Constant-time 2D and 3D through-bond correlation NMR spectroscopy of solids under 60 kHz MAS

Establishing connectivity and proximity of nuclei is an important step in elucidating the structure and dynamics of molecules in solids using magic angle spinning (MAS) NMR spectroscopy. Although recent studies have successfully demonstrated the feasibility of proton-detected multidimensional solid-state NMR experiments under ultrafast-MAS frequencies and obtaining high-resolution spectral lines of protons, assignment of proton resonances is a major challenge. In this study, we first re-visit and demonstrate the feasibility of 2D constant-time uniform-sign cross-peak correlation (CTUC-COSY) NMR experiment on rigid solids under ultrafast-MAS conditions, where the sensitivity of the experiment is enhanced by the reduced spin-spin relaxation rate and the use of low radio-frequency power for heteronuclear decoupling during the evolution intervals of the pulse sequence. In addition, we experimentally demonstrate the performance of a proton-detected pulse sequence to obtain a 3D ¹H/¹³C/¹H chemical shift correlation spectrum by incorporating an additional cross-polarization period in the CTUC-COSY pulse sequence to enable proton chemical shift evolution and proton detection in the incrementable t₁ and t₃ periods, respectively. In addition to through-space and through-bond ¹³C/¹H and ¹³C/¹³C chemical shift correlations, the 3D ¹H/¹³C/¹H experiment also provides a COSY-type ¹H/¹H chemical shift correlation spectrum, where only the chemical shifts of those protons, which are bonded to two neighboring carbons, are correlated. By extracting 2D F1/F3 slices (¹H/¹H chemical shift correlation spectrum) at different ¹³C chemical shift frequencies from the 3D ¹H/¹³C/¹H spectrum, resonances of proton atoms located close to a specific carbon atom can be identified. Overall, the through-bond and through-space homonuclear/heteronuclear proximities determined from the 3D ¹H/¹³C/¹H experiment would be useful to study the structure and dynamics of a variety of chemical and biological solids.