Towards uniform enhancement in solid-state cross polarization magic [corrected] angle spinning NMR: a scheme...

A recently proposed experimental scheme for achieving uniform cross polarization enhancement of low-gamma nuclear species in solids under magic angle spinning, termed quantitative cross polarization (QUCP) [Hou et al., Chem. Phys. Lett. 421, 356 (2006)], is described, supported with comprehensive theoretical analysis, numerical simulation, and experimental investigation with both uniformly labeled and naturally abundant solids. This method combines cross polarization with dipolar-assisted rotational resonance (DARR) [Takegoshi et al., Chem. Phys. Lett. 344, 631 (2001)] broadband homonuclear recoupling technique to achieve quantitative CP spectra under fast magic angle spinning. In addition to the correct and systematical interpretation on the phenomenon we reported in the previous Letter, a number of general guidelines for performing QUCP experiments are presented in this work. It is firmly established that while the enhancement factor in QUCP depends on the CP contact time, uniform enhancement can nevertheless be realized for all types of carbon group. For natural abundance samples, the polarization transfer rate is generally slower than that in labeled samples, but quasi-equilibrium among dilute spins in the mixing period can always be reached and uniform enhancement can be achieved albeit the DARR irradiation time needed can be much longer. For labeled samples, the time gain of QUCP experiment is almost the same as that of conventional CP. For natural abundance samples, it is generally much better than single-pulse experiment. Various representative systems, including uniformly (13)C-labeled DL-alanine and (13)C, (15)N labeled L-tyrosine, as well as naturally abundant alanine, tyrosine, and monoethyl fumarate, are used to verify the validity of our theoretical analysis and numerical simulation and to demonstrate the utility and advantages of the present approach.