17O quadrupole double resonance in samples of natural isotopic abundance

Abstract

It has become possible in recent years to detect 17O quadrupole resonance signals in polycrystalline solids with the isotope in its natural abundance (0.0037%). The present experimental methods are mostly based on adiabatic demagnetisation, in which the strong magnetic resonance signal from another nucleus present in the sample, usually protons (P) of spin 1 2 , is monitored in high field, and the 17O spin system (Q) irradiated in zero field a short time later. Repetition of the magnetic field cycle together with simultaneous step wise changes in the Q-Crequency produces a plot of the Q quadrupole resonance spectrum in terms of the change in the P polarisation. Two important conditions are necessary for the successful outcome of a double resonance experiment. Firstly the P-spin-lattice relaxation time roust be long, at least of the order of the cycle time. Secondly, in zero-field, the P and Q spin systems must be brought into repeated thermal contact. Ac least four different experimental techniques are available for ensuringchis. The first utilizes the Harcmann-Hahn equation, in which the precessional frequency of the Q-spins about the applied r.f. field in the rotating frame is sec equal to the P-spin frequency in its own local field; frequent phase reversals of the Q irradiation frequency establish almost continuous thermal contact between the two spin systems. The second requires three different types of spin system, one of which is a nucleus such as 35Cl whose nuclear quadrupole resonance in zero field is used to cool-the P system. The third and fourth methods are particularly useful for the study of oxygen either directly bonded or hydrogen-bonded to hydrogen. They depend on ehe “solid-effect”, in which simultaneous P and Q spin flips occur in high r.f. fields when the Q spin states are sufficiently strongly perturbed by a magnetic dipolar interaction with some of the P spins. This is an efficient mechanism for establishing continuous thermal contact: between the P and Q spins provided chac the P spins attached to oxygen are in good thermal contact with the remainder of the protons, the socalled “dipolar bath”. If they are not, two-frequency irradiation must be used, in which two equally incense radiofrequencies are applied to the sample, separated by the dipolar frequencies. Well-resolved dipolar structure is observed, from which the orientation of ehe electric field gradient: censor at. the 17O nucleus can in principle be derived. These new techniques are currently being used in the study of many different kinds of oxygen-containing compound; the review concludes with a brief survey of their application to hydrogen bonded C 17O and C- 17OH groups in many molecules and ions, including Chose containing symmetrical hydrogen bonds.