Spin-locking cross-polarisation

Relaxation times Ti were obtained using the inversion-recovery technique (7T-T-7r/2-acq), while the F Tip relaxation times were measured by means of the variable-time spin-lock technique, and Tip was obtained via F resonance by a variable spin-lock followed by H F cross polarisation. In all cases decoupling of the complementary nucleus was not implemented during the variable time allowed for relaxation. The F relaxation times all show single-exponential decay of the magnetisation as a function of... 

The solid state NMR measurements on the hard coke concentrates were carried out at 25 MHz on a Bruker DSX spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca. 6-7% of the central aromatic bands. A contact time of 1 ms was used for the cross polarisation (CP) measurements and the H decoupling and spin-lock field was ca. 60 kHz. The FIDs were processed using a Lorentzian linebroadening factor of 50 Hz. To determine the fraction of protonated and non-protonated carbon, four delay periods between 1 and 100 is were employed in dipolar dephasing experiments. 

Figure 3.14 The pulse sequence for cross-polarisation. The sequence is initiated with a 90° proton pulse followed by spin locking (the contact time) of the carbon and proton magnetisations. This leads to the efficient polarisation of the carbons via the protons when the carbon and proton field strengths are matched. The signals are then observed with high-power proton decoupling.

With the advent of high resolution techniques in solids it became of interest to measure site-specific relaxation times. For the typical double-resonance type of experiments, there are no real problems in measuring T s under MAR conditions, except possibly the often very large values. When it comes to Tj p, however, for the dilute spin species, then there can be difficulties in interpretation. Suppose we have created some spin-locked magnetisation say for in a solid, probably via cross polarisation. To measure T p we would switch off the... 

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