Sensitivity enhancement, assignment, and distance measurement in 13C solid-state NMR spectroscopy for paramagnetic systems under fast magic angle spinning

Despite success of previous studies, high-resolution solid-state NMR (SSNMR) of paramagnetic systems has been still largely unexplored because of limited sensitivity/resolution and difficulty in assignment due to large paramagnetic shifts. Recently, we demonstrated that an approach using very-fast magic angle spinning (VFMAS; spinning speed ⩾20 kHz) enhances resolution/sensitivity in 13C SSNMR for paramagnetic complexes [Y. Ishii, S. Chimon, N.P. Wickramasinghe, A new approach in 1D and 2D 13C high resolution solid-state NMR spectroscopy of paramagnetic organometallic complexes by very fast magic-angle spinning, J. Am. Chem. Soc. 125 (2003) 3438-3439]. In this study, we present a new strategy for sensitivity enhancement, signal assignment, and distance measurement in 13C SSNMR under VFMAS for unlabeled paramagnetic complexes using recoupling-based polarization transfer. As a robust alternative of cross-polarization (CP), rapid application of recoupling-based polarization transfer under VFMAS is proposed. In the present approach, a dipolar-based analog of INEPT (dipolar INEPT) methods is used for polarization transfer and a 13C signal is observed under VFMAS without 1H decoupling. The resulting low duty factor permits rapid signal accumulation without probe arcing at recycle times (∼3 ms/scan) matched to short 1H T1 values of small paramagnetic systems (∼1 ms). Experiments on Cu(dl-Ala)2 showed that the fast repetition approach under VFMAS provided sensitivity enhancement by a factor of 8-66 for a given sample, compared with the 13C MAS spectrum under moderate MAS at 5 kHz. The applicability of this approach was also demonstrated for a more challenging system, Mn(acac)3, for which 13C and 1H paramagnetic shift dispersions reach 1500 and 700 ppm, respectively. It was shown that effective-evolution-time dependence of transferred signals in dipolar INEPT permitted one to distinguish 13CH, 13CH2, 13CH3, 13CO2- groups in 1D experiments for Cu(dl-Ala)2 and Cu(Gly)2. Applications of this technique to 2D 13C/1H correlation NMR under VFMAS yielded reliable assignments of 1H resonances as well as 13C resonances for Cu(dl-Ala)2 and Mn(acac)3. Quantitative analysis of cross-peak intensities in 2D 13C/1H correlation NMR spectra of Cu(dl-Ala)2 provided distance information between non-bonded 13C-1H pairs in the paramagnetic system.