Through-bond coupling trimethylene

A variation of a, all the other parameters being unchanged, is a pure 1—4 stretch, during which a level crossing similar to that observed in trimethylene is found. Fig. 10 shows the energies of the A and S levels as a function of a, and a through-bond orbital coupling rationale similar to the one for trimethylene. 

Figure 4.25. Sum-over-atoms factor in the spin-orbit coupling vector // a) in orthogonally twisted ethylene and b) in (0, 90°) twisted trimethylene biradical, using Equation (4.12) and (4.13) most localized orbitals x - Xh and nonvanishing atomic vectorial contributions from Xh (white through-space, black through-bond).

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