Internal rotation and interconversion motions in weakly bound complexes of sulfur dioxide.

Abstract

The microwave spectra of toluene$\cdot$SO$\sb2$, benzene$\cdot$SO$\sb2$ and (SO$\sb2)\sb2$ have been investigated using a pulsed molecular beam Fourier-transform microwave spectrometer. Evidence of internal motions was observed in the spectra of these three complexes and their isotopic species. In toluene$\cdot$SO$\sb2$, the internal motion is associated with an internal rotation of the methyl group about its C$\sb3$ axis. In benzene$\cdot$SO$\sb2$, the internal motion is identified as an internal rotation of benzene about its C$\sb6$ axis. In (SO$\sb2)\sb2$, the two monomers undergo a high barrier interconversion tunneling motion analogous to that displayed by (H$\sb2$O)$\sb2$. In the case of toluene$\cdot$SO$\sb2$ and benzene$\cdot$SO$\sb2$ the principal-axis method (PAM) internal rotation Hamiltonian was applied to the analysis of their spectra. The barriers to internal rotation are V$\sb3$ = 84(1) cm$\sp{-1}$ and V$\sb6$ = 0.28(1) cm$\sp{-1}$ for toluene$\cdot$SO$\sb2$ and benzene$\cdot$SO$\sb2$, respectively. An identical Hamiltonian has also been utilized to estimate the internal rotation barrier for ethylene$\cdot$SO$\sb2$. The internal rotation corresponds to a rotation of ethylene in its molecular plane and the barrier is V$\sb2$ = 30(2) cm$\sp{-1}$. The structures of toluene$\cdot$SO$\sb2$, benzene$\cdot$SO$\sb2$ and (SO$\sb2)\sb2$ were also determined from analyses of moments of inertia of their respective isotopic species. In toluene$\cdot$SO$\sb2$ and benzene$\cdot$SO$\sb2$, the SO$\sb2$ moiety sits above the aromatic ring. In (SO$\sb2)\sb2$, an ac-plane of symmetry was established where one monomer lies in the plane of symmetry while the oxygens of the second monomer straddle it. The electric dipole moments of these three complexes were also derived from Stark effect measurements. Electrostatic calculations using distributed multipoles were carried out to rationalize the internal rotation barriers, the binding energies and the structures of the three complexes. The van der Waals stretching force constants were also estimated for each complex based on the pseudo-diatomic model.