Amplitude modulation, heteronuclear spin coupling

Aluminum complexes, 2D NMR, 116 Amplitude modulation, heteronuclear spin coupling, 99... 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Figure 7.2. Amplitude modulation of the X-spin signal by heteronuclear coupling evolution during ti, illustrated for a coupled X-H pair. The doublet vectors evolve throughout ti but are frozen during t2 by the application of proton decoupling so the resultant X-spin signal displays pure amplitude modulation according to the heteronuclear coupling constant.

MAS does have an effect on the cross-polarization efficiency for mobile systems like elastomers [72]. For a static sample, the transfer rate is a function of u>c and has a maximum polarization-transfer rate at the Hartman-Hahn match and a slower polarization transfer with increasing mismatch. However, when the sample is spun with MAS, there is a very slow transfer of polarization at the Hartman-Hahn match, where the transfer is expected to be most rapid. A series of maxima in the transfer rate occurs when co c and co w are mismatched by a multiple of the spinning firequency. Thus, mismatching the cuic and )ih produces high signals at multiples of the MAS firequency. A series of maxima in Tch are observed as a result of an amplitude-modulated pattern due to C-H heteronuclear coupling [73]. 

The decoupling efficiency depends on two factors [Engl, Mehl] (1) The amplitude (Oil = - y fii/ of the decoupling field in comparison with the strength of the heteronuclear dipole-dipole interaction. (2) The modulation of the heteronuclear dipole-dipole coupling by flip-flop transitions in the system of the abundant / spins, which communicate by the homonuclear dipole-dipole interaction. In addition to this, the influence of thermal motion has to be considered [Mehl]. 

---