Knowledge Bank

A recent investigation of cyclopropanecarboxaldehyde by electron $diffraction1$ showed that the compound exists in the gas phase as two conformers of approximately equal concentration. Each of the two forms has the aldehyde group in a plane of symmetry, one having the oxygen atom cis and one having it trans to the cyclopropane ring. We have examined the microwave spectrum of cyclopropanecarboxaldehyde and have assigned a- and b-type transitions in the cis species and a-type transitions in the trans species. In each case the rotational constants are very close to those calculated using the structural parameters given by Bartell and $Guillory1$. Stark effect measurements lead to near-zero values of $\mu e$ for the cis species and $\mu b$ for the trans species which would be predicted for conformers with a plane of symmetry. Transitions in excited vibrational states have been observed, and the vibrational satellite pattern will be described.