14N NMR as a General Tool to Characterize the Nitrogen-Containing Species and Monitor the Nitration Process.

The usefulness of 14N NMR spectroscopy was highly underestimated compared with 15N NMR, which usually required tedious and expensive 15N-labeling manipulations. It is of great significance to make the 14N NMR spectroscopy convenient and useful considering 14N nuclei's high natural abundance of 99.6%. Herein, lots of efforts have been made to generalize routine 14N NMR to characterize nitrogen-containing species by tuning the balance between the solubility and viscosity of the samples. Satisfactory 14N NMR spectra of more than 60 nitrogen-containing compounds have been recorded, and the chemical shifts and the peaks' full width at half-maxima of more than 10 nitrogen-based functionalities have been summarized. Successful monitoring of the ortho-selective nitration of aniline has been demonstrated using the 14N NMR protocol developed in this paper, which will help realize the visualization of nitration processes in the industry.

15N Solid-State NMR as Bright Eyes to See the Isomerization of the Azo Bond: Revision of Tris(ß-hydroxyl-azo)-benzene to Tris(ß-keto-hydrazo)-cyclohexane in Porous Organic Polymers.

Porous organic polymers (POPs) have aroused great and wide attention from the materials community, while the identification of their precise structures is still very challenging. The well-defined structures are of great importance in understanding the relationship between the structure and function of the polymer materials, though they are sometimes ignored and do not receive enough attention. In this letter, an efficient 15N labeling technique and 15N solid-state NMR (15N-SSNMR) were combined to obtain strong evidence for the presence of the azo bond and keto-hydrazone structure in the solid state. Thus, the structure of tris(ß-hydroxyl-azo)-benzene in previously proposed hydroxylazobenzene polymers was revised to tris(ß-keto-hydrazo)-cyclohexane in TKH-POPs for the first time. In contrast, similar tautomerization did not occur in the azo coupling polymerization of 1,3,5-triaminobenzene and diazonium salts, i.e., tris(ß-amino-azo)-benzene remained in Azo-POPs. This work will open up a window to develop innovative porous organic polymers more efficiently with the aid of 15N-SSNMR.