Singlet-triplet splittings in carbenes

Electronic Excitation Energies and the Singlet/Triplet Splitting in Carbenes... 

In November I submitted a manuscript [19] to the Journal of the American Chemical Society titled Electronic Structure of Carbenes I, CH2, CHF, and CF2 in which I calculated a CH2 angle of 132.5°. I was certain of the CH2 angle and began to study the effect of substituents on the geometry and singlet-triplet splitting in carbenes. 

Both calculations and experiments have shown that substituents can affect the singlet-triplet splitting in carbenes by changing the difference between the energies of the a and n orbitals. Since the Ai state essentially has a pair of electrons localized in the a MO whereas the state has one electron in the a and one in the n MO, increasing the energy difference between these two orbitals should selectively stabilize... 

Carbenes are a fascinating class of chemical species My own interest in this area was first inspired by Shi Shavitt s seminar on methylene in 1979—the material presented was subsequently published as a review article in Tetrahedron [1]. It is a comprehensive summary of the most important experimental and theoretical determinations of the geometry and singlet-triplet splitting in methylene to date, i.e. 1984.1 have read that paper many times. Its clarity is typical of Shavitt s publications, where pioneering research was beautifully presented and discussed. It is an honour to contribute a piece of work on carbenes to this memorial issue that honours the life and scientific contributions of Isaiah Shavitt. 

Singlet-triplet Energy Splittings in Carbenes and Diradicals 229... 

When one estimates the heats of formation of aromatic nitrenes and carbenes from those of NH and CH2 with the aid of group additivity, the nitrenes will automatically become 10—15 kcal/mol more stable than the isomeric carbenes. These estimates are of necessity for the triplet species we do not know the resonance energies in the singlets. Nor do we know the singlet-triplet splittings, which may be different in carbenes and nitrenes. We have, therefore, also performed semi-... 

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