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 C 4H 4O · 34SO 2, C 4H 4O · S 18O 2, C 4D 4O · SO 2, and two different C 4H 4O · SO 18O isotopic species were assigned. Stark effect measurements gave dipole moment components of μ a = 1.79(1) D, μ b = 0.50(2) D, μ c = 0.73(1) D, and μ total = 1.99(2) D. The moment of inertia data show that the two monomers are separated by 3.43(1) Å ( R cm), with the SO 2 lying above the furan plane. The two C 2 axes of the monomer units are skewed by about 65° and the plane of the SO 2 is tipped considerably from parallel to the furan plane, with the sulfur atom closest to the furan. While one oxygen in SO 2 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.