Scheme for Sensitive Nuclear Double-Resonance Detection: Deuterium and 13C

Abstract

An experiment which involves the participation of three nuclear spin species is carried out to accomplish sensitive double-resonance detection of nuclei. An ordered nuclear quadrupole spin system (A) is used to establish and monitor a metastable ordered state of a second abundant spin system (B). A double-resonance coupling is brought about between the B spin system and a third rare or weakly interacting system (C) to accomplish the detection of the C resonance. The usefulness of this scheme lies in the choice of an A system with a short spin–lattice relaxation time (T1A) and a B system with a long spin–lattice relaxation time (T1B). The choice of a long T1B increases the sensitivity of the experiment in that a long time is available during which effective nuclear dipole–dipole cross relaxation can occur between the B and C systems. With a short T1A, the cycling rate at which information is gathered to detect the C system is enhanced by a factor ∼T1B / T1A over that of the earlier two species double-resonance method. For increased sensitivity T1B can be chosen arbitrarily long, while for the purposes of an increased rate of data acquisition, T1A can be chosen arbitrarily short, thus producing an efficient double-resonance scheme. This scheme is applied to a 12% deuterium enriched sample of paradichlorobenzene. The nuclear quadrupole coupled Cl35 reservoir acts as the A system. The zero field nuclear quadrupole resonance system (at 34.78 mHz at 77°K) supplants the usual high field stabilized magnet and power supply required in an NMR double-resonance experiment. The B proton system has a zero field lifetime of 46 sec, and approximately a 1 h lifetime at 135 G. The C systems investigated are (1) deuterium and (2) the naturally abundant 13C nuclei in paradichlorobenzene. An analysis of double-resonance detection of I = 1 spin systems is given and shows that for a well resolved nuclear quadrupole asymmetry η splitting, a double resonance cannot be detected because of the nonsecular character or quenching of cross-relaxation interaction terms. The analysis is extended to include the unquenching effect of constant magnetic fields and the integer I > 1 quadrupole system is discussed. The observation of the 13C resonance is the first reported observation through the nuclear magnetic resonance techniques of the 1.1% naturally abundant 13C in a solid.