Electronic structure of dicarbenes

The temperature dependences of the ESR signals (Fig. 9.10) show one eommon feature the ESR intensities vanish for T 0, i.e. the ESR signals are thermally activated and the ground state is spinless. But the temperatures for the maximum intensities are different for each ESR signal, indicating different activation energies. 

If Assq is smaller than or somewhere in the order of the magnetic dipole-dipole coupling then the singlet and quintet states are mixed. The spin Hamiltonian Hs for this general case has been derived in the elegant work of Benk and Sixl [35]  

The first term of Eq. 4 represents the electronic Zeeman term, Si and 2 the spin operators for tvro triplet carbenes, and the second term represents the electronic exchange interaction. If Si and 2 couple to a quintet state (S = 2), ( 1 + 2) = S = S(S + 1) = 6. If they couple to a singlet, S = 0. Therefore, this term directly results in the energy level scheme, indicated in the inset of Fig. 9.13. The pure singlet and the pure quintet states are split by Assq which turns out to be the characteristic property of each dicarbene. The third term of Eq. 4 represents the magnetic dipole-dipole coupling of the two triplet carbenes  

D and E are the fine structure parameters of the identical triplet carbene chain ends X, y, and z are the principal axes of the corresponding fine structure tensors. The intercar-bene magnetic dipolar interaction is represented by the parameter 

The geometrical factors A and a are only dependent on the orientation p of the fine structure tensor z-axis with respect to the fe-axis of the erystal. 

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