Triplet vinylidene

Worthington, S. E., Cramer, C. J., 1997, Density Functional Calculations of the Influence of Substitution on Singlet-Triplet Gaps in Carbenes and Vinylidenes , J. Phys. Org. Chem., 10, 755. 

Conrad, M.P. Schaefer, H.F., III. Absence of an energetically viable pathway for triplet 1,2 hydrogen shifts. A theoretical study of the vinylidene-acetylene isomerization. J. Am. Chem. Soc. 1978, 100, 7820-7823. 

Table 2. Singlet-triplet splittings AEg. and electron affinities EA of the parent and the fluorinated carbenes and vinylidenes.

It is interesting to compare the influence of the fluorine substituents on the singlet triplet splitting AEgj and the ionization potential EA in the vinylidenes R2C=C with that in the carbenes R2C (Table 2). [29,30] The a accepting and... 

The high electrophilicity of lb is also demonstrated by the barrierless insertion into H2, T>2, CH4, and CD4 to produce the difluoroolefins 11 and 12, respec-tively. A rapid insertion into H-H and C-H bonds is only expected for highly electrophilic singlet carbenes. [14] Recently Zuev and Sheridan reported that triplet carbenes and open-shell singlet carbenes also insert into H2 in low temperature matrices, but not into D2. [42] Since calculations predict an activation barrier of several kcal/mol for these reactions, a tunneling mechanism was proposed. Singlet closed-shell carbenes were not observed to insert into H2 under similar conditions. This clearly demonstrates the exceptional reactivity of viny-lidene lb, which readily inserts into both H2 and D2. This is backed by calculations at the MP2 or DFT level of theory which predict for the insertion of lb into H-H and C-H bonds very shallow or no activation barriers, while for the parent vinylidene la substantial barriers are expected. [41]... 

In contrast to the singlet vinylidene lb, the triplet carbene 2b does not form a complex with xenon that exhibits a distinct IR spectrum. The formation of such a weakly bound complex requires a vacant, low lying orbital of a closed-shell singlet carbene as in lb. The lowest closed shell singlet state of 2b is 3.9 kcal mol above the triplet, which is about the expected stabilization energy of the complex. 

A final class of carbene complexes includes vinylidenes (E in Figure 2.14). Vinylidenes possess one mediylene substituent at the carbene carbon instead of two substituents connected by single bonds. Free vinylidene is electrophilic, and the singlet state is calculated to lie below the triplet state by 43.1 kcal/mol. The electrophilicity of free vinylidene is similar to that of a dihalocarbene. It is highly reactive toward rearrangement to the much more stable alkyne tautomer. 

This analysis extends to vinylidene complexes. When free and uncoordinated from the metal center, the singlet state of vinylidene is more stable than the triplet state by 42 kcal/mol. (Of course, either form of the parent vinylidene [rCCH ] is much less stable than its acetylene tautomer. The difference in energy between acetylene and the singlet vinylidene is estimated to be 45 kcal/mol. ) The oxidation state of the metal center in most... 

The 13c NMR of 4a includes a low-field singlet at 346.8 ppm, which is assigned to the carbide unit of the vinylidene fragment. An upfield resonance at 7.5 ppm appears as a triplet in the gated-decoupled l C NMR... 

Ab initio calculations of the potential energy surface of triplet oxygen-assisted isomerization of acetylene to vinylidene suggest that it cannot proceed in a unimolec-ular fashion." ... 

Figure 8 Geometries of singlet (left) and triplet (right) 1-propylidene, cyclopropylidene, cyclobutylidene, cyclopentylidene, cyclopropylcar-bene, cyclopropenylidene, cyclopentadienylidene, and vinylidene at B3LYP/6-311+G ...

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