The microwave spectrum of beta-butyrolactone was observed in the ground state and first three excited states of the ring puckering mode of vibration. Rotational constants were obtained for the ground state (A₀ = 9275.1 ± 14.1, B₀ = 2698.32 ± 0.11, and C₀ = 2379.94 ± 0.10) and each of the excited states. The potential energy function for the ring puckering mode has no barrier at the planar configuration, as indicated by the linear variation of B and C with the vibrational quantum number for the ring puckering mode. The Stark effects were quantitatively measured and the dipole moment for beta-butyrolactone was found to be μₐ= 3.90 ± 0.07 D, μb = 2.07 ± 0.05 D, and μc = 0.2 ± 0.3 D (assumed), which results in a total dipole moment of 4.46 ± 0.09 D. This result is slightly higher than the total dipole moment for beta propiolactone which has a value of 4.17 ± 0.03 D (16).
