Spin-echoes homonuclear

Dipole-dipole (homonuclear) Afjfhomo) (mean-squared local field) Spin-echo NMR Intemuclear distances, number of surrounding nuclei... 

SPIN-ECHO FORMATION IN HOMONUCLEAR AND HETERONUCLEAR SYSTEMS... 

Spin-Echo Formation in Homonuclear and Heteronuclear Systems... 

No. Since the direction of precession of the vectors is reversed during the second half of the sequence, they continue to diverge from each other. The effect of a spin-echo experiment on spin-spin coupling in a first-order homonuclear spin system is shown in the following vector representation ... 

Another 2D homonuclear shift-correlation experiment that provides the coupling information in a different format is known as SECSY (spin-echo correlation spectroscopy). It is of particular use when the coupled nuclei lie in a narrow chemical shift range and nuclei with large chemical shift differences are not coupled to one another. The experiment differs... 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

Several modifications have been proposed for the basic HNN-COSY experiment. For example, frequency separations between amino and aromatic 15N resonances are typically in the range 100-130 ppm and therefore much larger than between imino 15N donor and aromatic 15N acceptor resonances. As has been pointed out by Majumdar and coworkers [33], such 15N frequency separations are too large to be covered effectively by the non-selective 15N pulses of the homonuclear HNN-COSY. They therefore designed a pseudo-heteronuclear H(N)N-COSY experiment, where selective 15N pulses excite the amino and aromatic 15N resonances separately to yield excellent sensitivity [33]. An inconvenience of this experiment is that the resonances corresponding to the amino 15N nuclei are not detected, and a separate spin-echo difference experiment was used to quantify the h2/NN values. A slightly improved version of this pseudo-heteronuclear H(N)N-COSY [35] remedies this problem by the use of phase-coherent 15N pulses such that both amino and aromatic 15N resonances can be detected in a single experiment. 

The spin-echo pulse sequence can also be applied to produce homonuclear (e.g., H) J-resolved spectra. For such a system it is not possible to apply broadband decoupling, so the F2 dimension might be expected to display the ordinary coupled H spectrum. However, because the coupling information is independently available, it is not difficult to process the data in such a way that only the chemical shifts are displayed in the F2 dimension, as illustrated in Fig. 10.7. 

FIG. 10. Amplitude modulation of spectra obtained by Fourier transformation of the last half of a spin-echo when there is a small unresolved homonuclear spin-spin coupling constant. The spectra are from the protons in 2,4,5- trichloronitrobenzene which has a para coupling of 0-4 FIz. From ref 145. 

The combination of homonuclear Hartmann-Hahn transfer with homonuclear double- or zero-quantum spectroscopy yields the so-called DREAM experiment (double-quantum relay enhancement by adiabatic mixing Berthault and Perly, 1989) and the zero-quantum-(ZQ) TOCSY experiment (Kessler et al., 1990a), respectively. Multiplet-edited HOHAHA spectra can be obtained by adding a spin-echo sequence to the Hartmann-Hahn mbdng period (Davis, 1989a). 

The spin echo decay under homonuclear couplings only (as opposed to decay by relaxation) is often called the slow beat, even if no periodic modulation is observed. A real beat usually occurs when a scalar coupling between unlike nuclei is the main decay mechanism, e.g., for Pt in dilute bulk alloys. Dipolar coupling usually leads to a monotonic decay, which initially is Gaussian (at the origin all its odd time derivatives are zero) and becomes more exponential at longer times. 

Fig. 2 Timing diagram of the spin echo decay spectroscopy method devised for the measurement of homonuclear dipole-dipole interactions...

Static spin echo decay spectroscopy also forms the basis for the measurement of magnetic dipole-dipole interactions between two unlike nuclei I and S. While this interaction is refocused by the Hahn spin echo, it can be recoupled by applying a 7i-pulse to the S-spins during the dipolar evolution period [12]. This manipulation inverts the sign of the heterodipolar Hamiltonian, and thereby interferes with the ability of the Hahn spin echo technique to refocus this interaction. The corresponding pulse sequence, termed SEDOR spin echo double resonance) shown in Fig. 4, compares the I-spin echo intensity as a function of dipolar evolution time (a) in the absence and (b) in the presence of the ti(S) pulses. Experiment (a) produces a decay F(2ti)/Fo, which is dominated by homonuclear dipole-dipole interactions, while experiment (b) results in an accelerated decay, reflecting the contribution from the heteronuclear I-S dipole-dipole interaction, which is now re-introduced into the spin Hamiltonian. For multi-spin systems, a Gaussian decay is expected ... 

For bone, the P spin-lattice relaxation time Tf is ca 100 s [36,39] and decreases by approximately 15% from the dried to the fully hydrated sample [36]. In contrast, for synthetic HA, Tf is considerably shorter and varies from 1 to 22 s [31, 36]. The cited results are for differently prepared samples, different magnetic fields and different MAS rates. Homonuclear 3ip.3ip spin-spin relaxation in the sohd state can be conveniently characterized by van Vleck second moments M [46,47].Wuetal. [41] measured Mf in bone, dental enamel and synthetic apatites, HA and CHA-B, using the P Hahn spin echo under proton decouphng. The initial P magnetization was prepared either by a nl2 pulse or... 

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