Hartmann-Hahn condition

The transfer of magnetization from the proton spins to the carbon spins occurs now when the Hartmann-Hahn condition, Eq. (2), is fulfilled. 

In addition, the technique of cross polarization introduced and developed by Pines, Gibby and Waugh (9) is used to increase the signal-to-noise ratio of the spectrum. The proton magnetization is spin-locked along the y axis with a spin-locking field % and the carbons subjected to an RF pulse chosen such that the two fields fulfill the Hartmann-Hahn condition (10), equation [3] (Figure 2). 

The first example, also being the example introducing optimal control to solid-state NMR [40] and further elaborated on later [161], is optimal control versions of the DCP experiment. This experiment was a natural choice for numerical improvements as it is widely used and it is well known that this experiment is sensitive to offsets, rf mismatch relative to the MAS-modified Hartmann-Hahn condition, and rf inhomogeneity. In particular the two latter effects may reduce significantly the performance of 15N to 13C transfers, severely complicate setup of such experiments, and render these critically sensitive to altered tuning/rf conditions in the course of potentially long experiments for biological samples. 

Under MAS, the Hartmann-Hahn condition for 3Q-CPMAS of spin-3/2 nuclei can be written as... 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

Hartmann-Hahn cross polarization between two low-y nuclei has been successfully used to record chemical-shift correlation spectra between 13C and 15 N nuclei. Cross polarization between two low-y nuclei suffers from a high sensitivity to the exact matching condition at one of the side bands of the Hartmann-Hahn condition [101]. Adiabatic methods (APHH-CP) can eliminate most of this sensitivity and lead to high transfer efficiencies [34, 62, 90]. 

Harmonic entrainment, Pt, 37 237-238 Hartmann-Hahn condition, 33 210 Hartree-Fock... 

Figure 3. Cross polarization magnetization for the PIP-cured epoxy under the SL (Hartmann-Hahn) condition. The cross polarization contact time is rcp. The decay corresponds to proton T,p relaxation.

These questions were resolved with the use of the same relatively simple epoxy system. All C-13 nuclei in contact with the proton bath were counted when moderate spinning rates were used and in spin-lock cross polarization in rf fields not close to any Tle minimum. The molecular motion determines the relaxation rate, under the Hartmann-Hahn condition when T, = T2. The spin-spin effects determine relaxation when Tle does not equal T2 under the same conditions 62). The spin-spin fluctuations are in competition with the spin-lattice fluctuations in producing an effective relaxation time. To discriminate against the spin-spin fluctuations large rf fields are mandatory. It was pointed out that, with great care, C-13 NMR spectra can reflect molecular motion. 

Cross-polarization in tilted frame has been proposed.275 In this scheme, the two effective fields matching the Hartmann-Hahn condition have a large offset comparable to two RF fields. This method was named SPECIFIC CP (spectrally induced filtering in combination with cross-polarization Fig. 20) because the matching depends on the resonance offset of the involved nuclei. This method is clearly useful for spectral simplification, as demonstrated in Fig. 21. Under fast spinning conditions, 7-coupling-mediated CP can be observed as shown by Verhoeven et al.216... 

To enhance the sensitivity of 13C solid state NMR, polarisation (magnetisation) transfer from the abundant proton spins to the rare carbon spins is applied. This technique is called cross polarisation (CP). This is achieved by first exciting the proton resonance and then simultaneously turning on the 13C and aH radio frequency fields. The so called Hartmann-Hahn condition should then be satisfied ... 

CP) required for normal FID methods XH to 13C under Hartmann-Hahn conditions. Sensitivity... 

Switching on the 13C RF transmitter is represented by opening the valve between the reservoirs H and 13C. The relative powers of the proton and 13C RF transmitters are adjusted to maximize interactions between the two types of precessing nuclei. Polarization can then be transferred between neighboring nuclei through spin flip-flop processes. Optimization is achieved when the Hartmann-Hahn condition is met, i.e., the H and 13C RF field strengths are in a ratio set close to 1 4 (Pines et al. 1973). Magnetization is then transferred with a time constant TCH-... 

N, or 29Si. In CP, both nuclear spin species are spin locked and the rf amplitudes adjusted so that their Larmor precession frequencies in their respective rotating frames are equal the Hartmann-Hahn condition. The single-contact CP enhancement of the rare spins is given by yj/ys. Additional enhancement normally occurs because the CP experiment may be recycled at a rate limited by the usually much faster spin-lattice relaxation rate of protons rather than that of the rare spins. 

The experimental method for obtaining ROESY is essentially the same as that for HOHAHA, application of a spin-lock pulse sequence for mixing at the end of the evolution period. HOHAHA effects can interfere with ROESY measurements but are minimized by using lower rf power and offsetting the pulse frequency to interfere with the Hartmann-Hahn condition. 

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