Unpaired electrons, singlet-triplet splittings

From the point of view of ESR spectroscopy, the distinction between molecules with one unpaired electron and those with more than one lies in the fact that electrons interact with one another these interactions lead to additional terms in the spin Hamiltonian and additional features in the ESR spectrum. The most important electron electron interaction is coulombic repulsion with two unpaired electrons, repulsion leads to the singlet-triplet splitting. As we will see, this effect can be modeled by adding a term, JS St, to the spin Hamiltonian,... 

The ESR results, along with kinetic results inferred from the stereospecificity observed in the coupling products of the two radicals, suggest that the singlet-triplet splitting produced by the interaction of the two unpaired electrons of the two radicals is very small and that the rate of spin inversion is quite rapid. 

This pattern seems to be fairly common for main group compounds. In Table 2, results with the same range of functionals are shown for the singlet-triplet splitting in methyl-silylene [24]. In this system, the unpaired electrons reside on a different atom (Si versus C), and the singlet is now the... 

The magnetic gap in Fig. 3, for example, becomes the singlet-triplet splitting associated with breaking the a-bond. The epr intensity I(T), or the static susceptibility yield AEp = 0.27 + 0.01 ev in the range 110 < T < 370 K. The usual occurrence of g 2 doublets in Fig. 7, which probably involve misfits in dimerized chains, does not complicate the TSE intensity in the resolved fine-structure lines. The reason is that magnetic dipole-dipole interactions of the two unpaired electrons... 

Keywords Singlet-triplet splitting MR-AQCC Unpaired electron density Natural orbitals... 

The exchange interaction J reflects the interaction between the unpaired electron spins of the RP which gives rise to the energy splitting between i(P+H") and (P+H") RP states. An estimate of this interaction resting on second order perturbation theory can be essentially based on the contributions of the states P and P [5] leading to specific shifts of the singlet and triplet RP states ... 

The size of the splitting between the open-shell singlet and the triplet state of a diradical will be small to the extent that, even in the triplet, the unpaired electrons occupy MOs that are disjoint. As discussed in Section 2.2, diradicals in which the MOs used by the triplet are disjoint will have an open-shell singlet state that has nearly the same energy as the triplet and electron correlation effects can and usually do make the... 

---