Spectroscopy and Photochemistry of Triplet Methylpentadiynylidene (Me−CC−C̈−CC−H)

Triplet carbene methylpentadiynylidene, MeC<sub>5</sub>H (<b>1</b>), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (λ > 497 nm) of the isomeric diazo compounds, 1-diazo-hexa-2,4-diyne (<b>2</b>) or 2-diazo-hexa-3,5-diyne (<b>3</b>), generates triplet carbene <b>1</b>. EPR spectra yield zero-field splitting parameters (|<i>D</i>/<i>hc</i>| = 0.62 cm<sup>−1</sup>, |<i>E</i>/<i>hc</i>| < 0.0006 cm<sup>−1</sup>), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet <b>1</b> is characterized by a weak absorption in the near-UV and visible region (350−430 nm) with vibronic progressions corresponding to excitations of the acetylenic stretching and the terminal CC−H bending modes. Chemical trapping of triplet <b>1</b> in an O<sub>2</sub>-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at λ > 399 nm, triplet <b>1</b> undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (<b>6</b>).