Weakly bound complexes of sulfur trioxide: The structure of ArSO ₃ and the dipole moment of N₂SO₃

Microwave and radio frequency spectra of ArSO₃ and radio frequency spectra of N₂SO₃ are measured by molecular beam electric resonance spectroscopy. Both complexes are symmetric rotors. The spectroscopic constants determined for Ar³²SO³ are B = 1449.089(5) MHz, D_J = 3.6(6) kHz, and μ = 0.2676(3) D. For Ar ³⁴SO₃, B = 1439.090(5) MHz. The stretching frequency of the weak Ar-S bond is 61.(5) cm^-1, and the force constant is thus 0.059(10) mdyn/Å. The Ar-S bond length in ArSO₃ is 3.350(1) Å. The Ar-S-O angle is found to be 90.5°(5), thereby indicating no significant out-of-plane distortion of the SO₃ moiety. Preliminary assignment of the N₂SO₃ spectrum yields a dipole moment of 0.46(1) D. Electric deflection studies show that (SO₃)₂ and (SO₃)₃ are polar species, while Xe₂SO ₃ is not.