THE 3540 {\AA} ELECTRONIC TRANSITION OF MALONALDEHYDE

$^{1}$ P.R. Bunker, B. M. Landsberg and B.P. Winnewisser, J. Mol. Spectrose. 74, 9-25 (1979).Author Institution:The 3540{\AA} vibronic band system of malonaldehyde (normal- and $d_{2}$-isotopes) has been partially analyzed and it represents an $n\to\pi^{*} (1^{1}B_{1}\leftrightarrow; 1^{1}A_{1})$ electronic transition with an origin band at 3538.3{\AA} ($28262cm^{-1}$) in the normal isotope. The vibrational structure consists of more than 100 vibronic components. Two principal progressions in $^{1}B_{1}$ state vibrations are found and they are of magnitude $185cm^{-1}$ and $1300cm^{-1}$. The $185cm^{-1}$ vibration apparently corresponds to an in-plane ring deformation and the $1300cm^{-1}$ vibration corresponds to the carbonyl stretching vibration reduced by $360cm^{-1}$ relative to its ground state value. Tunneling effects are prominent in the spectrum and these features may be explained by using a treatment of malonaldehyde vibrations similar to the semirigid bender Hamiltonian model advanced by Bunker, et al.(1) for non-rigid moledules with one large-amplitude vibration. Rotational band contours and Herzberg-Teller intensity found in this band system are also discussed