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


Tetrabutylammonium ion (1) forms tight ion pairs with small anions (Cl-, BH4-) in CDCl3solution. 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 and15N 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 of1 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, and13C 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.



Publication Date


Journal Title

Journal of the American Chemical Society


American Chemical Society

Digital Object Identifier (DOI)


Document Type



Copyright © 1997 American Chemical Society