Knowledge Bank

The hindered internal rotation motion in the 3-fluropropene molecular has been investigated through a study of the far infrared spectrum of the gas at ambient temperature using FT-IR spectroscopy. The asymmetric torsion for the cis conformer was observed at $164.6cm-1$ with four accompanying hot bands, and the corresponding torsion of the gauche conformer was observed at $108.4cm-1$ with three additional hot bands. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: $V2=403\pm 27,V3=815\pm 6,V4=42\pm 8$, and $V6=-36\pm 3$ $cm-1$. From these data it has been determined that the cis conformer is the predominant form at ambient temperature and the enthalpy difference between the cis and gauche conformers is $126cm-1$ (360 cal/mol). The calculated cis-gauche barrier is $1156cm-1$ (3.31 kcal/mol) and the gauche-gauche and gauche-cis barriers are $496cm-1$ (1.42 kcal/mol) and $839cm-1$ (2.40 kcal/mol), respectively. These data will be discussed in relation to the corresponding information for several other similar molecules.