Electronic configuration, triplet carbenes

The states can be approximated as the combination of a triplet carbene with a nitrene in which its ct- and -rr-electrons have opposite spins (Fig. 14). If the latter configuration is taken to correspond to an open-shell singlet nitrene (as... 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Calculations predict that the lowest state of PN has an open-shell electronic configuration." " The Salem-Rowland Rule for ISC promoted by spin-orbit coupling (SOC) predicts that singlet to triplet relaxation will have its maximum rate when the singlet state is closed-shell. This is the case with diaryl carbenes where the absolute rate constants of ISC are in the order of Michl has recently pointed out the importance of donor-... 

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

On the other hand, the cyanomethylene H—C—C=N triplet as well as a number of odd alternant methylene compounds, propargylene and its homologues, H— —C=CR (R = H, CH3, C6H5), and H—C— C=CC=CR [R = CH3, C(CH3)3, C6Hs] have been predicted to be linear on the basis of the zero-field splitting parameters from electron spin resonance (ESR) experiments (15). The linear nature of the first named H—C—C=N triplet carbene has been confirmed by a micro-wave study in the gas phase. However, if the potential well in which the linear configuration lies is shallow, the ground state may not show the effects of nonlinearity (142). 

Methylene is the simplest example of a carbene, a molecule containing a carbon formally bearing only six valence electrons. Of these, four electrons are involved in the C-H bonds. The orbital occupation of the last two electrons defines the specific electronic state of methylene. If we assume a bent structure, we can use the simple model of an sp -hybridized carbon. The four bonding electrons occupy two of these sp hybrids. This leaves the third sp hybrid (3ai) and the p-orbital (l i) available for the last two electrons (see Figure 5.1). Placing one electron in each of these orbitals with their spins aligned creates a triplet state. The electronic configuration of this triplet state is... 

A carbene may exist as a singlet or a triplet, depending on whether the two nonbonded electrons are, respectively, in the same molecular orbital with paired spins, or in two orbitals of identical energy with parallel spins. Depending upon the mode of generation, a carbene may initially be formed in either state, no matter which is the lower in energy. These two electronic configurations should be reflected... 

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