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Carbene radicals, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character. Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates. The "second generation" of the Grubbs catalysts for alkene metathesis features an NHC ligand.
Phenyldiazirine produces the singlet carbene whereas trifluoromethylphenyldiazirine and 3-chloro-3-[(4-nitrophenyl)methyl]diazirine produce triplet state carbenes. Carbenes produced from diazirines are quickly quenched by reaction with water molecules, [ 20 ] and hence yields for photoreactive crosslinking assays are often low.
As opposed to the now ubiquitous NHC (N-heterocyclic carbene) systems based on imidazole rings, these carbenes are structured from triazole rings. 1,2,4-triazol-5-ylidene can be thought of as an analog member of the NHC family, with an extra nitrogen in the ring, while 1,2,3-triazol-5-ylidene is better thought of as a mesoionic carbene (MIC). [1]
The ground state is a triplet radical with two unpaired electrons (X̃ 3 B 1), [10] and the first excited state is a singlet non-radical (ã 1 A 1). With the singlet non-radical only 38 kJ above the ground state, [10] a sample of methylene exists as a mixture of electronic states even at room temperature, giving rise to complex reactions. For ...
Carbynes can be seen as derivatives of the simplest such compound, the methylidyne radical or unsubstituted carbyne H−C· or H−C 3·, in which the functional group is a hydrogen atom. Reported for the first time back in 1967 by Kasatochkin, carbyne is an infinite sp1 hybridized long linear chain of carbon, where each link is just a single ...
Examples include (CO) 5 W=COMePh and (OC) 5 Cr=C(NR 2)Ph. Orbital interaction in a Fisher carbene. The carbene electrons are donated to a sigma bond, and weak pi-backbonding occurs. Fisher carbene complexes are related to the singlet form of carbenes, where both electrons occupy the same sp 2 orbital at the carbon. This lone pair donates to a ...
This effect is evident in the higher percent buried volume (%VBur) of CAACs compared to diamino NHCs at a distance of 0 Å from the carbene. For carbenes bearing a diiopropylphenyl group at the N substituent(s), the %VBur for CAACs (79.0-83.1) is markedly higher than the classical NHC (70.3). [3]