SPIN-ORBIT-COUPLING IN BIRADICALS .1. THE 2-ELECTRONS-IN-2-ORBITALS MODEL REVISITED

The two-electrons-in-two-orbitals active space model of electronic str ucture of biradicals and biradicaloids is extended to a Hamiltonian th at incorporates the usual kinetic and electrostatic energy terms, and also outside electric and magnetic fields, spin-spin dipolar terms, an d one- and two-electron spin-orbit coupling terms. It leads to a more rigorous version of the Salem-Rowland rules for the dependence of T-1- S-0 spin-orbit coupling (SOC) in biradicals and biradicaloids on molec ular structure and conformation. For large T-1-S-0 SOC in a bitopic bi radical, (i) the localized orbitals A and B that are singly occupied i n the T-1 state either interact covalently or one of them is sufficien tly lower in energy to have nearly double occupancy in the S-0 state, (ii) on at least one atom of reasonably high atomic number one p orbit al contributes strongly to A and another to B, and (iii) the atomic co ntributions add constructively rather than destructively. The nature o f this addition is such that inverse heavy atom effects on SOC are pos sible. Through-bond coupling is essential and its effects are apparent from simple resonance structures, illustrated on alpha,omega-alkanedi yl biradicals. Implications for the zero-field splitting parameters of triplet states are noted.