Forbidden spin exchange

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

Energy transfer by the following route is forbidden both by exchange (spin) and resonance transfer ... 

Exchanging the coordinates of the two electrons changes the wave function to tlf(x2, yi, z2) Pa(x, yi, Zi), which is the same as before. Since the wave function does not change sign, it is forbidden by the Pauli principle. Hence two electrons with the same spin cannot be described by the same wave function, or, in other words, an orbital cannot contain two electrons with the same spin. As we shall see, this form of the Pauli principle is used in describing atoms and molecules in terms of orbitals. 

Understand that triplet-triplet energy transfer by the dipole mechanism is forbidden, but that the application of the Wigner spin rule shows that it can occur by the exchange mechanism. 

CsNiCl3-type). Table 6 summarizes results on electronic spectra. The three spin-allowed transitions are usually observed and extensive vibrational contribution has been observed in single crystal spectra.120,121 Intense spin-forbidden transitions may also be observed122 due to exchange interactions in the solid phase. The diiodide123,124 and the complex halides121 show trigonal distortion. 

Spin-spin relaxation (time T2) can result from both intermolecular homo-nuclear exchange coupling and dipole-dipole interactions, but only the latter is observable at Mn2+ concentrations <10-2 M. Mn2+ forms both inner- and outer-sphere complexes. In symmetrical inner-sphere complexes like Mn(H20)62+, the spin-spin coupling is strongly forbidden, T2 is long, and lines remain narrow. When nonsymmetric inner-sphere complexes form, the resulting anisotropy of the electric field leads to allowed spin-spin transitions that produce very small values of T2 and very broad, perhaps even unobservable, lines (56). 

In accordance with the Pauli exclusion principle, electron exchange can take place only between electrons with antiparallel spins. Therefore, from schemes EE1 and EE2, only the EE2 type is permitted. Interactions according to schemes EE3 and EE4 are real, as well. Scheme EE3 corresponds to the ferromagnetic orientation of spins. Scheme EE5 (one-electron transfer from the cation radical to the anion radical) is energetically forbidden. 

As elaborated in detail in Ref. (5) there are two principal intensity mechanisms for dimer excitations. The single-ion mechanism is based on the combined action of spin-orbit coupling and an odd-parity ligand field potential at the Cr center. It is by this mechanism that spin-forbidden transitions obtain their intensity in mononuclear complexes. The pair mechanism, on the other hand, is restricted to exchange-coupled systems. It leads to the selection rules AS = 0,... 

Exchange energy transfer from the lowest spin forbidden excited state is expected for singlet-triplet intersystem-crossing (ISC) reactions presented further in the following parts of this thesis. 

The absorption bands at 18,450 cm-1 and 20,300 cm-1 (fig. 4.16c) represent crystal field transitions within Ti3+ ions, and the weaker band near 12,500 cm-1 may represent a Ti3+ - Ti4+ IVCT transition between cations in face-shared octahedra. The peaks in the spectra of the yellow and blue sapphires clustered at 22,200 cm-1 and near 26,000 cm-1 represent spin-forbidden 6A, - 4AxfE G) and 6A[ — 4A2,4E(D) transitions in octahedrally coordinated Fe3+ ions (fig. 3.10), intensified by exchange interactions between adjacent Fe3+ ion pairs in the corundum structure ( 3.7.3). Other spin-forbidden Fe3+ bands occur at... 

In the same way, a process which is spin-forbidden by an exchange mechanism can obviously become important if other competing processes of dissipation of the donor energy are similarly forbidden. For a process which is spin-forbidden by coulombic and exchange interactions, there are no such compensating effects when the transition the acceptor undergoes is forbidden. Thus the process... 

---