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The microwave spectrum of the title compound has been determined in the gas phase at room temperature, and the Raman spectrum has been determined on the liquid at room temperature and on the solid at 73 K. From the experimental data it is concluded that there are two conformers separated in energy by approximately 0.6 kcal/mol, and only the more stable one persists in the crystal. The more stable conformer has a very small (<0.1 D) µ<. component of the dipole moment, while this component is much greater for the less stable conformer. The rotational constants for the two conformations have been determined and are substantially different. The use of the molecular mechanics (mm2) program indicates two stable conformers, a half-chair and a boat form. The former is calculated to be 0.54 kcal/mol more stable and to have a value of µ0 about 0.02 D. The value for µ0 predicted for the boat conformer is approximately 1 D. The rotational constants calculated for the two conformations are in agreement with the experimental ones only if the half-chair conformation is the more stable.

The microwave spectrum of bicyclo[3.1.0]hexane has been studied in the range 26.5-40.0 GHz ( band) with a Hewlett-Packard Model 8400 Stark-modulated microwave spectrometer. The rotational constants (MHz) for the ground vibrational state have been determined to be A = 5542.

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