NMR imaging of paramagnetic centers in solids via dynamic nuclear polarization

Abstract

During the past few years the scope and importance of NMR imaging techniques, especially in medicine, have grown enormously. Recent developments have extended NMR imaging into the realm of solid materials (Z-d), and include a technique reported from this laboratory that takes advantage of the dilute spin behavior of 13C in natural abundance (1). O f course, the sensitivity limitations encountered with 13C in natural abundance in typical solids can be severe, and techniques that can enhance 13C spin polarization are therefore of interest. One method that has been shown to provide major enhancements of nuclear spin polarization in paramagnetic samples with favorable relaxation properties is dynamic nuclear polarization, DNP (5-8). Hence, it appeared worthwhile to combine the DNP technique for generating nuclear spin polarization with the dilute-spin 13C technique for NMR imaging introduced earlier (I). This approach, which is the subject of this paper, has the further potential of providing a method for obtaining a spatial image of the paramagnetic centers responsible for the DNP phenomenon. In dynamic nuclear polarization (5) the electron spin resonance of a paramagnetic center is irradiated (ideally saturated) at the resonance microwave frequency. The resulting perturbation of the electron spin polarization is transmitted via electron spin/nuclear spin interactions to the nuclear spin populations. When NMR signals are detected the magnitude of the observed NMR signal intensity is enhanced by a factor up to the ratio of the magnitudes of the electron spin and nuclear spin magnetic moments. The actual value of the enhancement depends upon the relative contributions of the three competing types of DNP mechanisms (Overhauser, solid-state effect, thermal mixing effect), the microwave frequency employed and the details of the dynamics of pertinent spin transition probabilities (i.e., relaxation times and the rate of microwave-stimulated electron spin transitions). Wind and co-workers (5-8) have observed DNP enhancement factors in the ‘H and r3C spectra of both static solids and solids rotated rapidly at the magic angle. In the work presented here, &, is 1.4 T, and the frequencies of the ‘H, ‘%Z, and ESR radiation were 60.0 MHz, 15.1 MHz, and 39.4 GHz, respectively. The DNP system is based upon a CP/MAS spectrometer that employs a Varian 1.4 T electro-