The microwave spectrum and structure of the furan · sulfur dioxide complex

Abstract

The microwave spectrum of 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 of the normal isotopic species are A = 3190.7285(11), B = 1119.3596(2), and C = 1015.6841(2) MHz. In addition, the rotational spectra of the C4H4O · 34SO2, C4H4O · S18O2, C4D4O · SO2, and two different C4H4O · SO18O isotopic species were assigned. Stark effect measurements gave dipole moment components of μa = 1.79(1) D, μb = 0.50(2) D, μc = 0.73(1) D, and μtotal = 1.99(2) D. The moment of inertia data show that the two monomers are separated by 3.43(1) Å (Rcm), with the SO2 lying above the furan plane. The two C2 axes of the monomer units are skewed by about 65° 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 β-carbon of the furan. Splittings of the c-dipole transitions for various isotopes suggest a tunneling motion between two equivalent forms of the complex.