Abstract
A novel method for accurate determination of small chemical shielding anisotropy (CSA) of the solid state nuclear magnetic resonance is described. The method is based on the adiabatically ramped spin-locking of transverse magnetization. The equation for the time evolution of the spin dynamics for a system on the presence of chemical shielding and the ramped spin-locking rf field is analogous to that for the case of homonuclear magnetic dipolar coupling. The time course of signal intensity with respect to the ramped spin-locking time is sensitive to magnitude of CSA (span). The asymmetrical parameter (skew) modulates the intensity profile a little. By carrying out the experiments under the on-resonance condition, the span and skew values were determined for the l-alanine C β-carbon. Asymmetry of the chemical shift anisotropy tensor for the C β-carbon is due to the asymmetrical molecular field, which is reasonably explained by the chemical shielding calculation with the ab initio coupled Hartree–Fock method.
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