Bond-breaking reactions, orbital

Electronic factors related to orbital overlap also appear to interfere significantly in the dynamics of concerted electron transfer/bond breaking reactions in donor-spacer-cleaving acceptor systems.94... 

Figure 6.6. Orbital directionality of C-H (left) and C-C (right) bond-breaking reactions.

If you specify a multiplicity of one (singlet), then you would most often choose the RHFmethod, unless the reactions result in bond breaking (see page 46). If the selected multiplicity is greater than one, then the system is open-shell and the usual choice is the UHF method, which uses different orbitals for electrons with different spins. 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

The difference in stereochemical outcome of these reactions is determined, therefore, by the relative phase of the lobes—at the terminal carbon atoms—of the MOs of these (and other similar) mre systems by the symmetry of their orbitals, that is. As we have seen, the orbital lobes, at the two terminal carbon atoms, have the same phase in the HOMO (tf 3) of the triene (67re), and in the HOMO after irradiation (i/i3) of the diene (4ire) while these orbital lobes have opposite phases in the HOMO (i/r2) of the diene and in the HOMO after irradiation (ifi4) of the triene. Two such terminal lobes with the same phase require disrotatory movement before bond-making/bond-breaking can occur, while two terminal lobes with... 

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