Practical Aspects of Hartmann-Hahn Experiments

In this section, aspects of Hartmann-Hahn experiments are discussed that are important for practical applications. There are obvious instrumental differences between heteronuclear and homonuclear Hartmann-Hahn experiments, such as the necessity for one or several heteronuclear rf channels and double- or triple-resonance probes. In addition, the rf amplitude of the channels must be matched, that is, the duration of the respective 90° pulses must be carefully adjusted such that the difference is not larger than a few percent. A detailed discussion of setup experiments for the calibration of hetero pulses has been given, for example, by Griesinger et al. (1994). 

Pulses of different attenuator settings usually have inherent phase differences. The degree of these phase errors depends on the spectrometer used. In experiments where pulses of different rf amplitudes are applied to the same nuclear species, the phase relationship must be determined experimentally and taken into account in the phase programming. 

In spite of technical differences, HOHAHA and HEHAHA experiments also share a number of potential practical problems, for example, phase anomalies in the spectra, water suppression, and sample heating. The discussion in this section will focus on examples of HOHAHA experiments, but special features that are characteristic for HEHAHA experiments will also be pointed out. For simplicity, only two-dimensional Hartmann-Hahn experiments are considered here, but the extension of the discussed principles to hybrid or multidimensional experiments (see Section XIII) is generally straightforward. 

In order to obtain optimum resolution and undistorted multiplet patterns, pure-phase absorptive two-dimensional HOHAHA or HEHAHA 

One way to transfer coherence order pity) = -t-1 as well as pity) = -1 to detectable magnetization with coherence order pit2) = -1 is to eliminate one of the transverse magnetization components that are present after the evolution period f,. For example, if the y component is eliminated, the terms I, -t- ily (coherence order p = 1) and / = / - ily (coherence order p = -1) are both reduced to I, = il + / )/2, which contains the detectable coherence order p = -1 (as well as coherence order p = -1-1). 

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