Hydrogen structures triplet carbenes

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. 

In order to determine whether QMT may contribute to the overall reaction of diarylcarbenes with hydrogen atom donors in solution at ambient temperature, kinetic isotope effects for the benzylic hydrogen atom abstractions of the triplet states of several diarylcarbenes with toluene-toluene-i g in fluid solution were determined over the temperature ranges of —75 to 135 °C. The results are very much dependent on the structure of the carbene (Table 9.11)." The differential... 

Considering the simplest carbene, methylene, it is clear that it could have a linear structure (D -symmetry) or be bent ((72s-symmetry). In the former case, using the ls-orbitals of hydrogen and the 2s- and 2 -orbitals of carbon, two a molecular orbitals can be constructed and these can clearly accommodate the C—H bond electrons. A degenerate pair of orbitals, similar to carbon atomic p-orbitals, are the ones of next-lowest energy. The linear structure of methylene would thus correspond to a triplet state. 

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