 29 Si{ 1 H} NOE, 13 C{ 1 H} NOE, and 13 C 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. JA9723139 log η ) -3.646 + 584.9t -1 + (7.22 × 10 -3 )t -(8.26 × 10 -6 )τ 2