( N 2 O ) 2 ⋅ SO 2 : Rotational spectrum and structure of the first van der Waals trimer containing sulfur dioxide

Abstract

The trimer (N2O)2⋅SO2 has been studied by Fourier transform microwave spectroscopy. The rotational constants for the normal species are A=1369.1014(11) MHz, B=1115.5816(11) MHz, and C=730.5790(4) MHz. Five isotopomers of the complex have been studied, allowing a determination of its structure. The configuration aligns the centers-of-mass of the three monomers in an approximate equilateral triangle. The monomers twist relative to this plane so that the two N2O’s are intermediate between T-shaped and crossed rather than the usual slipped-parallel planar. The SO2 is oriented so that its oxygens are straddling one N2O (similar to the N2O⋅SO2 dimer), while its sulfur is closest to the oxygen end of the other N2O. The dipole moment of (15N2O)2⋅SO2 was measured, giving μa=0.606(1) D, μb=1.256(2) D, μc=0.058(2) D, and μtotal=1.396(2) D. A semiempirical model has been used to predict the trimer structure, and comparison is made with this model as well as with the dimer N2O⋅SO2.