PROPARGYLENE

A theoretical investigation has been performed on triplet and singlet propargylene (C3H2, ethynylcarbene) using various levels of ab initio theory (MP2, MP4, QCISD, QCISD(T), and multireference CI). According to the results, and in contradiction to earlier calculations, neither the singlet nor the triplet propargylene is a carbene. Vibrational frequencies of the 1,3-diradicaloid C-2 structure of triplet propargylene are in much better agreement with experiment than those of the C-s carbene structure. The hitherto assumed transition state (C-2 upsilon) of the singlet propargylene automerization is a minimum, and the singlet (like the triplet) carbene structure (C-s) is not a stationary point on the energy hypersurface at the QCISD(T)/6-31G* level. The singlet-triplet gap is much larger than previously assumed, and the rotational lines of triplet propargylene are shifted to lower frequencies compared to earlier;calculation.