The almost bottleable triplet carbene: 2,6-dibromo-4-tert-butyl-2 ',6 '-bis(trifluoromethyl)-4 '-isopropyldiphenylcarbene

Computations on 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isop ropyldiphenylcarbene ( 1) using ab initio and density functional theory met hods underscore the unusual stability of the tripler over the singlet state . At the B3LYP/6-311G(d,p) level. the triplet state had a slightly bent cen tral C-C-C bond angle of 167 degrees, whereas this angle in the singlet was 134 degrees. The B3LYP singlet-triplet splitting (12.2 kcal/mol) was large r than that of the parent molecule (5.8 kcal/mol), diphenylcarbene (2), whi ch also has a triplet ground state. The energy of a suitable isodesmic reac tion showed the triplet and singlet states of 1 to be destabilized, by 6.3 and 12.5 kcal/mol. respectively, due to the combined effects of the CF3, Br , and alkyl substituents. The linear-coplanar form of (3)1, which might fac ilitate dimerization or electrophilic attack ar the more exposed diradical center, was prohibitively (35.9 kcal/mol) higher in energy. Our results con firm Tomioka's conclusion that the triplet diarylcarbene, ortho-substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. Dimerization and other possible reaction pathways are inhibited, not only by the bulky ortho substituents but also by the Fa ra alkyl groups. The increase in stability of the triplet ((3)1) state rela tive to the singlet((1)1) state does not influence the reactivity directly.