Knowledge Bank

The infrared and Raman spectra of methylenecyclopropane have been studied. As an aid in the assignment of the vibrational frequencies an analysis of the normal modes (exclusive of the CH stretching modes) has been carried out, using force constants transferred from ethylene, isobutene, and spiropentane. In this highly strained molecule a double bond is attached directly to the 3-carbon ring. The strong infrared bands at $890cm-1$ and $1032cm-1$ characteristic of the methylene $CH2$ group and the cyclopropane ring, respectively, are observed. However, there is no significant, infrared absorption or Raman displacement in the region of the normal double-bond stretching frequency ($1660cm-1$ for asymmetrically disubstituted ethylenes). Both the normal coordinate calculations and the Raman spectra indicate that that in methylene cyclopropane the carbon double-bond stretching frequency lies in the neighbourhood of $1800cm-1$. The abnormally high value of this frequency is a result of kinetic interaction between the double bond and symmetric ring stretching modes rather than an increase in the force constant for the double bond.