Microwave spectra have been observed for N 2 -SO 3 , OC-SO 3 , Ar-SO 3 -CO, and several of their isotopic derivatives. All three complexes are symmetric tops with little or no out-of-plane distortion of the SO 3 . In Ar-SO 3 -CO, the Ar and CO lie on opposite sides of the sulfur, an arrangement which has not been previously observed for trimers involving SO 3 . In N 2 -SO 3 and OC-SO 3 , the N-S and C-S bond lengths are 2.934(12) and 2.854(12), respectively. In Ar-SO 3 -CO, the C-S distance is 2.849(4)
A A, which is identical, within the estimated uncertainties, to that in OC-SO 3 . The Ar-S distance, on the other hand, is 3.411(11)
A A, which represents a small but distinct lengthening of 0.061(12)
A A relative to that previously determined for Ar-SO 3 . Stark effect measurements for OC-SO 3 and Ar-SO 3 -CO give dipole moments of 0.8488(13) and 0.602(15) D, respectively. The latter is very nearly equal to the difference between the dipole moments of OC-SO 3 and Ar-SO 3 , suggesting that the dipole moment of the trimer is simply the vector sum of the unperturbed dimer dipole moments. Counterpoise corrected optimized geometries and binding energies have been computed at the MP2/aug-cc-pVTZ level for Ar-SO 3 (1.08 kcal/mol), N 2 -SO 3 (2.60 kcal/mol), OC-SO 3 (3.92 kcal/mol), and Ar-SO 3 -CO (4.90 kcal/mol). The binding energy of Ar-SO 3 -CO is nearly equal to the sum of the Ar-SO 3 and OC-SO 3 binding energies, indicating that the two-body interactions on opposite sides of the SO 3 plane are not strongly coupled. Taken together, the experimental and theoretical results indicate that Ar-SO 3 -CO is best regarded as a composite of Ar-SO 3 and OC-SO 3 moieties.