Frequency filtration and the elucidation of scalar coupling networks in zero-quantum coherence spectra

Abstract

The lim itations of static magnetic field homogenei ty usually confine high-resolution NMR spectroscopy to sample volumes of no more than a few cubic centimeters. One way to overcome this lim itation is to observe zero-quantum coherences (ZQCs) (I4), since they have linewidths which are independent of magnetic field homogeneity. Unfortunately, ZQC spectra are difficult to interpret, and it is this problem that we address in this communication. We have developed a method whereby it is possible to extract from a ZQC spectrum subspectra which contain only those ZQCs produced by a single spin system. Zero-quantum coherences are phase coherences between sets of coupled, but chemically inequivalent, spins which obey the transition rule AA4 = 0. Since they are, therefore, spin-forbidden they can neither be created directly by the action of a single pulse on the equilibrium magnetization of a spin system nor be detected directly as they have no net magnetization in any direction. Fortunately, both of these problems can be overcome by using an appropriate series of pulses and delays in the form of a two-dimensional experiment (I, 2). The biggest drawback to using ZQCs is the unfamiliarity of their spectral parameters (4). Both their precessional frequencies, ueff, Eq. [ 11, and their scalar couplings, Jeff, Eq. [2], are such as to preclude conventional spectroscopic analysis: