Microwave spectrum of benzene⋅SO2: Barrier to internal rotation, structure, and dipole moment

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Abstract
The microwave spectrum of the benzene⋅SO2 complex was observed with a pulsed beam Fourier‐transform microwave spectrometer. The spectrum was characteristic of an asymmetric‐top with a‐ and c‐dipole selection rules. In addition to the rigid‐rotor spectrum, many other transitions were observed. The existence of a rich spectrum arose from torsional–rotation interactions from nearly free internal rotation of benzene about its C 6 axis. Transitions from torsional states up to m=±5 were observed. The principal‐axis method (PAM) internal rotation Hamiltonian with centrifugal distortion was used to assign the spectrum. Assuming six‐fold symmetry for the internal rotation potential, the barrier height was determined as V 6=0.277(2) cm−1. The spectrum of C6D6⋅SO2 was also assigned. Analysis of the moments of inertia indicated that the complex has a stacked structure. The distance R cm separating the centers of mass of benzene and SO2, as well as the tilt angles of the benzene and SO2 planes relative to R cm were determined. The values obtained were R cm=3.485(1) Å, θC6H6 =±12(1)° and θSO2 =44(6)°. While SO2 is certainly tilted with the sulfur end towards benzene, the sign of the benzene tilt angle could not be unambiguously determined. The dipole moment of C6H6⋅SO2 was determined as μ a =1.691(2) D, μ c =1.179(2) D, and μ T =2.061(2) D.