Closed-Shell Ion Pairs: Cation and Aggregate Dynamics of Tetraalkylammonium Salts in an Ion-Pairing Solvent Huaping Mo, Anping Wang, ² Patricia Stone Wilkinson, ‡ and Thomas C. Pochapsky* Contribution from the Department of Chemistry, Brandeis UniVersity, Waltham, Massachusetts 02254-9110 ReceiVed July 11, 1997. ReVised Manuscript ReceiVed September 26, 1997 X Abstract: 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. Introduction Ion pairing is a phenomenon of considerable interest to physical scientists in a variety of fields, and has been under intense investigation since Bjerrum first introduced the concept in the early part of this century. 1-3 Much of what is known or conjectured concerning ion pairs is the result of measuring electrical and colligative properties of ion pair-containing solutions. 4 However, as spectroscopic methods, particularly magnetic resonance techniques, have become more sophisti- cated, it has become possible to obtain more direct information concerning the structure and dynamics of ion pairs. 5-10 We have previously described the use of steady-state 1 H{ 1 H} nuclear Overhauser effects (NOEs) to characterize the time-average structure of tetraalkylammonium tetrahydridoborate (R 4 N + , BH 4 - ) ion pairs in nonpolar solvents. 11,12 This structure places the BH 4 - anion in a trigonal pyramidal site created by three alkyl chains of the tetraalkylammonium ion, and shows close contact between the anion and the protons on the methylene group adjacent to the quaternary nitrogen of the cation. We also used steady-state 11 B{ 1 H} and 10 B{ 1 H} NOEs as well as 10 B and 11 B relaxation measurements to characterize the motions of the BH 4 - anion in these ion pairs. 12 We found that in solutions of tetrabutylammonium tetrahydridoborate (1a) in CDCl 3 , the anion reorients rapidly (τ c ∼ 10 -12 s) relative to the overall motion of the ion pair. Furthermore, anion reorientation is sufficiently fast to average the local electrical environment, suppressing quadrupolar relaxation of the boron nucleus to a large extent. 10 B and 11 B relaxations for 1a do not show significant differences when measured in nonpolar and dis- sociative solvents (such as water), so ion pairing does not appear to significantly affect the electronic environment of the boron. 12 The motion of the cation, as well as the overall motion of the ion pair, is somewhat more complicated. The results of interionic NOE measurements for 1a and for the related compounds tetraisoamylammonium tetrahydridoborate (1b) and tetraoctylammonium tetrahydridoborate (1c) in chloroform over a range of temperatures suggested to us that these ion pairs aggregate in nonpolar solution. 13 NMR self-diffusion measure- ments performed with solutions of 1a in CDCl 3 support the conclusion that ion pairs formed by 1a aggregate in chloroform and also confirm that the ion aggregate is the primary diffusing species. 14 Self-diffusion measurements for tetrabutylammonium chloride (1d) permitted us to estimate aggregate size by comparing the self-diffusion of the cation to that of a nonionic internal reference of similar shape and nominal mass, tetra- butylsilane (2). 14 Similar estimates were made for for 1a as well. 15 The present work is aimed at characterizing the motions of the tetrabutylammonium (TBA + ) cation and ion aggregates in ² Current address, General Electric Inc., Waterfront, NY. ‡ Current address, Bruker Instruments, Inc., Billerica, MA. 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Soc. 1993, 115, 11084-11091. (13) Stone, P. M.; Pochapsky, T. C.; Callegari, E. J. Chem. Soc., Chem. Commun. 1992, 178-179. (14) Pochapsky, S. S.; Mo, H.; Pochapsky, T. C. J. Chem. Soc., Chem. Commun. 1995, 2513-2514. (15) Mo, H.; Pochapsky, T. C. J. Phys. Chem. B 1997, 101, 4485. 11666 J. Am. Chem. Soc. 1997, 119, 11666-11673 S0002-7863(97)02313-5 CCC: $14.00 © 1997 American Chemical Society