15min:
MILLIMETER-WAVE SPECTROSCOPY OF THE VINYL RADICAL GENERATED BY UV LASER PHOTOLYSIS IN A PULSED SUPERSONIC JET EXPANSION: DETERMINATION OF THE PROTON TUNNELING SPLITTING.

MASAAKI TOSHIMITSU, KENSUKE HARADA, KEIICHI TANAKA AND TAKEHIKO TANAKA, Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812-8581, Japan.

The rotational and proton tunneling spectra of the vinyl radical have been observed by millimeter-wave spectroscopy combined with a pulsed supersonic jet technique. The vinyl radical was generated by the 193 nm excimer laser photolysis of vinyl bromide. The pure rotational transitions, N_KaKc=101-000, 202-101, 303-202, and 404-303, observed in the frequency region of 60-250 GHz, consisted of two components, 0+ and 0-, confirming the proton tunneling motion of the vinyl radical. The observed rotational lines were split into fine and hyperfine components due to the spin-rotation interaction and the spin-nuclear spin interaction of the acetylenic (CH) as well as methylenic (CH2) protons. The pure rotational spectra for Ka=1 were also observed both for the 0+ and 0- components. The rotatinal constants, spin-rotation coupling constants, and hyperfine coupling constants for each tunneling components obtained were consistent with the results of the ESR spectroscopy in the Ar matrix and the infrared diode laser spectroscopy. The b -type Q -branch lines for the 0+ leftarrow 0- proton tunneling transition were observed around 200 GHz. The proton tunneling splitting was determined to be about 0.54 cm-1, and the barrier height of the double minimum potential to be about 1250 cm-1 .