CLOSED-SHELL ION-PAIR STRUCTURE AND DYNAMICS - STEADY-STATE H-1(H-1), B-10(H-1, AND B-11(H-1) NUCLEAR OVERHAUSER EFFECTS AND B-10, B-11 NUCLEAR-RELAXATION OF TETRAALKYLAMMONIUM TETRAHYDRIDOBORATES IN ION-PAIRING AND DISSOCIATIVE SOLVENTS

The magnitudes of selective steady-state H-1{H-1} interionic nuclear Overhauser effects (NOEs) and nonselective B-10{H-1} and B-11{H-1} NOEs as well as B-10 and B-11 spin-lattice relaxation times were measured for solutions of (butyl)4N+,BH4- 1a in (HCCl3)-H-2, (H2O)-H-2, and [H-2(6)]dimethyl sulfoxide. Interionic NOEs resulting from the selective saturation of cationic alkyl resonances were used to evaluate the time average structure of the ion pair la in various solvents. The largest NOEs are observed in (HCCl3)-H-2, while smaller specific interionic NOEs were observed in [H-2(6)]-dimethyl sulfoxide and [H-2(6)]benzene. Only small nonspecific interionic NOEs were observed in (H2O)-H-2 Solutions of 1a. B-10 and B-11 T1 relaxation times and nonselective steady-state B-10{H-1} and B-11{H-1} NOEs were also measured for solutions of NaBH4 2 in 2H2O and [H-2(6)]dimethyl sulfoxide in order to determine the effect of counterion on observed relaxation behavior for BH4-. Dipolar and quadrupolar contributions to observed spin-lattice relaxation of both boron nuclei were calculated. B-10{H-1} and B-11{H-1} NOEs obtained upon broad-band presaturation of H-1 resonances were in all cases similar to the values predicted from dipolar contributions to B-10 and B-11 relaxation. Quadrupolar coupling constants of 5.2 x 10(5) Hz for B-10 and 2.5 x 10(5) Hz for B-11 in the BH4- anion were calculated. Quadrupolar contributions to boron relaxation in ion pair la correlate with measured solution viscosity as a function of temperature under ion pairing conditions, but interionic H-1{H-1} NOEs do not, showing a negative deviation from intensity expected as a result of viscosity. Increasing the size of the cation increases the deviation. It is concluded that magnitude and sign of interionic NOEs reflect the motion of the ion pair as a unit, and hence the effects of multiion aggregation, while the relaxation of the boron nucleus reflects more rapid motion independent of the ion pair. The correlation time of the interionic vector is used to predict a lower limit on the mean lifetime of a discrete ion binding mode.