~(13)C-~1H dipolar-driven ~(13)C-~(13)C recoupling without ~(13)C rf irradiation in nuclear magnetic resonance of rotating solids

摘要

Two recently proposed ~(13)C-~(13)C recoupling methods under magnic angle spinning (MAS), resonant interference recoupling (RIR), and ~(13)C-~1H dipolar-assisted rotational resonance (DARR), are examined on a common theoretical foundation using the average Hamiltonian theory. In both methods, a rf field is applied on not ~(13)C but ~1H to recouple the ~(13)C-~1H dipolar interactions, and spectral overlap necessary to conserve energy for ~(13)C-~(13)C polarization transfer is achieved by the ~(13)C-~1H dipolar line broadening. While DARR employs time-independent ~(13)C-~1H interactions recoupled by suitable rf irradiation to ~1H spins, PIR uses time-dependent ~(13)C-~1H interactions modulated appropriately by ~1H rf irradiation. There are two distinct cases where ~(13)C-~1H line broadening realizes ~(13)C-~(13)C spectral overlap. For a pair of a carbonyl or aromatic carbon and an aliphatic carbon, spectral overlap can be achieved between one of the spinning sidebands of the former ~(13)C resonance and the ~(13)C-~1H dipolar powder pattern of the latter. On the other hand for a pair of spins with a small chemical shift difference, the two center bands are overlapped with each other due to ~(13)C-~1H dipolar broadening. For the former, we show that both RIR and DARR occur in the first order, while for the latter, DARR recoupling is appreciable for time-independent ~(13)C-~1H interactions. We refer to the former DARR as the first-order DARR recoupling and the latter as the second-order DARR. Experimentally, we examined the following ~(13)C-~1H recoupling methods for DARR: ~1H CW irradiation fulfilling a rotary-resonance condition or a modulatory-resonance condition, and ~1H pi pulses applied synchronously to MAS. For RIR, the FSLG-m2mm sequence is applied to ~1H. Several one-dimensional DARR and RIR experiments were done for N-acetyl[1,2-~(13)C, ~(15)N] DL-valine, and [2,3-~(13)C] L-alanine. It was found that the polarization transfer rate for RIR is larger than that for DARR except for fast spinning, while the rate for DARR is less sensitive to the spinning speed. Further, we showed that the efficiency of the second-order DARR recoupling is not significantly less than that of the first-order DARR. Among the ~(13)C-~1H recoupling methods examined, CW irradiation at the n=1 rotary-resonance condition is superior for DARR because it gives a larger ~(13)C-~1H dipolar broadening, leading to broadband recoupling. We showed that a broadband-recoupling experiment with the first and the second-order DARR by CW irradiation at the n=1 rotary-resonance condition is applicable to signal assignment as well as structural determination of a multiply/uniformly ~(13)C labeled molecule as demonstrated by two-dimensional ~(13)C-~(13)C DARR polarization-transfer experiments of uniformly ~(13)C, ~(15)N-labeled glycylisoleucine.