Investigation of an Unusual Frequency Dependence of Nuclear Magnetic Relaxation Rates in Aqueous Solutions of Tetramethylammonium Chloride

Abstract

The 1H, 14N and 35Cl nuclear magnetic relaxation rates of (CH3)4NCl in D2O were studied as a function of salt concentration at different resonance frequencies of the respective nuclei. At aquamolal salt concentrations c > 3 m, the 1H spin-lattice relaxation rate is frequency dependent even at room temperature. The intramolecular and the intermolecular parts of the dipole-dipole relaxation rate of the (CH3)4N+ protons were measured separately by means of the method of isotopic dilution. With exception of very high salt concentrations (c > 9 m), only the intermolecular part of the 1H relaxation rate (1/T1)inter is found to be frequency dependent, while for the intramolecular part, the extreme narrowing condition remains fulfilled. The observed frequency dependence of (1/T1 )inter is compared with results of theoretical approaches to the intermolecular relaxation rate; satisfactory agreement is found only if the propagator for the translational molecular motion is modified by including a slow-motion process. The correlation time of the corresponding longtime part of the time correlation function for the interaction between 1H spins on different (CH3)4N+ ions was found to be τc ≍ 10−9 s. For the 14N and 35Cl spin-lattice relaxation rates in the same system, no frequency dependence was observed. In relaxation studies of liquid systems, where intermolecular relaxation contributions play a significant role, a check for a possible frequency dependence is recommended even in systems.