Closed-shell Ion Pair Structure and Dynamics


Ion pairing is a phenomenon of considerable importance in chemical and biological systems, and has been studied by physical and spectroscopic methods for many years. NMR spectroscopy permits us to examine the structures and dynamics of ion pairs. We are interested in the closed-shell ion pairs formed in non-polar solvents by large lipophilic cations such as quaternary ammonium ions and small hard anions such as chloride and tetrahydridoborate (BH4-). These ion pairs are important in organic phase-transfer chemistry, and insight into their behavior will be used to design improved catalysts for phase-transfer reactions. We observed interionic NOEs in the tetrabutylammonium tetrahydridoborate (TBABH4) ion pair in CDCl3 showing that structural studies of ion pairs in non-polar media by NMR are feasible, and used pulsed field gradient NMR methods to  estimate ionic aggregate size as a function of temperature and concentration. We found that the cation and anion diffuse at the same rate using borohydride anion diffusion as a probe. The dynamics of the anion reorientation in the TBABH4 ion pair under a variety of conditions were characterized by 11B{1H}, 10B{1H} NOE and 11B, 10B relaxation measurements. We have analyzed the results of 13C{1H} and 15N{1H} NOE and 13C,15N relaxation experiments in order to characterize the dynamics of the cation and overall aggregate motion in the TBABH4 ion pair.   We applied this information to the design of more selective phase transfer catalysts (PTCs). 


"Closed-shell ion pairs:  Cation and aggregate dynamics of tetraalkylammonium salt in an ion-pairing solvent" (H. Mo, T. C. Pochapsky , A. Wang and P. Stone Wilkinson), J. Am. Chem. Soc. 119, 11666-11673 (1997).

"Self-diffusion coefficients of paired ions", (H. Mo and T. C. Pochapsky), J. Phys. Chem. B. 101, 4855-4486 (1997).

"Closed-Shell Ion Pair Aggregation in Non-Polar Solvents Characterized By NMR Diffusion Measurements", (S. S. Pochapsky, T. C. Pochapsky and H.P. Mo) J. Chem. Soc., Chem. Comm. 2513 (1995).

"Closed-Shell Ion Pair Structure and Dynamics: Steady State 1H{1H}, 10B{1H} and 11B{1H} Nuclear Overhauser Effects and 10B, 11B Nuclear Relaxation of Tetraalkylammonium Tetrahydridoborates in Ion-Pairing and Dissociative Solvents", (T. C. Pochapsky , A.-P. Wang and P. M. Stone) J. Am. Chem. Soc.  115, 11084-11091 (1993).

"A Study of Ion Pair Solution Structure Using Nuclear Overhauser Effects: Interionic 1H{1H} and 11B{1H} NOEs in the (n-Butyl)4N+,BH4- Ion Pair" (T. C. Pochapsky  and P. M. Stone) J. Am. Chem. Soc. 112, 6714-15 (1990).  (Communication)