Homonuclear coupled spin systems

A simple pulse sequence using transverse relaxation is the spin-echo (90 - s-180°-rs-FID) and here the signal intensity is proportional to exp(—2rs/72). Unlike the inversion-recovery method, the spin-echo method can allow the spectra to be phased to give positive peaks for all spins, but phase distortion is expected for homonuclear coupled spin systems when longer spin-echo times are used or where larger coupling constants are present in this method. 

In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

Vega, and coworkers as a sequence of well-placed ideal (i.e., infinitely strong) 7r-pulses serving to reintroduce the zero-quantum (ZQ) part of the homonuclear dipole-dipole coupling operator in a homonuclear two-spin system. The coherent averaging due to MAS is in the toggling frame of the n-pulses partially disrupted by a differential chemical shift term and thereby recoupling takes place. 

Homonuclear Hartmann-Hahn transfer functions for off-resonant CW irradiation have been derived for two coupled spins 1 /2 (Bazzo and Boyd, 1987 Bothner-By and Shukla, 1988 Elbayed and Canet, 1990) and for the AX 2 spin system (Chandrakumar et al., 1990). In the multitilted frame, Hartmann-Hahn transfer functions under mismatched effective fields are related to polarization- and coherence-transfer functions in strongly coupled spin systems (Kay and McClung, 1988 McClung and Nakashima, 1988 Nakai and McDowell, 1993). Numerical simulations of homonuclear... 

The most widely used form of integral projection is not however onto the fi or f2 axis, but onto the axis fi = f2 weakly coupled spin systems, homonuclear 2D J spectroscopy gives rise to muliplets which lie along lines with gradient df2/dfi = l.tS Thus,... 

The measurements of homonuclear /-couplings in solids are an important experimental method for studying conformational and structural effects as well as hydrogen bonding. The principles of spin-echo modulation by homonuclear /-couplings in systems containing isolated pairs of spin have been studied by Duma et The theoretical conclusions have been supported by numerical simulations and experimental results obtained for three representative samples containing spin pairs. 

The interaction of interest in RR is the dipolar interaction, an orientationally dependent through-space spin-spin coupling, which leads to a perturbation of the CS-perturbed Zeeman energy levels. For a homonuclear two-spin system, the truncated dipolar Hamiltonian operator is given by the following ... 

The experiment just discussed represents a heteronuclear spin system. In a homonuclear case, the separate 90° C pulse necessary in the heteronuclear system is not required, since the second 90° H pulse affects the coupled partner H nucleus as well. The nucleus detected therefore has its two transitions antiphase with respect to each other, corresponding to the states represented in Fig. 2.7, Ilg, Ilh, etc. at detection. 

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 ... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

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. 

More recently, homonuclear correlation techniques relying on J-couplings have also been developed [86, 87, 89] and applied to the assignment of spin systems of amino-acid residues in uniformly labeled proteins and peptides [90]. These have, in some cases, a higher information content than the comparable dipolar-mediated experiments, as relayed correlations throughout the continuous 13C-13C network are more easily realized at high B0 fields [90]. 

In the pulse sequence for a homonuclear COSY experiment, the first 90° pulse flips the z magnetization into the x direction and into the xy plane. Considering an AX spin system (one in which the nuclei have very different chemical shifts) having two doublets due to spin-spin coupling, the magnetization will include four components processing at different frequencies. During a... 

Performing the CPMG experiment on a spin system with homonuclear coupling introduces the complication of echo modulation by the scalar couplings [4, 5, 7, 36]. This makes transverse proton relaxation measurements... 

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