Off-resonance nutation

Whereas the on-resonance nutation spectra are amplitude-modulated and can be properly phased in the FI dimension, the off-resonance nutation spectra are phase-modulated and can no longer be phased properly. Therefore, a magnitude calculation is performed after Fourier transformation. Furthermore, because the magnetisation evolves during t around the effective field, the sine and cosine components of the modulation are different and the amplitudes of positive and negative FI signals are unequal. 

---

Figure 3.20 shows an example of the strength of the off-resonance nutation technique applied to various samples of y-alumina impregnated with phosphorus and/or molybdenum. The nutation spectrum of the tetrahedral resonance is very similar to the nutation spectrum of 7-AI2O3. The octahedral resonance however shows a nutation... 

Figure 3.20. Off-resonance nutation experiments on Mo(12)p(2)/y-Al203 with an rf field strength of 41 kHz. The normal ID MAS spectrum A. is shown and the nutation spectra of the tetrahedral B. and octahedral region C. with the tetrahedral region made up of only 7-AI2O3 but the octahedral region requiring a superposition of 7-AI2O3 and Al2(Mo04).3 from Krauss et. al. (1996) with permission of the American Chemical Society.

Peelers, M.P.J. and Kentgens, A.P.M. 1997. A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials. Solid State Nucl. Magn. Reson. 9 203-217. Peinador, R.I., Calvo, J.I., Prddanos, R, Palacio, L., and Hernandez, A. 2010. Characterisation of polymeric UF membranes by Uquid-liquid displacement porosimetry. J. Membr. Sci. 348 238-244. 

The case of off-resonance pulses is a little bit more involved, since now the axis of nutation is given by the effective magnetic field in Equation (2.3.6) and, therefore, is not contained in the transverse plane. The extent to which this axis is tilted out of the transverse plane and the sense of the tilt is determined by the frequency offset ( >l — 2). Also the nutation frequency is no longer given by co = y Bi, but depends on the offset. It can be shown that the rotation operator describing such off-resonance pulse can be written as a product of five rotations about orthogonal axes [5] ... 

---