Microwave spectrum, structure, and dipole moment of bicyclo ) 72.1.0{pentane. )]

The microwave spectra of the normal and three singly substituted 13C isotopic species of bicyclo[2.1.0]pentane have been assigned and analyzed. Using the moments of inertia derived from these spectra the following structural parameters have been obtained for the carbon skeleton: C1 – C2=C3 – C4=1.528 Å, C2 – C3=1.565 Å, C1 – C4=1.536 Å, C1 – C5=C4 – C5=1.507 Å, and the dihedral angle α=67.26°. The disagreement of these results with those of an earlier electron diffraction study has been discussed. It seems likely that a reanalysis of the electron diffraction data will lead to concurrence with the results of the present study. We have compared our structural parameters to those of other cyclic systems and have discussed the essential constancy of the average C – C distances in cyclobutyl systems and also in cyclopropyl systems. Finally, Stark effect measurements have yielded a total dipole moment of 0.27 D.