Low-versus High-Spin States in Polyenes

7t 7t 2, we increase the coefficients of the Cl and C3 atoms, and decrease the coefficient of the C2 atom, in the up-spin orbitai 7T by orbitai mixing. 

For square cyclobutadiene, the lowest lying singlet state is described by the electron configuration 12.15, in which the degenerate orbitals are singly occupied with opposite spins. The alternative singlet state configurations 12.16 and 12.17, 

In discussing the relative stabiiity of the singiet and triplet states of a diradical system, it is convenient to define their energy difference as follows (for further discussion see Section 24.4.1)  

As mentioned earlier, K 2 is the self-repulsion involving the overlap density and increases with the overlap density (p (p2- When the orbitals f and p2 interact, their energies are split by Ae, which is proportional to the overlap integral S 2 between the orbitals 0 and 02- The antiferromagnetic term Jaf is essentially proportional to —(Ae) (see Section 24.4.) so that 

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