Interchange mechanisms definitions

But cation diffusion always can occur by the interchange mechanism, whereas in pore—fissure diffusion it depends on the presence of unoccupied cation positions in a certain number and a definite distribution. Accordingly, the coefficients of internal diffusion are maximal when both diffusion mechanisms are operating at the same time, and minimal when diffusion is occurring only by the interchange mechanism. 

The notion of electrons in orbitals consists essentially of ascribing four distinct quantum numbers to each electron in a many-electron atom. It can be shown that this notion is strictly inconsistent with quantum mechanics (7). Definite quantum numbers for individual electrons do not have any meaning in the framework of quantum mechanics. The erroneous view stems from the original formulation of the Pauli principle in 1925, which stated that no two electrons could share the same four quantum numbers (8), This version of the principle was superseded by a new formulation that avoids any reference to individual quantum numbers for separate electrons. The new version due to the independent work of Heisenberg and Dirac in 1926 states that the wave function of a many-electron atom must be antisymmetrical with respect to the interchange of any two particles (9,10). 

The definition of chemical shift equivalence given for protons also applies to carbon atoms interchangeability by a symmetry operation or by a rapid mechanism. The presence of equivalent carbon atoms (or coincidence of shift) in a molecule results in a discrepancy... 

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