15N NMR is widely used because of the importance of nitrogen in chemistry, materials, biology, environment etc. However, very low abundance of 15N (<0.4%) results in poor sensitivity and thus makes observation time-consuming. On the other hand, the rest of nitrogen atoms are also NMR sensitive nucleus of 14N. Despite the high abundance of 14N (>99%), it's application is rather limited due to the huge quadrupolar interactions and its spin quantum number I = 1. The introduction of very fast MAS (>70 kHz) enabled the 1H detected 14N/1H correlation spectroscopy (Nishiyama et al.,JMR 208 (2011) 44-48). However, it demands precise magic-angle adjustment, moreover, the sensitivity is largely affected by the molecular motion. It was shown that the use of 14N overtone transition avoids these difficulties under MAS (O'Dell et al. Chem. Phys. Lett. 514 (2011) 168-173) with a cost of sensitivities. Since the overtone transition between -1 and +1 energy level is forbidden, the low sensitivity fundamentally comes from small transition moment between these levels. Here, we have developed 1H detected 14N overtone/1H correlation spectroscopy under ultrafast MAS conditions > 70 kHz to achieve highly sensitive 14N NMR spectroscopy of rigid solid.
Y. Nishiyama, M. Malon, Z. Gan, Y. Endo, T. Nemoto, J. Magn. Reson. 230 (2013) 160-164.
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