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Mass-selective threshold photoelectron spectroscopy in the gas phase was employed to characterize the dialkynyl triplet carbenes pentadiynylidene (HC 5 H), methylpentadiynylidene (MeC 5 H), and dimethylpentadiynylidene (MeC 5 Me). Diazo compounds were employed as precursors to generate the carbenes by flash pyrolysis. The R 1 -C 5 -R 2 carbon chains were photoionized by vacuum ultraviolet (VUV) synchrotron radiation in photoelectron photoion coincidence (PEPICO) experiments. High-level ab initio computations were carried out to support the interpretation of the experiments. For the unsubstituted pentadiynylidene (R 1 = R 2 = H) the recorded spectrum yields an adiabatic ionization energy (IE ad ) of 8.36 ± 0.03 eV. In addition, a second carbene isomer, 3-(didehydrovinylidene)cyclopropene, with a singlet electronic ground state, was identified in the spectrum based on the IE ad of 8.60 ± 0.03 eV and Franck-Condon simulations. We found that multireference computations are required to reliably calculate the IE ad for this molecule. CASPT2 computations predicted an IE ad = 8.55 eV, while coupled-cluster computations significantly overestimate the IE. The cyclic isomer is most likely formed from another isomer of the precursor present in the sample. Stepwise methyl-substitution of the carbene leads to a reduction of the IE to 7.77 ± 0.04 eV for methylpentadiynylidene and 7.27 ± 0.06 eV for dimethylpentadiynylidene. The photoionization and dissociative photoionization of the precursors is investigated as well.

the journal of physical chemistry a

Pentadiynylidene and Its Methyl-Substituted Derivates: Threshold Photoelectron Spectroscopy of R1-C5-R2 Triplet Carbon Chains