Microwave Spectrum, Dipole Moment, and Ring-Puckering Vibration of β-Propiolactone

The microwave spectrum of β-propiolactone has been observed in the vibrational ground state and in the first four excited states of the ring-puckering vibrational mode. The variation of the rotational constants with vibrational quantum number and the relative intensities of the observed vibrational satellites indicate that the vibration is essentially harmonic with ν0=160 cm−1 and with no appreciable barrier at the planar configuration. The dipole moment is 4.17±0.03 D, with μa=3.67±0.03 D, μb=1.99±0.02 D, and μc=0 in both the ground and first excited states.