Passive spin

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

Fig. 7.3 Pul se sequence of the constant-time (ct) HNCA-COSY experiment. The passive spin is C. Obviously, C is affected by only one 180° pulse,...

HAp is a good model compound for the study of DQ coherence excitation profile.85 The closest P-P distance in HAp is 4.143 A. The numerical simulations and the experimental data were shown in Figure 5. The maximum DQ coherence excitation efficiency decreases as the spin cluster size increases. The reason is simply due to the fact that all the spins in the cluster will contribute (different combinations of two spins) to the DQ coherence. The passive spins in close proximity would provide a strong dipolar dephasing of the excited DQ coherence. As expected, the simulation results converge towards the experimental data as the cluster size increases. 

IXSZTZ Ix magnetization in antiphase with respect to 5 and T 4IXSXTZ Two-spin coherence in antiphase with respect to T, the passive spin 4IXSXTX Three-spin coherence... 

If the I pulse at time 4 is 9, rather than 90°, the only change in the preceding pathway is to convert the final Sx to Sx sin 0. Some multiple quantum coherence remains but is not observed. For 13CH2 and 13CH3 systems, the pathway is much the same. The 13C can be considered as being coupled to one of the magnetically equivalent protons as described, and the other protons behave as passive spins, each of which can be shown to contribute an additional modulation of cos 9. Thus, the general expression becomes... 

The two-dimensional PICSY experiment (pure-in-phase correlation spectroscopy Vincent et al., 1992, 1993) can also be regarded as a highly selective E.TACSY experiment. In these experiments, coherence transfer is accomplished by doubly selective rf irradiation with relatively low rf amplitudes, which are on the order of 50 Hz for each sideband. Hence, the polarization state of a passive spin remains essentially undisturbed if mir where is the smallest frequency difference between... 

Willker and Leibfritz (1992a) introduced an extension of the E.COSY principle that yields additional flexibility. In addition to coherence transfer between the active spins i and j, polarization of spin p, which is passive during is transferred to a spin q, which plays the role of the passive spin during t - Hence, in general, the E.COSY triad is opened up. The two- and three-dimensional JHH-TOCSY experiments for the determination of coupling constants of Willker and Leibfritz (1992a) use a combination of homonuclear TOCSY transfer and two BIRD (bilinear rotation decou-... 

Chapter 2 considers how we can understand the form of the NMR spectrum in terms of the underlying nuclear spin energy levels. Although this approach is more complex than the familiar successive splitting method for constructing multiplets it does help us understand how to think about multi-plets in terms of active and passive spins. This approach also makes it possible to understand the form of multiple quantum spectra, which will be useful to us later on in the course. The chapter closes with a discussion of strongly coupled spectra and how they can be analysed. 

Transition 3-4 is similar to 1-2 except that the passive spin (spin 1) is in the state this transition forms the second line of the doublet for spin 2. This discussion illustrates a very important point, which is that the lines of a multiplet can be associated with different spin states of the coupled (passive) spins. We will use this kind of interpretation very often, especially when considering two-dimensional spectra. 

The two transitions in which spin 1 flips are 1-3 and 2 1, and these are associated with spin 2 being in the a and fi spin states, respectively. Which spin flips and the spins states of the passive spins are shown in Fig. 2.7. 

What happens is the coupling is negative If you work through the table you will see that there are still four lines at the same frequencies as before. All that changes is the labels of the lines. So, for example, transition 1-2 is now the right line of the doublet, rather than the left line. From the point of view of the spectrum, what swaps over is the spin state of the passive spin associated with each line of the multiplet. The overall appearance of the spectrum is therefore independent of the sign of the coupling constant. 

Fig. 2.11 Energy levels for a three-spin system showing by the arrows the four allowed transitions which result in the doublet of doublets at the shift of spin 1. The schematic multiplet is shown on the right, where it has been assuming that v0,i = -100 Hz, J 2 = 10 Hz and Ji3 = 2 Hz. The multiplet is labelled with the spin states of the passive spins.

In the product operator representation of multiple quantum coherences it is usual to distinguish between active and passive spins. Active spins contribute transverse operators, such as Ix, I and 7+, to the product passive spins contribute only z-operators, Iz. In a sense the spins contributing transverse operators are "involved" in the coherence, while those contributing z-operators are simply spectators. 

Fig. 14. Contour plot of the 2D 120 MHz P/ H HETCOR experiment for tris(tri-methyl-silyl)phosphane, P(SiMc3)3, in C5D5. The experiment was modified by using a z-filter in order to suppress most of the intensity of the central signal. This allows to observe the satellite signals almost undisturbed. The path of PT is indicated for each isotopomer. The active spins are P and in each case, and the passive spin is either Si or C. In the case of Si, there are two types of pairs of cross peaks for Si satellites shown. The pair with lower intensity has a significant positive tilt which means that A i( P, Si) and Al( Si, HMe) have alike signs. Since 2 ( Si, HMe)<0, it holds that A ( P, Si) < 0 ( J( P,2 Si)>0 ). The pair of crosspeaks with higher intensity results from PT across magnetically non-active silicon isotopes, and there is no tUt because of the small magnitude of Al( Si,P,Si,C, H). A very small tilt is indicated for C satellites. (B. Wrackmeyer and U. Klaus, unpublished results).

Multiple-quantum coherences do not exhibit scalar couplings between their active spins. Their scalar couplings to passive spins are linear combinations of those of their individual active spins to the passive spin concerned. Thus a double-quantum coherence between spins k and / would exhibit a scalar coupling constant of (Jkm Jim) to a passive spin m the corresponding zero-quantum coherence scalar coupling constant would be (Jkm JlJ-... 

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