Ethylene singlet-triplet energy-difference

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

The ethylene molecule is stable with highly occupied orbitals. The triplet state is high and, as may be expected, the UHF calculations performed show the same result the energy does not vary and remains in the singlet state with a zero spin. The total energy U and the reacting bond length Rcc calculated by different methods are listed in Table 1. 

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