Triplet carbenes defined

The rate constants kTS and kST define an equilibrium constant (ATeq) connecting the singlet and triplet carbenes. An estimate of Ktq, and hence AGSX, for BA can be obtained from the experiments described above. The time resolved spectroscopic measurements indicate that BA reacts with isopropyl alcohol with a rate constant some five times slower than the diffusion limit (Table 7). This, in conjunction with the picosecond timescale measurements, gives a value for ksr. The absence of ether formation from the sensitized irradiation, when combined with the measured rate constant for reaction of 3BA with isopropyl alcohol, gives an upper limit for k-. These values give Keq and thus AGST 2 5.2 kcal mol-1 (Table 8). 

Two different quantities, D which is defined as the energy needed to form the ground state radical and carbene and the "Simonized quantity D in which the triplet carbene is utilized so that the two half-filled a orbitals of the fragments can combine to form the new C-C bond are shown in Table III. While we can offer no explanation for the changes in the bond strengths of the variously substituted radicals upon fluorination, it is nonetheless readily seen that the "Simonized" D values have removed most of the wide swings of the values of D associated with increasing fluorination. 

Carbenes are defined as species containing divalent carbon [1], and they may display either electrophilic or nucleophilic reactivity depending on whether the two unshared electrons on the carbon center are unpaired (triplet carbene) or paired (singlet carbene). Metal-carbene complexes can be classified in a similar way based on their reactivity toward electrophiles and nucleophiles. The resonance forms shown in Fig. 4.1 define the limiting structures, and the formal charge on the carbene carbon indicates the preferred reactivity. Those that are nucleophilic at carbon are called Schrock-type complexes or alkylidenes, and they generally... 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

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 is defined as divalent carbon atom, the latter is linked to two adjacent groups by covalent bonds. The two carbon-centered electrons may have their spins antiparallel (singlet state) or parallel (triplet state) [see 8.1.4 for reviews]. 

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