Knowledge Bank

The radio frequency and microwave spectrum of $(SO2)2$ was first reported by Nelson $etal.1$ They showed that this dimer undergoes a low frequency inversion tunneling and determined the center-of-mass distance $(Rcm)$ and the $\mu a$ dipole moment. However, it was not possible to determine the relative orientation of each subunit and the inversion motion was not very well understood. We have reinvestigated the microwave spectrum of $(SO2)2$ using the FTMW technique. New transitions for the normal species arising from the $Ka\le 3$ states and the spectra of 8 isotopically substituted species were measured. The tunneling motion $treatment2$ used for $(H2O)2$, has been applied to $(SO2)2$. The latter undergoes an inversion motion similar to the water dimer. From analysis of the moments of inertia of the various isotopic species, an ac plane of symmetry is established for the dimer and the tilt angles of the $C2$ axis of each subunit relative to the distance joining their centers of mass were determined. From Stark effect measurements. $\mu a$ was remeasured and $\mu c$ shown to be nearly zero. These components are consistent with the structure determined for $(SO2)2$.