15min:

T. BABA AND T. TANAKA, Department of Chemistry, Kyushu University, Fukuoka, 812-8581 Japan; I. MORINO AND K. M. T. YAMADA, National Institute for Advanced Interdisciplinary Research (NAIR), Tsukuba, 305-8562 Japan; KEIICHI TANAKA, Institute for Molecular Science (IMS), Okazaki, 444-8585 Japan.

Due to the proton tunneling motion, the ground state of malonaldehyde is split into a doublet. The tunneling-rotation transitions connecting the lower(0⁺) and upper(0^-) components of the tunneling doublet were observed by submillimeter-wave spectroscopy employing BWO tubes. So far, more than two hundred Q- and R-branch tunneling-rotation transitions were identified in the frequency region of 642-745 GHz together with about fifty pure rotational lines for both the 0⁺ and 0^- sublevels.

The present submillimeter-wave data were analyzed together with the reported pure rotational lines by the millimeter-wave^a and TuFIR^b spectroscopy. The proton tunneling splitting in the ground state ₀ = 647046.208 \pm 0.019 MHz, and the tunneling-rotation interaction consatnt F = 45.8965 \pm 0.0082 MHz, were determined as well as the rotational and centrifugal distortion constants for each tunneling sublevels. >From the line intensities, the a-component of transition moment, responsible >to the tunneling-rotation transitions, turned out to be about one tenth of the b-component of dipole moment (2.58 D^a), responsible to the pure rotational transitions.

^a S. L. Baughcum, Z. Smith, E. B. Wilson, and R. W. Duerst, J. Am. Chem. Soc., \underline106,2265 (1984). ^b D. W. Firth, K. Beyer, M. A. Dvorak, S. W. Reeve, A. Grushow,and K. R. Leopold, J. Chem. Phys., \underline94,1812 (1991).