Distance measurements in spin-1/2 systems by 13C and 31P solid-state NMR in dense dipolar networks.

In this article solid-state NMR methods for the determination of internuclear dipole-dipole couplings between homonuclear spin-1/2 nuclei are presented. They are suitable for relatively dense dipolar networks which are still dominated by 2-spin interactions. C-/R-symmetry theory is applied to create a double-quantum average Hamiltonian using phase-modulated radio-frequency irradiation and magic-angle sample-rotation. Symmetry derived pulse sequences with improved compensation against chemical shift anisotropies were found assuming a small isotropic chemical shift difference and using numerical calculations of the spin dynamics. Moreover it is shown that a constant time procedure can be used to acquire reliable double-quantum build-up curves even in systems in which damping obscures oscillations in their symmetric build-up curve. This technique is demonstrated on four crystalline model compounds with 31P and 13C spin systems typical for inorganic and biological applications. Comparison to crystal structure data indicates that the distances derived this way from 31P and 13C double-quantum NMR carry only small systematic errors caused for example by anisotropic J-coupling, dipolar contributions from adjacent spins and relaxation.