Instrumentation R. G. Griffin Francis Bitter National Magnet Laboratory Massachusetts Institute of Technology Cambridge, Mass. 02139
In the late 1960's the prevailing view among many NMR spectroscopists was that the field had reached maturity, and except for routine ap plications, few problems of real inter est remained. This feeling led to an exodus of physicists and chemists from the field. Of those that continued to practice the art, many involved themselves in applications of the subject to problems in materials science or to chemical or biological problems. However, during that peri od several technical and scientific ad vances were made which opened new areas of scientific endeavor. Specifi cally, the development of supercon ducting solenoids, together with Fou rier transform techniques (i), made observation of spectra of low abun dance and/or sensitivity nuclei a rou tine matter. The most notable exam ple of this is the observation of spectra of 1.1% abundant 13C nuclei in liquids; the wealth of information on molecu lar conformation, dynamics, etc., available from these spectra is enor mous. In addition, during these and later years, techniques used for obtaining high resolution NMR spectra of solids were developed. It is these techniques and their present and future applica tions that we will discuss here. In Sec tion I we outline the problem of high resolution NMR in solids; that is, we discuss the origin of the broad line spectra one customarily observes in solids and then outline three ap proaches to regaining resolution in the solid state. The approaches are con sidered in order of their chronological development, i.e., magic angle sample spinning, multiple pulse techniques, and magnetic dilution. Given that one can obtain high resolution spectra in solids, we consider in Section II what can be learned from such spectra. High resolution liquid NMR spectra are routinely employed in structural determinations and in the examina tion of chemical dynamics in solution. The observation of chemical shift, quadrupolar, and dipolar tensors in solids can also provide structural data, and when molecular motion is present in a solid sample, the spectra can, in principle, elucidate the rate and mechanism of that motion. In the final section, III, we discuss some new ex periments which combine cross polar -
High Resolution NMR in Solids
ization, multiple pulse, and sample spinning techniques. Each of these ex periments is designed to obtain a par ticular type of information, and these hybrid approaches may develop into the most exciting and generally appli cable of the various experiments. We should mention at the outset of this review that in the last year two mono graphs concerned with this subject have appeared. One, by U. Haeberlen (2), is concerned primarily with multi ple pulse techniques; the second, by M. Mehring (3), discusses both multi ple pulse and magnetic dilution exper iments. This author highly recom mends both of these monographs to the serious student of solid state NMR. A review on sample spinning techniques by Andrew (4) appeared some time ago. I. Approaches to High Resolution NMR in Solids NMR spectra of solids are with few exceptions characterized by broad featureless lines. We can understand the source of this broadening and how to cope with it if we examine the size and form of the various terms in the nuclear spin Hamiltonian. A typical nuclear spin Hamiltonian is given in Equation 1 and contains four "inter esting" terms: Ή = 9ics + ΉΛ + JiO + ftq (1) Liquids 103 10 0 0 Solids 103 io 5 χ 10* 105-106 The table indicates the expected sizes of the various interactions for liquids and solids in Hz; in liquids the dipolar (Ήΐ>) and quadrupolar (WQ) terms vanish, and only chemical shift {Jics) and scalar coupling terms (féj) contribute. The reason that Ή-ç, (and likewise Ή^) vanishes in liquids can be understood by examining Equation 2 which is the dipolar Hamiltonian »Ώ = 72Λ2 Σ •
