Scalar coupling phenomenon

There is a simple and well-known model for the description of DNMR phenomenon, provided no scalar coupling is involved.11 In Section 3.2.1, the macroscopic magnetisation vector of the vector model gives the detected signal (the FID). This vector rotates in the xy plane perpendicular to the direction of the external magnetic field during the detection its frequency is determined by the shielding effect of the chemical environment of the nuclei. 

In many respects, at a superficial level, the theory for the chemical reaction problem is much simpler than for the velocity autocorrelation function. The simplifications arise because we are now dealing with a scalar transport phenomenon, and it is the diffusive modes of the solute molecules that are coupled. In the case of the velocity autocorrelation function, the coupling of the test particle motion to the collective fluid fields (e.g., the viscous mode) must be taken into account. At a deeper level, of course, the same effects must enter into the description of the reaction problem, and one is faced with the problem of the microscopic treatment of the correlated motion of a pair of molecules that may react. In the following sections, we attempt to clarify and expand on these parallels. 

Besides integrals (Chapter 3) and the chemical shift (Chapter 4), NMR spectroscopy provides another key piece of information in the form of how spins affect one another through the electrons of the molecule. This intramolecular communication is the phenomenon known as 1-coupling, scalar coupling, or spin-spin coupling. 

It is possible for more than two forces to couple. There exists a criterion which allows one to deduce a priori the number of effective couplings. This is Curie s principle of symmetry. The principle states that a macroscopic phenomenon in the system never has more elements of symmetry than the cause that produces it. For example, the chemical affinity (which is a scalar quantity) can never cause a vectorial heat flux and the corresponding coupling coefficient disappears. A coupling is possible only between phenomenon which have the same tensor symmetry. Thus Onsager reciprocity relation is not valid for a situation when the fluxes have different tensorial character. 

Firstly the so-called Curie s law must be respected and the membrane system show dissymmetry in order to allow a vectorial phenomenon to appear through coupling of a chemical reaction which is scalar in its nature. Secondly, as for mechanical pumps, some sort of valve is useful in the system to increase the yield or add a complementary selectivity. Finally, calculations on some early systems [76] showed high instability which can be avoided by the creation of convenient geometrical or functional structures, for example. 

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