Two-dimensional J spectra in solids

Abstract

Various two-dimensional NMR techniques exploiting the scalar coupling between nuclear spins have been demonstrated in liquids (I). These methods have proved to be valuable tools in the application of NMR to various problems, such as structure determination, kinetics, molecular dynamics, etc. Such techniques, applied in conjunction with those that are rapidly developing for obtaining high resolution spectra of solids (2, 3), offer many new avenues for investigation of complex solids. Work recently published from this laboratory (4) and simultaneously from another (5) demonstrated that high resolution 13C spectra of certain solid materials can be obtained which exhibit the proton-carbon scalar coupling. An obvious extension of this work leads to 2D spectra similar to those referred to as heteronuclear .I spectra (1) in liquids. Chemical shifts are displayed along one frequency axis, and scalar couplings are displayed along a perpendicular axis. Such a spectrum of polycrystalline camphor is shown in Fig. 1. This spectrum was obtained using the pulse sequence shown in Fig. 2. The sample is spun at the magic angle. Carbon transverse magnetization is generated using the usual cross polarization technique (2). This is followed by multipulse decoupling using the MREV-8 pulse sequence (3) for a period t, /2 so that the carbon magnetization evolves with scalar coupling but dipolar interactions are suppressed. The carbon magnetization is then inverted by a ?r pulse and allowed to evolve for an additional time t,/2 under continuous proton decoupling. The isotropic part of the chemical shift refocuses during this period so that the amplitude of the echo is modulated only by scalar coupling. The second half of this echo is then digitized still under proton decoupling. A data matrix indexed by 2, and t2, the time variable of the FID, is then Fourier transformed with respect to t, and f2 to yield the spectrum shown. The combination of MREV-8 irradiation and magic-angle spinning removes the effects of dipolar interactions, both proton-proton and proton-carbon, as well as chemical shift anisotropy during the first half of the evolution period tl . During the second half of the evolution period switching to continuous proton irradiation removes the effect of scalar coupling as well. The overall effect is then very similar to carbonproton .I spectroscopy using the gated decoupler method (1). However, in contrast to the liquid experiment where the scalar coupling is reduced only by a factor of onehalf, the couplings are scaled by an additional factor of about 0.488 due to the MRFV8 pulse sequence (3). This scaling by almost a factor of one-fourth limits the usefulness