Multiple Quantum NMR: Studies of Molecules in Ordered Phases

Abstract

The object of this article is to describe recent applications of multiple quantum spectroscopy to the study of molecular systems in ordered phases. I make no attempt to review the phenomenon of multiple quantum spectroscopy comprehensively since several general reviews have already appeared in the literature (1, 2). For the purposes of this article, it is necessary only to discuss that class of nonselective multiple quantum experiments commonly employed by several groups to elucidate molecular structure and dynamics in ordered media. In conventional NMR spectroscopy one observes only transitions for which the change in the absolute value of the magnetic quantum number is one. Such transitions are referred to as single quantum transitions. Multiple quantum transitions occur when states in nonadjacent Zeeman manifolds are placed in coherent superposition. These various transitions are illustrated in Figure 1, which shows, in schematic form, the energy level diagram of a system of N spin 1/2 nuclei. In this paper I discuss how, in such a system, multiple quantum transitions from order zero to N may be excited and detected. Although it is possible to observe multiple quantum transitions in slow passage experiments (3, 4), all of the work described in this review involves the use of pulsed techniques to excite and detect multiple quantum transitions. Therefore I confine theoretical discussion to pulsed techniques. Attention is also confined to those experiments in which the objective is to obtain physical information through an analysis of the multiple quantum