Sigma orbitals rearrangement

The frontier orbital method is capable of extension to include for all peri-cyclic reactions. The analysis of a few representative allowed per icy die processes is given in Fig. 4.3. Note that the transition state of the [3,3] sigma-tropic rearrangement is likened to the intimate interaction of two ally radicals the strongest bonding interactions will be between the non-bonding (SOMO) levels of these species. In many cases there are alternative HOMO-LUMO interactions, but these have not been illustrated in the figure. 

For examples of the use of these terms see CYCLOADDITION and SIGMATROPIC REARRANGEMENT. See also ANTI PI (jl) ORBITAL SIGMA (o) ORBITAL. 

Although both of these diradical extremes have the same dominant electronic configuration as the aromatic TS [4], a proper wave function for each of these diradical extremes requires a different second configuration. Therefore, at least a three-configuration wave function is demanded for an unbiased study of the Cope rearrangement. Even better is a (6/6)CASSCF wave function in which three virtual orbitals are used to correlate the six electrons in the three bonding orbitals (two pi and one sigma) that are active in this reaction. 

An explanation for the orbital symmetry-forbidden stereochemistry of the [1,51-sigma-tropic shift of substituted norcaradienes has been reported, and ab initio methods have been used to study the [l,5]-hydrogen shift in cycloheptatriene and the [1,51-carbon shift in norcaradiene. A degenerate rearrangement consisting of a formal... 

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