Matrix-isolation and ab initio molecular orbital study of 2,2,2-trifluoroethylidene

Article abstract of DOI:10.1021/ja00105a049

Photolysis of 2,2,2-trifluorodiazoethane (2) in an argon matrix at 12 K generates triplet 2,2,2-trifluoroethylidene (1) in addition to a significant amount of trifluoroethylene (3) and small amounts of trifluoromethyldiazirine (4). These compounds were identified by IR and UV spectroscopy. Short-wavelength - photolysis of the carbene 1 converts it to trifluoroethylene, while slowly warming the matrix to 35 K results in dimerization to the isomeric hexafluorobut-2-enes. High-level ab initio calculations (QCISD(T)6-311(2D,2P)//MP2-FC/6-31G**) are reported for the singlet and triplet states of 2,2,2-trifluoroethylidene as well as for methylene and ethylidene. The calculated IR spectrum for triplet 2,2,2-trifluoroethylidene is in good agreement with the experimental one, but the UV/vis spectrum calculated using the CIS method does not match very well. The transition structures for the 1,2-fluorine atom rearrangement of the single and triplet states of carbene 1 to trifluoroethene were calculated at the QCISD(T)-FC/6-311(2D,2P)//MP2-FC/6-31G** level of theory. The calculated barrier for 1,2-fluorine atom migration in the singlet carbene, 21.5 kcal/mol, is less than suggested by recent experimental results (29 ± 4 kcal/ mol). The calculated barrier for the corresponding rearrangement in the triplet system was 51 kcal/mol. Previous reports concerning the energies and geometries of these calculated transition structures are shown to be in error.