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

Triplet carbene methylpentadiynylidene, MeC₅H (1), 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 (2) or 2-diazo-hexa-3,5-diyne (3), generates triplet carbene 1. EPR spectra yield zero-field splitting parameters (|D/hc| = 0.62 cm⁻¹, |E/hc| < 0.0006 cm⁻¹), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet 1 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 1 in an O₂-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at λ > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).