Correspondence diagram for thermolysis of diazomethane

The reader who deplores pictorializations of this kind can be reassured by the fact that the formal requirements of spin-orbit coupling are fulfilled. 

Like other pyrazolines, 4-methylene-1-pyrazoline (MP) undergoes thermal extrusion of dinitrogen to form methylenecyclopropane (MCP) [35], but it does so much more rapidly the energy of activation is about 9 kcal/mol lower than that of the parent 1-pyrazoline, more than enough to offset a hundredfold reduction in the pre-exponential factor [40, 41]. This kinetic behaviour is prima facie evidence that the reaction proceeds stepwise via a triplet intermediate. The obvious choice was trimethylenemethane (TMM), that had been shown to have a triplet ground-state [36, 37, 38], in confirmation of numerous theoretical predictions [39], [18, pp. 141ff.j. 

The much more elaborate correspondence diagram for generation of TMM [14, Fig. 9] is displayed in Fig. 9.6. As in previous examples in which CH bonds occupy different planes in the reactant and product, the occupied acH orbitals are taken into account explicitly. They can be brought into correspondence by either a conrotation or a disrotation, respectively 02 -nd 62 in the axis convention adopted. The symmetry coordinates that have to be incorporated in the two reaction coordinates are illustrated schematically in Fig. 9.7 In one (a2) the extruded N2 molecule rotates about the symmetry axis as it departs and the methylene groups move into plane conrotatorily. In the other (62) if recedes above (or below) the molecular plane as the methylenes disrotate into plane. 

The correspondence diagram does not reject either pathway  

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Figure9.4. Correspondence diagram for thermolysis of Diazomethane (from Fig. 2 of reference [14])...

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