Spin-isotope-labelling techniqu

Various 2H, 13C and 31P NMR techniques were used to follow the slowing down of molecular dynamics during the glass transition.3 In particular, 2H NMR proved very powerful since solely molecular reorientation is probed and isotopic labeling is easily achieved. Fig. 7 shows the correlation time windows of the major 2H NMR techniques. Two reference frequencies exist The Larmor frequency col determining the sensitivity of the spin-lattice relaxation and the coupling constant 8q fixing the time window of line-shape experiments. 

The remark has been made that compounds of tin can be studied by more techniques than those of any other element. The fact that it has more stable isotopes that any other element gives it very characteristic mass spectra, and isotopic labelling can be used to interpret vibrational spectra, and for spiking samples in trace analysis two of the isotopes have spin 1/2 and are suitable for NMR spectroscopy, and their presence adds information to the ESR spectra of radical species. Further, the radioactive isotope 119mSn is appropriate for Mossbauer spectroscopy. The structural complications that are referred to in the previous chapter have therefore been investigated very thoroughly by spectroscopic and diffraction methods, and structural studies have always been prominent in organotin chemistry. 

Given the above, it was of interest to establish the direction of rotation of the triptycene in 37b. Preparation of isotopically labeled rotamers of 37b should be possible, but would require extensive synthetic, rotamer-separation, and structure-determination efforts. Fortunately, the spin polarization transfer NMR technique [43] affords the same information at a small fraction of the effort. In short, if one has a system that is conformationally mobile, but that mobility is slow on the NMR time scale, then one can polarize the spin of a slowly conformationally mobile atom, wait an appropriate time, and assay where (if anywhere) that polarization has moved to. 

Spin contrast variation will probably bear fruit in in situ structure determinations of arbitrary parts of biological structures. For this purpose a selected region of the macromolecular structure has to have the usual hydrogen isotope H whereas the larger remainder is preferably completely deuterated. This technique of specific isotopic labelling is now well established with ribosomes and other subcellular particles which allow in vitro recombination of the constituents... 

In the rotational-echo double-resonance (REDOR) [60—62] technique, the distance between two heteronuclei is determined by comparing the signal intensity in two closely related experiments. The interpretation of the experimental results assumes the existence of isolated spin pairs, and there is thus usually a requirement for selective isotopic labelling at the two sites, the distance between which is of interest. 

NMR has been a powerful technique for structural analyses of macromolecules. However, ID NMR spectra of PDMS are usually complicated due to signal overly. Their complete characterization often requires combinations of several techniques. Multidimensional NMR techniques, especially inversely detected 3D heteronuclear shift correlation experiments, offer the opportunity to obtain the complete structural characterization by using NMR experiments alone. Biological 3D-NMR experiments are usually performed in conjunction with uniform and isotopic labeling. In polymer chemistry, when isotopic labeling is possible, it is often very difficult and expensive. By modifying the 3D-pulse sequence used for biopolymers, triple resonance 3D-NMR techniques have been adapted for sbufying the structures of polymers, which involve H- C- P, H- C- Si spin... 

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