Selective determination of relaxation times in high resolution NMR

Abstract

A technique is described for investigating the longitudinal and transverse relaxation times of individual lines of a high-resolution NMR spectrum. Pulse methods are used exclusively, high selectivity being achieved through the use of a weak irradiation level ( γH 1/2 π = 1 Hz) and long pulse durations (0.25–1 sec). This permits the nuclear induction signal to be monitored during the pulse. Instrumentation is considerably simplified by detecting signals in the form of modulation sideband responses excited by pulses of audio-frequency modulation applied to the magnetic field; in all other respects the equipment resembles a conventional high-resolution spectrometer. Evidence is presented that the measurements are not seriously perturbed by the presence of near-neighbor lines, while the influence of finite instrumental line width, which can be appreciable in spin echo experiments, can be effectively compensated through a simple modification to the phase of the excitation signal. If high selectivity is to be maintained, the method is restricted to the measurement of relaxation times longer than about 2 sec; most substances examined by high-resolution techniques fall into this category. Four basic experiments are proposed: spin-lattice relaxation, T 1, spin echo measurements for T 2, forced transitory precession for ( T 1) ϱ, and rotary spin echoes for Trot. All four techniques have been applied to a single practical example—relaxation of the ring protons of 2,4-dimethoxy-5-chloronitrobenzene. The spin-lattice relaxation time of the proton at position 3 is found to be determined predominantly by its dipole-dipole interaction with the methoxy protons, a result confirmed by double irradiation “internuclear Overhauser” experiments. The spin-spin relaxation times of the ring protons have been measured under single and multiple resonance conditions, permitting an evaluation of the different contributions to the natural line width, notably from the spin coupling to the 14N nucleus and the other protons in the molecule.