Experimental aspects of 14N overtone RESPDOR solid-state NMR spectroscopy under MAS beyond 60 kHz

Nitrogen-14 (14N) overtone (OT) spectroscopy under fast magic angle spinning (MAS) conditions (>60 kHz) has emerged as a powerful technique for observing correlations and distances between 14N and 1H, owing to the absence of the first-order quadrupolar broadenings. In addition, 14NOT allows selective manipulation of 14N nuclei for each site. Despite extensive theoretical and experimental studies, the spin dynamics of 14NOT remains under debate. In this study, we conducted experimental investigations to assess the spin dynamics of 14NOT using the rotational-echo saturation-pulse double-resonance (RESPDOR) sequence, which monitors population transfer induced by a14NOT pulse. The 14NOT spin dynamics is well represented by a model of a two-energy-level system. Unlike spin-1/2, the maximum excitation efficiency of 14NOT coherences of powdered solids, denoted by p, depends on the radiofrequency field (rf-field) strength due to orientation dependence of effective nutation fields even when pulse lengths are optimized. It is also found that the p factor, contributing to the 14NOT spin dynamics, is nearly independent of the B0 field. Consequently, the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B0 when the 14NOT pulse length is optimized. The study also identifies the optimal experimental conditions for 14NOT/1H RESPDOR correlation experiments

Bottom-Up Synthesis of Crystalline Covalent Organic Framework Nanosheets, Nanotubes, and Kippah Vesicles: An Odd-Even Effect Induction

Few-layer organic nanosheets are becoming increasingly attractive as two-dimensional (2D) materials due to their precise atomic connectivity and tailor-made pores. However, most strategies for synthesizing nanosheets rely on surface-assisted methods or top-down exfoliation of stacked materials. A bottom-up approach with well-designed building blocks would be the convenient pathway to achieve the bulk-scale synthesis of 2D nanosheets with uniform size and crystallinity. Herein, we have synthesized crystalline covalent organic framework nanosheets (CONs) by reacting tetratopic thianthrene tetraaldehyde (THT) and aliphatic diamines. The bent geometry of thianthrene in THT retards the out-of-plane stacking, while the flexible diamines introduce dynamic characteristics into the framework, facilitating nanosheet formation. Successful isoreticulation with five diamines with two to six carbon chain lengths generalizes the design strategy. Microscopic imaging reveals that the odd and even diamine-based CONs transmute to different nanostructures, such as nanotubes and hollow spheres. The single-crystal X-ray diffraction structure of repeating units indicates that the odd-even linker units of diamines introduce irregular-regular curvature in the backbone, aiding such dimensionality conversion. Theoretical calculations shed more light on nanosheet stacking and rolling behavior with respect to the odd-even effects.11Nsciescopu