MICROWAVE-SPECTRUM OF TOLUENE SO2 - STRUCTURE, BARRIER TO INTERNAL-ROTATION, AND DIPOLE-MOMENT

The microwave spectrum of toluene .SO2 was observed with a pulsed beam Fourier-transform microwave spectrometer. The spectrum displays a-, b-, and c-dipole transitions. The transitions occur as doublets arising from the internal rotation of the methyl group. The transitions were assigned using the principal-axis method (PAM) internal rotation Hamiltonian with centrifugal distortions. Assuming a threefold symmetry for the internal rotation potential, the barrier height was determined as V3 = 83.236(2) cm-1. The torsional-rotational spectra of toluene-CD3.SO2 and toluene-d8.SO2 were also assigned. Additional small splittings of the c-dipole transitions for the normal species and toluene-CD3.SO2 suggest a reorientation tunneling motion Of SO2 with respect to the aromatic plane. The moment of inertia data show that the two monomer units are separated by R(cm) = 3.370 (1) angstrom, with the SO2 located above the aromatic ring. The projection of the C2 axis of SO2 on the aromatic plane makes an angle of tau = 47.0(1)-degrees with the C3 axis of toluene. The dipole moment of the complex is mu(T) = 1.869 (27) D.