Vibrational Modes of the Vinyl and Deuterated Vinyl Radicals

Following the initial report of the detection of fundamental transitions of all nine vibrational modes of the vinyl radical [ Letendre L. ; Liu D.-K. ; Pibel C. D. ; Halpern J. B. ; Dai H.-L. J. Chem. Phys. 2000, 112, 9209 ] by time-resolved IR emission spectroscopy, we have re-examined the assignments of the vibrational modes through isotope substitution. Precursor molecules vinyl chloride-d 3, vinyl bromide-d 3, and 1,3-butadiene-d 6 are used for generating vibrationally excited vinyl-d 3 through 193 nm photolysis. The nondeuterated versions of these molecules along with vinyl iodide and methyl vinyl ketone are used as precursors for the production of vinyl-h 3. IR emission following the 193 nm photolysis laser pulse is recorded with nanosecond time and ∼8 cm−1 frequency resolution. A room-temperature acetylene gas cell is used as a filter to remove the fundamental transitions of acetylene, a photolysis product, in order to reduce the complexity of the emission spectra. Two-dimensional cross-spectra correlation analysis is used to identify the emission bands from the same emitting species and improve the S/N of the emission spectra. Isotope substitution allows the identification of several low-frequency vibrational modes. For C2H3, the assigned modes are the ν4 (CC stretch) at 1595, ν5 (CH2 symmetric bend) at 1401, ν6 (CH2 asymmetric + α-CH bend) at 1074, ν8 (CH2 + α-CH symmetric out-of-plane (oop) bend) at 944, and ν9 (CH2 + α-CH asymmetric oop bend) at 897 cm−1. For C2D3, the modes are the ν5 (CD2 symmetric bend) at 1060, ν6 (CD2 asymmetric + α-CD bend) at 820, and ν8 (CD2 + α-CD symmetric oop bend) at 728 cm−1.