Relative energies, lone-pair stabilization

Such an enhancement of the fluorescence quantum yield can be explained in terms of the relative locations of the singlet n-n and n-n states. In the absence of cation the lowest excited states has n-n character, which results in an efficient intersystem crossing to the triplet state and consequently a low fluorescence quantum yield. In the presence of cation, which strongly interacts with the lone pair of the carbonyl group, the n-n state is likely to be shifted to higher energy so that the lowest excited state becomes n-n. An outstanding selectivity of Na+ versus K+ was found the ratio of the stability constants is 1300 in a mixture of ethanol and water (60 40 v/v). 

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Kamey and Borden explained that the reason why the N—H BDEs in RNH radicals are smaller than the C—H BDEs of RCH radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.The lone pair of electrons of a nitrene reside in a low-energy sp hybrid orbital. This effect dramatically stabilizes nitrenes relative to carbenes in which the nonbonding electrons reside in either pure p or pseudo sp orbitals. 

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