Singlet-triplet splitting, nitren

The imidogen anion (NH ) can ionize to form either the singlet or triplet nitrene, thus, the difference in the kinetic energies of the photoelectrons leading to NH and NH is just the singlet-triplet splitting of NH. 

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-... 

The singlet-triplet splitting in nitrene (NH) is experimentally found to be 36 kcal mol", four times larger than that in CH2. A C-H bond in CH2 becomes a lone pair of electrons on N in NH. The nitrogen 2s orbital is distributed between this lone pair and the N-H bond, leaving a degenerate pair of pure 2p orbitals partially occupied in NH. Thus, the major difference between CH2 and NH is that the hybridized a MO, which is doubly occupied in CH2, becomes a 2p AO in NH. 

S.V. Chapyshev, Zero-field splitting parameters of triplet nitreno-s-triazines a new insight into the geometry of the nitrene centres of triplet and singlet mtisaes, Mendeleev Commun., 2002,12,227-229. 

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