Cross-polarization dynamics and spin diffusion in some aromatic compounds.

The inversion-recovery cross-polarization (IRCP) magic-angle spinning experiment has been applied to study the 13C-1H cross-polarization dynamics of protonated aromatic carbons in ferrocene, 5,6-dimethoxyindole (DMI) and some indole derivatives. Using the 13C-detected proton spin diffusion (SD) experiment recently developed by Zhang et al. [Solid State Nucl. Magn. Reson., 1 (1992) 313], the slow decaying or incoherent stage of the IRCP experiment is shown to be controlled by the spin diffusion process at the directly bound proton. Moreover, a simple phenomenological model treating spin diffusion as a relaxation process provides an excellent agreement with both the IRCP and SD experimental data for all the different C-H pairs of DMI and its derivatives. The resulting time constants of the non-exponential spin diffusion decays are related to the local intra- and intermolecular network of dipolar interactions. This model is nevertheless found to be inadequate for ferrocene because intramolecular spin diffusion then has an inhomogeneous character.