A method for dynamical characterization and high resolution 1H-NMR in dipolar coupled systems: application to liquid...

We study the variation of 13C spectra as function of off-resonances in protons during decoupling, for continuous wave (cw) and small phase incremental alternation with 64-step (SPINAL-64) schemes in the liquid crystals 4-n-octyl-4'-cyanobiphenyl (8CB) and 4-n-pentyl-4'-cyanobiphenyl (5CB). The self-decoupling mechanism induced by the strong homonuclear dipolar interactions provides a method to study the dynamics of the proton system through the 13C spectra. In the n-cyanobiphenyl (nCB) liquid crystals each nonquaternary carbon is coupled through dipolar interactions to more than one proton constituting a SI(N) group (with N> or =2). We extend the analytical treatment of the variation of the 13C spectrum with the off-resonance, described for SI groups, to SI(N) under cw decoupling. The dependence of the maxima of the 13C spectra as a function of proton off-resonance follows a Lorentzian line that depends on the rate of exchange among proton spin states. From the fitting parameters of this curve and the heteronuclear interaction measured in cross-polarization experiments, we extract dynamical information of the intramolecular 1H-1H interactions. In the case of SPINAL-64 we experimentally observe the same behavior. Under both kinds of decouplings, we characterize the chemical shift of the protons through the NMR spectra of carbons. The resulting values are in very good agreement with those obtained by other methods.