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 H2O-SO3 have been observed by pulsed nozzle Fourier transform microwav e spectroscopy. The complex, which has long been regarded as an import ant precursor to H2SO4 in the atmosphere, has a structure in which the oxygen of the water approaches the sulfur of the SO3 above its plane, reminiscent of a donor-acceptor complex. The intermolecular S-O bond length is long (2.432 +/- 0.003 Angstrom), and the out-of-plane distor tion of the SO3 is small (2-3 degrees). The C-2 axis of the water form s an angle of 103 +/- 2 degrees with the intermolecular bond. For an e clipsed configuration, this structure places the protons 2.67 Angstrom from the SO3 oxygens, indicating that a rather long distance must be traversed in order to transfer a proton to form sulfuric acid. The suc cess of these experiments depended critically on the use of a molecula r source in which liquid water was evaporated directly into the supers onic expansion. Such a source should be general for liquids of moderat e vapor pressure, and its design is described.