Spectroscopy and Photochemistry of Triplet 1,3-Dimethylpropynylidene (MeC₃Me)

Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11) affords triplet 1,3-dimethylpropynylidene (MeC₃Me, ³3), which was characterized spectroscopically in cryogenic matrices. The infrared, electronic absorption, and electron paramagnetic resonance spectra of MeC₃Me (³3) are compared with those of the parent system (HC₃H) to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Quantum chemical calculations (CCSD(T)/ANO1) predict an unsymmetrical equilibrium structure for triplet MeC₃Me (³3), but they also reveal a very shallow potential energy surface. The experimental IR spectrum of triplet MeC₃Me (³3) is best interpreted in terms of a quasilinear, axially symmetric structure. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (|D/hc| = 0.63 cm^-1, |E/hc| = β 0 cm^-1), while theoretical analysis suggests that the methyl substituents confer significant spin polarization to the carbon chain. Upon irradiation into the near-UV electronic absorption (λ_max 350 nm), MeC₃Me (³3) undergoes 1,2-hydrogen migration to yield pent-1-en-3-yne (4), a photochemical reaction that is typical of carbenes bearing a methyl substituent. This facile process apparently precludes photoisomerization to other interesting C₅H₆ isomers, in contrast to the rich photochemistry of the parent C₃H₂ system.