Hartman-Hahn matching

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]. 

Cross-polarization is a double-resonance experiment in which the energy levels of the H and 15N spins are matched to the Hartman-Hahn condition in the rotating frame [47], Under this condition the energy between the two coupled spin systems may be exchanged and the 15N magnetization (S) develops as a function of time at the expense of the proton magnetization according to the relation ... 

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