Estimation of Carbon−Carbon Bond Lengths and Medium-Range Internuclear Distances by Solid-State Nuclear Magnetic Resonance

Abstract

We describe magic-angle-spinning NMR methods for the accurate determination of internuclear dipole-dipole couplings between homonuclear spins-(1/2) in the solid state. The new sequences use symmetry principles to treat the effect of magic-angle sample-rotation and resonant radio frequency fields. The pulse-sequence symmetries generate selection rules which reduce the interference of undesirable interactions and improve the robustness of the pulse sequences with respect to chemical shift anisotropies. We show that the pulse sequences may be used to estimate distances between 13C spins in organic solids, including bond lengths in systems with large chemical shift anisotropies, such as conjugated systems. For bond-length measurements, the precision of the method is +/-2 pm with a systematic overestimate of the internuclear distance by 3 +/- 1 pm. The method is expected to be a useful tool for investigating structural changes in macromolecules.

Symmetry‐Based Pulse Sequences in Magic‐Angle Spinning Solid‐State NMR

“…All inequivalent solutions in the range N ≤ 20, n, ≤ 10 and ν ≤ 10 are shown bond lengths and medium-range distances in 13 C 2 -labelled systems, even in the presence of large CSA interactions. 85 As discussed above, the orientation-dependence of the recoupling under the m = ±1 sequences (Tables 5 and 7) is different from that under the m = ±2 sequences (Table 6). This is expected to improve the information content of the NMR spectroscopy of rotating oriented samples.…”

“…85 A combination of electronic simulations and experimental measurements has led to the development of a simple analytical model for the transient behavior of the rf field at the sample. This model was used for accurate spin dynamical simulations during symmetry-based sequences, including the transient probe response.…”

“…Even small deviations from this precise phase shift value may be disastrous. 85 A phase shift accuracy of ±0.1 • or better is necessary for many applications of RN ν n sequences. In particularly demanding applications, such as long-range distance measurements, the phase shift accuracy should be an order of magnitude better still.…”

“…The symmetries R14 6 2 and R22 9 4 with R = 90 180 270 0 have been mainly used so far. 41, 85 Measurement of the excitation dynamics of the doublequantum coherences allows the accurate estimation of 13 …”

Sensitivity Enhancement and Heteronuclear Distance Measurements in Biological 17O Solid-State NMR

“…69 It has been shown before that the robustness of RN n ν sequences with respect to isotropic chemical shifts, chemical shift anisotropies, rf inhomogeneity, and instrumental errors in the rf phase setting can be further improved by supercycling. 60,61,65,67,70 We used a combination of inversion supercycles, in which all rf phases are inverted in adjacent pulse sequence blocks, and "multiple-quantum" (MQ) phase cycles, in which the overall rf phase of consecutive pulse sequence blocks is incremented. 59,61,65,67,70 The nested supercycles are denoted …”