Spectroscopy and photochemistry of triplet methylpentadiynylidene (Me-C[triple bond]C-:C-C[triple bond]C-H).
Triplet carbene methylpentadiynylidene, MeC(5)H (1), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (lambda > 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[triple bond]C-H bending modes. Chemical trapping of triplet 1 in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at lambda > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).
Thomas
Phillip S
PS
Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, USA.
Bowling
Nathan P
NP
McMahon
Robert J
RJ