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 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 b y comparing self-diffusion coefficients of the aggregates with that of an internal nonaggregating standard of the same shape and nominal siz e, tetrabutylsilane (2). The magnitudes of steady state interionic H-1 {H-1} NOEs observed between 1 and the BH4- anion in CDCl3 as a functio n of temperature in solutions of fixed concentration are well fit to t he standard theoretical expression by assuming a single aggregate size that is independent of temperature. A simplified model-free analysis was applied to steady state N-15 {H-1} NOE and N-15 T-1 measured at se veral magnetic field strengths, using N-15-labeled 1 to obtain estimat es for reorientational correlation times for the ion aggregates. A sim ilar analysis of C-13{H-1} NOE and C-13 T-1 gives local effective corr elation times for C-H bond vectors of the 1-CH2 carbon of 1 and order parameters relating the local motion to overall cation motion. Compari son of these correlation times with those obtained from analysis of Si -29{H-1} NOE, C-13{H-1} NOE, and C-13 T-1 for silane 2 provides an est imate of aggregate size which is independent of that obtained by diffu sion, with good agreement between the different approaches.