Transverse magnetisation observable

Figure 5.10 Radio frequency variation of My(t) transverse magnetisation observed, acquired and stored digitally with time is known as a Free Induction Decay (FID). Stored FID either singly or averaged, are processed by fourier series transformation (FT) from time domain signal information, SnoCti), into frequency domain (spectral) information, /nmr( i)- Only chemically equivalent nuclei without spin-spin coupling and with an equivalent Lamor frequency, V, are being observed here hence only a single signal will result of frequency Vi.

Mx is the quantum operator associated with the transverse magnetisation. With regard to the proton magnetic relaxation, the probe determined by the end-to-end vector r, is substituted for any chemical unit attached to the chain segment. The observation is thus delocalised over the space scale defined by the distance r = 5 nm. From the spectroscopy point of view, HD represents a dispersion of non-coherent broadening frequencies and... 

The principle of the NMR approach to semi-local properties of polymeric melts is considered in Section 2 it is shown how the existence of a temporary network structure is detected from the relaxation of the transverse magnetisation of protons attached to chains. The observation of segmental motions from the longitudinal relaxation of proton magnetisation is described in Section 3 it is also shown how local motions in concentrated polymeric solutions can be probed from the diffusion process of small molecules. Section 4 is devoted to the analysis of the effect of entanglement relaxation on NMR properties. 

The transverse magnetisation we observe directly in an NMR experiment is known as single quantum coherence. Multiple quantum coherence, however, cannot be directly observed because it induces no signal in the detection coil. For multiple quantum coherence to be of use to us, it must be transferred back into signal quantum coherence by the action of rf pulses. The concept of coherence is developed further in Chapter 5. 

Figure 5.27. Net transverse magnetisation is produced from the bunching of individual magnetic moments which gives rise to an observable NMR signal. These spins are said to posses phase coherence, and because only single-quantum spin transitions (a <-> ) ate involved in generating this state, it is termed single-quantum coherence.

The pulse sequence of a general 2D COSY experiment is shown in Figure 5.12. In time q, transverse magnetisation is generated that evolves with chemical shift. Coherence transfer between spin-coupled nuclei then takes place during the second pulse afterwards, final coherence is observed and acquired over time t2- The coherence transfer between spin-spin coupled nuclei is the key element since the purpose of this experiment is to determine those homo nuclei of interest (e.g. C- C) that are i or spin-spin coupled with respect to... 

Figure 5.17 Cartoon diagram to represent general structure of 4D correlation experiments. This is the same as for 3D correlation experiments (Fig. 5.14) except that an extra resonant population of heteroatom nuclei are involved in generation of transverse magnetisation (in time ts) and magnetisation transfer (during M3). Final pulse sequence generates transverse magnetisation in the Destination Nuclei S that is observed, acquired and digitised in time t/,. Fourier series transformation is used to transform time domain signal information Sfid (ti, ta, ts, 4) into frequency domain (spectral intensity) information, /NMR(fi, F2,...

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