DETERMINATION OF THE PROTON TUNNELING SPLITTING OF MALONALDEHYDE IN THE GROUND STATE BY MILLIMETER-WAVE SPECTROSCOPY

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 $(O^{-})$ 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 $millimeterwave^{a}$ and $TuFIR^{b}$ spectroscopy. The proton tunneling splitting in the ground state $\\Delta_{0} = 647046.208 \\pm 0.019 MHz$, and the tunneling-rotation interaction constant $F = 45.8965 \\pm 0.0082 MHz$, 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., 106,2265 (1984). $^{b}$ D.W. Firth, K. Beyer, M.A. Dvorak, S.W. Reeve, A. Grushow, and K.R. Leopold, J. Chem. Phys., 94.1812 (1991).