Spectroscopic Identification of the Resonance-Stabilized cis- and frans-1-Vinylpropargyl Radicals

摘要

The cis-1-vinylpropargyl (cis-1 VPR, cis-pent-4-en-1-yn-3-yl) and frans-1-vinylpropargyl (trans-1VPR, frans-pent-4-en-1 -yn-3-yl) radicals, produced in a supersonically cooled hydrocarbon discharge, have been identified by a synergy of 2-dimensional fluorescence and ionization spectroscopies, revealing their electronic origin transitions at 21 232 and 21 645 cm~(-1) respectively. These assignments are supported by an excellent agreement between calculated ground state frequencies of cis-1 VPR and frans-1 VPR with those obtained by dispersed fluorescence spectroscopy. In addition, high-resolution rotational contours of the two bands are well simulated using calculated X- and A-state frans-1VPR and cis-1VPR rotational constants. Finally, computed origin transition energies of these two isomers are within several hundred wavenumbers of the observed band positions. With the 1-phenylpropargyl radical, the 1VPR isomers are the second 1-substituted propargyl species to have been observed abundantly from a hydrocarbon discharge, while no 3-substituted analogue has been positively identified. This is likely due to the greater resonance stabilization energy of the 1 -substituted species, arising from concerted delocalization of the unpaired electron over the vinyl and propargyl moieties.