Knowledge Bank

The infrared spectra of 1,3 butadiene and several derivatives (including fluoroprene, isoprene, and chloroprene) have been investigated in the $250-600cm-1$ region in an effort to establish the low-frequency skeletal modes. The lowest in-plane fundamental in butadiene itself has been located at $303cm-1$; the analogous fundamental in the derivatives falls between 250 and $290cm-1$. The C-C single bond torsional fundamental in fluoroprene is found to be $156cm-1$ from the microwave spectrum, and probably has a similar frequency in the other compounds. The correlation of the other skeletal bending modes will be discussed. The $Au$ fundamental of butadiene at $525cm-1$ shows well-resolved rotational structure which confirms the planar trans conformations of the molecule.