The microwave spectrum and structure of the H2O–SO2 complex

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Abstract
The microwave spectrum of H2O–SO2 has been observed with a pulsed beam, Fabry–Perot cavity, Fourier‐transform microwave spectrometer. In addition to the normal isotopic form, we have observed the spectra of H2O–34SO2, HDO–SO2, and D2O–SO2. For the normal and dideuterated forms we observe two states with a‐ and c‐type spectra which are split by internal rotation of the water unit, while for the 34 S and HDO species two states are observed but only the a‐type spectrum was assigned. Rotational analysis of each spectrum for H2O–SO2 provides the constants A=8763.071(3) MHz, B=3819.655(2) MHz, and C=2900.899(2) MHz for the I=0 state and A=8734.268(4) MHz, B=3821.281(2) MHz, and C=2900.837(2) MHz for the I=l state, where I is the resultant proton nuclear spin. Stark effect measurements give electric dipole components μa =1.984(2) D and μc =0.488(4) D for H2O–SO2. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units tilted approximately 45° from the parallel orientation with the oxygen atom of the water closest to the S atom of SO2, giving an S–O distance of 2.824(16) Å and a center‐of‐mass distance Rc.m. =2.962(5) Å. The (SO2)2 species was also produced with the same nozzle expansion conditions as used for H2O–SO2. New measurements on (SO2)2 are reported and fitted with the measurements from Nelson, Fraser and Klemperer [J. Chem. Phys. 8 3, 945 (1985)], providing improved rotational and centrifugal distortion constants.