Hartman-Hahn cross polarization

In order to study the molecular aggregation during the volume relaxation of network epoxies, CP/MAS carbon-13 (natural abundance) NMR was utilized. The Hartman-Hahn cross-polarization technique 129) was used with a cross contact time of 1 mi llisecond for transfer of proton polarization to carbon nuclei. The protondecoupling was achieved at the radio frequency of 56.4 MHz. Carbon-13 14.2 MHz spectra were measured in a 1.4 Tesla magnetic field. Room temperature (23 °C) experiments were performed at 54.7° MAS at 1 KHz. The spinner was constructed using an Andrew-type rotor driven by compressed air. 

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 ... 

Carbon-13. Carbon-13 solid state NMR is particularly difficult on non-enriched samples because of the low density of the sites, in addition to the small natural abundance (1.1%) and weak sensitivity of carbon-13 (Tc/Yh = 0-25). The acquisition of NMR spectra is normally realized with sequences implying a polarization transfer from the protons to the carbons, cross polarization (CP, Scheme 2) under the Hartman-Hahn optimal conditions coh = YhBh = YcBc = c- This has the advantage to greatly enhance the intensity of the carbon signals. However, in the case of the characterization of surface species, it is still necessary to enrich samples with carbon-13 (10-100%). Yet, despite these problems, it has become a crucial technique for the identification of surface species. 

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]. 

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