MITSUHIKO OHTSUKI, MASATO HAYASHI, KENSUKE HARADA, KEIICHI TANAKA, Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka 812-8581, Japan.

The tunneling-rotation transitions of the vinyl-d3(D2CCD) radical produced by the 193 nm excimer laser photolysis of vinyl-d3 chloride(D2CCDCl) have been observed by millimeter-wave spectroscopy combined with a pulsed supersonic jet technique. The b-type transitions, NK_aK_c=111- 000, 110-101, 212-101, for both the 0- leftarrow 0+ and 0+ leftarrow 0- tunneling subbands were observed in the frequency region of 101-183 GHz, split into fine and hyperfine components due to the spin-rotation interaction and the spin-spin interaction of the alpha(CD)- and the beta(CD2)-deuterons. The molecular constants such as rotational constants, spin-rotation interaction constants, and hyperfine interaction constants were determined by a simultaneous analysis of the observed spectrum and previously reported J=1 leftarrow0 pure rotational transition together with the tunneling splitting Delta E0 = 771.843(23) MHz between the 0+ and 0- states.

The tunneling splitting for D2CCD is less than 1/20 of that for H2CCH (16 271.842 9(59) MHz) due to the mass effect of the alpha-H and D, and it is about 2/3 of H2CCD (1 186.820(26) MHz) indicating the mixing of the vibrational modes for alpha-H/D and beta-H2/D2. From the observed tunneling splitting, the barrier height h of the double minimum potential for D2CCD was estimated to be 1549 cm-1 using one dimensional model. The barrier height h for D2CCD is almost the same as those for H2CCH and H2CCD, 1580 and 1520 cm-1, respectively, as expected by B.O. approximation and the isotopic effect due to zero point energies. The off-diagonal Fermi interaction constant, delta aF, which is responsible to the mixing of the wavefunctions of ortho( I_ beta = 0, 2) and para( I_ beta = 1) states, has been determined to be 19.8(30) MHz. The off-diagonal Fermi interaction may cause the nuclear spin conversion between the ortho- and para-states for D2CCD.