Microwave Spectrum of y-Thiobutyrolactone

The microwave spectrum of y-thiobutyrolactone has been observed and measured in the vibrational ground state and in several excited states of the ring-bending and ring-twisting modes. From the value of the /ic component of the dipole moment and inertial-defect considerations the ring skeleton was shown to be non-planar. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state of the ring-bending and ring-twisting modes are ~ 120 c n r 1 and ~ 250 c m 1 above the ground state respectively. These two out-of-plane ring vibrations are essentially independent and the ringbending vibration is governed by a nearly harmonic potential energy function. The components of the dipole moment were determined by the Stark effect to be /ia = 3.770 ± 0.001 D, = 1.818 ± 0 . 0 4 4 D and /ic = 0.832 ± 0 . 0 1 8 D, leading to /«total = 4.268 ± 0.023 D. The rigidity of the ythiobutyrolactone ring is discussed in relation to those of related molecules.