Knowledge Bank

This paper deals with torsions around the single bond in molecules containing the group $=C-C=$. Butadiene $(CH2=CH-CH=CH2)$ and glyoxal (OCH-CHO) are prototypes. Benzaldehyde is included as an example of the system [FIGURE]-C=. Torsional frequencies have been observed between 40 and $170cm-1$ for a dozen such compounds in the vapor state. The evaluation of a potential function from the infrared frequency is difficult, if not impossible. If it is written as the usual series [FIGURE] at least two of the three parameters $V1,V2$, and $V3$ are important for the butadiene and glyoxal derivatives. Since there is only one experimental datum, the dilemma is obvious. Nevertheless some interesting comparisons can be made between compounds. For benzaldehyde, $V1$ and $V2$ are zero by symmetry and $V2$ can be evaluated. The barrier for benzaldehyde is very different in the gas and the liquid states.