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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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The excitation profiles for the ν1 and ν2 vibrations of trans- β-carotene have been studied by means of coherent anti-Stokes Raman spectroscopy (CARS). The experimental profiles are compared to those calculated using the Albrecht theory modified for CARS. In order to obtain meaningful results it was necessary to drastically lower the laser power, specifically the power of ω1, the beam in resonance with the electronic absorption. If the laser power was too high, the intensity of the β-carotene lines did not scale with the square of the power in the pump beam due to saturation effects and, in fact, if the power was high enough the CARS signal was lost in the background emission. Due to the necessity of using low powers, photomultiplier detection was employed. The methods developed made it possible to detect β-carotene in benzene solution at concentrations of ∼5×10−7M, which is considerably lower in concentration than previously observed by CARS.

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