Zero-field splitting triplet carbenes

Table 4. Zero-field splitting parameters and derived bond angles for triplet carbenes...

EPR Spectroscopy. The structure of triplet carbenes is unequivocally characterized by EPR zero-field splitting (ZFS) parameters, which are analyzed by D and E values. In a simple model, the ZFS parameters D and E for a triplet depend on the distance between electrons with parallel spins as given by... 

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). 

Oxidation of the precursor hydrazones to the corresponding diazo compounds 10 and 12 can be carried out with active Mn02 but not with yellow HgO [8]. When photolyzed in 2-methyltetrahydrofuran (MTHF) glass at 10 K, they show EPR fine structures characteristic of unoriented triplet species. The zero-field splitting (zfs) parameter D/hc of a carbene is governed by the dipolar interaction between the unpaired electrons in the a- and n-orbitals at the carbenic center. The smaller D/hc value of thep isomer 11 than that of the m isomer 13 shows the extended delocalization of the n-spin in the former. The corresponding value diminishes further in the p-substituted 1,3-butadiyne 21 [8]. 

S. Sinnecker, F. Neese. Spin-Spin Contributions to the Zero-Field Splitting Tensor in Organic Triplets, Carbenes and Biradicals — A Density Fimctional and Ab Initio Study. /. Phys. Chem. A, 110 (2006) 12267-12275. 

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