¹⁴N overtone NMR spectroscopy

Nitrogen-14 has a spin quantum number of I = 1. Besides the allowed single-  quantum transitions between I_z = 0 and I_z = +/- 1, there is a forbidden transition  between I_z = +1 and -1. This transition is weakly allowed in the presence of  quadrupolar couplings and can be excited by the rf-field with either the Larmor  frequency (DQ: double quantum transition) or the twice of the Larmor frequency  (OT: overtone transition). Here we show ¹H-¹⁴N OT correlation spectra can easily be  observed by ¹H-¹⁴N OT HQMC experiments at ultrafast MAS rate. Since ¹⁴N OT is  free from the first order quadrupolar broadening, the line shape are relatively  narrow and affected only by 2^nd order quadrupolar broadening. Although there still  remain several issues in ¹⁴N OT spectroscopy, such as limited bandwidth, small  transition moments, ¹⁴N OT spectroscopy opens a new world for us!

Reference

Y. Nishiyama, M. Malon, Z. Gan, Y. Endo, T. Nemoto, J. Magn. Reson. 230 (2013) 160-164.