1) Observation of remote 1H/14N correlations [1]: Observation of 14N NMR through 1H under fast MAS conditions has become more accessible. While J-HMQC and D-HMQC were initially utilized, T-HMQC has proven to be a robust method for observing covalently-bonded 1H/14N correlations. However, observing correlations between non-bonded 1H-14N pairs remains a challenge. In this study, we demonstrate that selective observation of 1H near an irradiated 14N nucleus can be achieved through 14N overtone (OT) irradiation in PM-S-RESPDOR. By varying the 14N OT frequency, a set of PM-S-RESPDOR 1H spectra can be generated to efficiently obtain 1H/14N correlation spectra, even for non-bonded 1H-14N pairs.
2)Determination of NH distances with NH filtering [2]: Internuclear distances between 14N and 1H can be easily determined by the PM-S-RESPDOR method owing to the high abundance of these nuclei. However, the data analysis assumes isolated 1H resonances. In other words, if the 1H(14N) signal is overlapped with other 1H peaks, the data analysis becomes obscured. To solve this problem, we applied the T-HMQC filter prior to PM-S-RESPDOR. Although the application of the T-HMQC filter introduces orientation-dependent filtering efficiencies, thus potentially modify the RESPDOR fraction curves, the resultant PM-S-RESPDOR curve is immune to the presence of overlapped signals, enabling the data analysis using the analytical equation.
3)Application to structural analysis of CONT [3,4]: Covalent organic nanotubes (CONTs) are one-dimensional (1D) extended networks formed from discrete molecular building blocks through covalent bonds. By properly choosing the building units, a CONT nanofabric can also be prepared. However, there are several possible reaction pathways. While CONTs rely on imine bonds, they may convert into imidazole rings. 1H/14N correlation experiments prove this reaction mechanism.
[1] N.T. Duong*, Y. Nishiyama*, Phys. Chem. Chem. Phys. 24 (2022) 10717-1-726. DOI: 10.1039/D2CP00155A
[2] Y. Ogaeri, N. Suzuki, T. Fukami, Y. Nishiyama*, J. Magn. Reson. in press. DOI: 10.1016/j.jmr.2023.107378
[3] K. Koner, S. Das, S. Mohata, N.T. Duong, Y. Nishiyama*, S. Kandambeth, S. Karak, C.M. Reddy*, R. Banerjee*, J. Am. Chem. Soc. 144 (2022) 16052-16059. DOI: 10.1021/jacs.2c06133
[4] K. Koner, S. Karak, Y. Ogaeri, Y. Nishiyama*, R. Banerjee*, Angew. Chem. In. Ed., in press. DOI: 10.1002/anie.202300652