Knowledge Bank

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 $millimeterwavea$ and $TuFIRb$ spectroscopy. The proton tunneling splitting in the ground state $\Delta 0=647046.208\pm 0.019MHz$, and the tunneling-rotation interaction constant $F=45.8965\pm 0.0082MHz$, 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 $(2.58Da)$, responsible to the pure rotational transitions. S.L. Baughcum, Z. Smith, E.B. Wilson, and R.W. Duerst, J. Am. Chem. Soc., 106,2265 (1984). D.W. Firth, K. Beyer, M.A. Dvorak, S.W. Reeve, A. Grushow, and K.R. Leopold, J. Chem. Phys., 94.1812 (1991).