TOWARD A COMPLETE STRUCTURE OF BUTADIENE WITH HIGH-RESOLUTION INFRARED SPECTROSCOPY

Although the structure of butadiene is used in initial discussions of delocalized $\\pi$-bonding in every organic textbook, the structure of this fundamental molecule is incompletely known. Information about the CCCC backbone comes from electron diffraction studies.$^{1}$ Caminati et al have proposed a partial structure from this data, a low-resolution infrared investigation of butadiene,$^{2}$ and their MW investigation of butadiene-1,1-$d_{2}.^{3}$ We have begun a high-resolution $(0.002 cm^{-1})$ infrared investigation of the complete structure of this nonpolar molecule, which is MW-silent. For the normal species, a C-type band due to out-of-plane CH flapping at $908.072 cm^{-1}$ and a perturbed A-type band due to antisymmetric C=C stretching at $1596.446 cm^{-1}$ have been analyzed. The ground state rotational constants are $A = 1.3903839(10), B = 0.1478862(2)$, and $C = 0.1336942(2) cm^{-1}$ for a Watson-type Hamiltonian. For the $2,3-d_{2}$ species, the C-type band at $908.044 cm^{-1}$ has been analyzed to give $A = 1.0203626(5)$, $B = 0.1472767(3)$, and $C = 0.1287171(3) cm^{-1}$ for the ground state rotational constants. Structural implications of these data will be discussed. Currently, we are attempting a stereospecific synthesis of the two $1,4-d_{2}$ isotopomers and have hopes of obtaining the two singly-substituted ${^{13}}C$ isotopomers. (1) Kveseth, K.; Seip, R.; Kohl, D. A. Acta Chem. Scand. A 1980, 34, 31. (2) Cole, A. R. H.; Mohay, G. M.; Osborne, G. A. Spectrochim. Acta 1967, 23A, 909. (3) Caminati, W.; Grassi, G; Bauder, A. Chem. Phys. Letters 1988, 148, 13.