Abstract
The influence of indirect spin-spin interaction (briefly J coupling) on the evolution of spin coherences becomes appreciable if the much stronger dipolar or quadrupolar interactions are largely averaged out by molecular motions. This is the realm of high-resolution NMR which applies to low-viscous isotropic liquids in particular. Similar conditions can also be produced in solids by experimental averaging procedures such as magic-angle spinning or multi-pulse line narrowing [336]. As outlined before, echoes are understood as coherence dephasing/rephasing phenomena. Therefore any interaction and any measure influencing the coherence evolution and the coherence pathways affect the formation of spin echoes.
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Notes
The rules for the interpretation of the spin-operator terms are summarized in Table 51.1 on page 482.
The terminology and symbols for the time intervals normally used in context with three-pulse two-dimensional spectroscopy are partially different from those common with ordinary spin-echo experiments. The 2D nomenclature corresponds to that used in the three-pulse schemes Fig. 7.3, for instance.
A treatment of a similar three-pulse sequence serving volume-selective spectral editing of an A3X spin system can be found in Sect. 37.1.2.
The subscript in parentheses indicates that we are now dealing with converted coherences. In the present case, σ(lm) means single-quantum coherences which can be traced back to longitudinal magnetization in the previous interval.
This also implies that no “exchange” takes place (compare Sect. 23.2).
See footnote 4 on page 63.
The operator terms can equivalently be represented by the single-transition expressions given by Eqs. 42.76.
This is the reason why zero-quantum coherences cannot be spoiled by external field gradients.
Compare footnote 4 on page 63.
Compare footnote 8 on page 34.
The operator terms standing for double-quantum coherences can also be expressed by the single-transition operators given by Eqs. 42.77.
According to the theory outlined above, double-quantum coherences as well as any other order of non-single-quantum coherences should not occur if spin-spin and dipolar couplings do not affect the evolution of spin-1/2 coherences. In the present case, where complete motional averaging is assumed, this is readily demonstrated by setting J = 0 in Eq. 7.45. However, long-range intermolecular dipolar interactions fluctuate more slowly than short-range and intramolecular couplings. Motional averaging may therefore be incomplete for long distances r. Multiple-quantam coherences might arise on these grounds [505]. Remember that dipolar interaction varies proportional to r-3, whereas the number of coupling partners increases proportional to r2. Also, one should keep in mind that radiation damping and the demagnetizing field can cause striking effects which mimic the coherence-transfer features of J coupled spin systems under motional averaging conditions [64, 65]. Radiation damping is the result of the feedback action of the currents induced in the probe RF coil by the precessing magnetization [66]. The demagnetizing field is caused by the magnetization of the sample as described in Sect. 2.4. The line shifts caused by this field are appreciable in high-resolution NMR at the strong flux densities of modern spectrometers. The demagnetizing field follows the magnetization in the course of a pulse sequence, and, hence, a feedback effect on the evolution occurs [59].
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© 1997 Springer-Verlag Berlin Heidelberg
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Kimmich, R. (1997). Coherence Transfer of J-Coupled Spins. In: NMR. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-60582-6_7
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DOI: https://doi.org/10.1007/978-3-642-60582-6_7
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-642-64465-8
Online ISBN: 978-3-642-60582-6
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