Knowledge Bank

The microwave spectrum of the furan-sulfur dioxide complex has been observed with a pulsed molecular beam Fourier transform microwave spectrometer. Transitions having a-, b- and c- dipole selection rules were all seen. The rotational constants are $A=3190$.7285(11). $B=1119.3596(2)$, and $C=1015.6841(2)MHz$. In addition the rotational spectra of the $C4H4O-34SO2$, $C4H4O18O2$, $C4D4O$, $SO2$, and two different $C4H4O$ $\cdot$ $SO18O$, isotopic species were assigned. The two monomers are separated by 3.43(1) \AA $(Rcm$), with the $SO2$ lying above the furan plane. The two $C2$ axes of the monomer units are skewed by about $65\circ$ and the plane of the $SO2$ is tipped considerably from parallel to the furan plane, with the sulfur atom closest to the furan. While one oxygen in $SO2$ lies approximately above the oxygen of the furan, the other is located closer to a $\beta$-carbon of the furan. Splittings of the c-dipole transitions for various isotopes suggest a tunneling motion between two equivalent forms of the complex.