15min:

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

The vinyl radical has tunneling motion of acetylenic (CH) proton. The proton tunneling transitions of the vinyl radical, both for the normal (H₂CCH) and deuterated species (H₂CCD), were observed by millimeter wave spectroscopy

% , and the proton tunneling splittings of H₂CCH and H₂CCD in the ground state have been determined 16271.8429(59) MHz and 1164.861(20) MHz, respectively. The HDCCH is a mono-deuteride of the methylenic (CH₂) proton of the vinyl radical, and may have two local potential minima; cis- and trans-forms, where cis-HDCCH has an unpaired electron on the same side of deuterium with respect to the C=C double bond, while trans-HDCCH on the opposite side. If the energy difference _ct between the ground state of trans-HDCCH and that of cis-HDCCH is small enough, the proton tunneling transition will be observed. On the other hand, if _ct is large ( _ct \ge 1 cm^-1), the b-type rotational transitions of K_a=\pm1 will be observed in cis- and trans-tautomers instead of the tunneling transition between the cis- and trans-forms.

In the present study, the rotational spectra of the cis-HDCCH have been observed by millimeter wave spectroscopy combined with a pulsed supersonic jet technique. The HDCCH radical was generated by 193 nm excimer laser photolysis of mono-deuteride vinyl chloride(HDCCHCl). The a-type rotational transitions of the cis-HDCCH, N_K_aK_c =2₀₂-1₀₁, 3₀₃-2₀₂, were observed in the frequency region of 109-164 GHz. The observed rotational lines were split into fine and hyperfine components due to spin-rotation interaction and spin-nuclear spin interaction. The molecular constants such as rotational constants, spin-rotation interaction constant, and hyperfine interaction constants were determined by least squares fitting. These molecular constants are consistent with the results of the millimeter wave spectroscopy of H₂CCH and H₂CCD in gas phase, and result of ESR study in Ar matrix . The rotational spectra of the trans-HDCCH have not been observed in supersonic jet (15 K). This fact suggests that the ground state of trans-HDCCH is much higher than that of cis-HDCCH. The zero point energy difference _ct between cis- and trans-forms, calculated to be 38 cm^-1 from the CCSD(T)/cc-pVTZ calculation, supports the present result by millimeter wave spectroscopy.