Bond shift mechanism structural effects

All these results are readily interpreted by assuming the existence of two bond shift mechanisms. The first one, which accounts for methyl shift, may be ascribed to the metallocyclobutane mechanism responsible for the group III reactions of n-pentane and isopentane. The second one, which accounts for chain lengthening (and chain shortening) is the same as the mechanism of higher activation energy (group II) responsible for the interconversion between n-pentane and isopentane. The first is very sensitive to alkyl substitution, while the latter seems relatively insensitive to structural effects. 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

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