MASATO HAYASHI, KENSUKE HARADA AND KEIICHI TANAKA, Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashiku, Fukuoka 812-8581, Japan.
~~The vinyl-2-d1 radical HDC=CH, mono-deuteride of the methylenic (CH2) proton, may have two forms, cis- and trans-HDCCH; for cis-HDCCH an unpaired electron is located on the same side of deuterium with respect to the C=C double bond, while on the opposite side for trans-HDCCH. If the barrier hight h between the cis- and trans-form is sufficiently low and the zero point energy differece between the two forms tc0 is small enough ( tc0 \ll 1 cm-1), the proton on the acetylenic side (CH) will easily transfer through the barrier by tunneling effect, giving us a chance to observe the tunneling transitions between the two forms (0+ and 0-). If not, the two forms will remain as two toutomers, cis-HDCCH and trans-HDCCH, and will give us a chance to observe the b-type rotational transitions within the cis and trans toutomers instead of the tunneling transitions.
~~In our previous study , we have observed the a-type rotational spectra of cis-HDCCH by millimeter wave spectroscopy in a supersonic jet, while no lines for trans-HDCCH were observed. In the present study, we have searched for either the tunneling transitions or the b-type rotational transitions of HDCCH generated by 193 nm excimer laser photolysis of HDCCHCl. The b-type rotational transitions ( N_KaKc =111-000, 212-101, 110-101 and 211-202) only for one form of HDCCH were observed in the frequency region of 158-258 GHz, but no the tunneling transitions. The rotational and hyperfine interction constants,
%, indicate that the observed toutomer is not trans-HDCCH but cis-HDCCH.
~~Our ab~initio calulation performed in the CCSD(T)/cc-pVQZ level supports the present observation. Althogh the calculated barrier hight between the cis- and trans-form is as low as h = 1770 cm-1w, the zero point energy gap between the two toutomers tc0 = 30.7 cm-1 is large, as the result we have two toutomers, cis-HDCCH and trans-HDCCH, to observe the b-type rotational transitions instead of the tunneling transitions. The zero point energy gap tc0 30.7 cm-1 is large enough to populate only to the vibrational ground state of cis-HDCCH in the ultra low temperature T = 15 K in the supersonic jet giving us no trans-HDCCH spectra.