Spin Network

The ID homonuclear Hartmann-Hahn (HOHAHA) experiment is an excellent way to determine complete coupled spin networks (18). The following pulse sequence is used ... 

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. 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

A 3D spectrum of a three-spin subsystem in which all the nuclei are coupled to one another, such as C(H/i)(Hb)-C(Hc), will lead to 27 peaks, comprising six cross-peaks, 12 cross-diagonal peaks (six at o) — o>2 and the other six at 0)2 = (O3), six back-transfer peaks, and three diagonal peaks. However, in the case of a linear three-spin network (e.g., CH -CHg-CHc), the number of peaks will depend on whether two equal (e.g., COSY-COSY or NOESY-NOESY) or unequal (e.g., COSY-NOESY) mixing processes are... 

Finally, interactions between neighbouring molecules and viscosity will alter the resolution in the spectrum via the spin/network relaxation time. 

However, in all the papers mentioned above the authors analyzed only three-dimensional (3D) systems, while a two-dimensional (2D) case is also experimentally observed surfaces of various absorbers, heterogeneous catalysts, photocatalysts, etc. In [137], Fel dman and Lacelle examined the quenched disorder average of nonequilibrium magnetization, i.e., a free induction decay G(t) and its relative fluctuations for dipolar coupled homonuclear spins in dilute substitutionally disordered lattices. The studies of NMR free induction decays and their relative fluctuations revealed that the functional form of the disorder average (G(t))c depends on the space-filling dimentionality D of the lattice. Explicit evaluations of these averages for dilute spin networks with D = 1, 2, 3 were presented in [137] ... 

Hence the pattern produced by the coupling gives important information about the local spin network and has proved very useful in liquids for identifying the local fragments of molecules, especially in large molecules such as proteins. If a spin-I is coupled to several spins S then including the Zeeman interaction the Hamiltonian is... 

R647 T. Hasegawa, NMR Spin-Spin Network Analysis , CACS Forum, 1999,19, 29... 

An approach to the determination of the orientation of the carbonyl CSA tensor in a dipolar coupled spin network has been proposed. " The... 

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