Triplet carbenes geometry

However, be aware that the ratio of E/D is not always a reliable guide to the structure of triplet carbenes. For example, diphenylcarbene and fluorenylidene have very different geometries but almost exactly the same D and E values.If one really wants to learn about the geometry, one must label the carbene carbon with and measure the hyperfine coupling constants. 

Like UV-vis spectra, IR spectra do not give concrete evidence for the multiplicity of carbene in question and, hence, comparison with EPR spectra is highly desired. However, IR spectroscopic data encode a lot of structural information and can be analyzed with the help of computational methods, thus aiding in the identification of the observed species. Since geometries of the singlet and triplet carbenes are usually different, their IR spectra are expected to be different. One can often assign a multiplicity of the carbene if one can calculate theoretical... 

Among the many triplet diarylcarbenes known, triplet di(9-anthryl)carbene (54) is unique as it shows the smallest Z) (0.113 cm i) and E (0.0011 cm ) values ever reported. [66] This means that the carbene has almost linear and perpendicular geometry with extensive delocalization of the unpaired electrons into the anthryl portions of the molecule. [67] The extensive delocalization is expected to stabilize this carbene thermodynamically, while the perpendicular geometry of the anthryl group stabilize the carbene center kinetically, through shielding with the four pm-hydrogens. That is, the electronic factor and the molecular structure of the molecule seem ideal for the formation of a stable triplet carbene. 

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