Microwave detection of a key intermediate in the formation of atmospheric sulfuric acid: The structure of H2O-SO3

The microwave spectra of five isotopically substituted derivatives of H[sub 2]O-SO[sub 3] have been observed by pulsed nozzle Fourier transform microwave spectroscopy. The complex, which has long been regarded as an important precursor to H[sub 2]SO[sub 4] in the atmosphere, has a structure in which the oxygen of the water approaches the sulfur of the SO[sub 3] above its plane, reminiscent of a donor-acceptor complex. The intermolecular S-O bond length is long (2.432 [+-] 0.003 A), and the out-of-plane distortion of the SO[sub 3] is small (2-3[degree]). The C[sub 2] axis of the water forms an angle of 103 [+-] 2[degrees] with the intermolecular bond. For an eclipsed configuration, this structure places the protons 2.67 A from the SO[sub 3] oxygens, indicating that a rather long distance must be traversed in order to transfer a proton to form sulfuric acid. The success of these experiments depended critically on the use of a molecular source in which liquid water was evaporated directly into the supersonic expansion. Such a source should be general for liquids of moderate vapor pressure, and its design is described. 29 refs., 2 figs., 4 tabs.