THE O-CENTER-DOT-CENTER-DOT-CENTER-DOT-H-O HYDROGEN-BOND IN THE GAS-PHASE - MICROWAVE STRUCTURE OF ETHYLENE-OXIDE WATER

The free jet millimeter wave spectra of five isotopomers of the 1:1 co mplex between ethylene oxide and water have been assigned. The water m olecule lies in the plane of symmetry of ethylene oxide perpendicular to the ring; the water hydrogen involved in the hydrogen bond points t oward the ring oxygen, while the ''free'' hydrogen is entgegen to the ring. The H bond parameters are r(O-ring... H> = 1.92 Angstrom. angle( O-ring... H-O) congruent to 163 degrees, and angle(ring-O-ring... H) c ongruent to 103 degrees, respectively. The large differences of the tw o angles with respect to those of 1,4-dioxane-water and tetrahydropyra n-water are mainly due to dipole-dipole interaction energy effects. Th e MP2 method using a 6-311++G(d,p) is shown to be a reliable approach for this kind of study. An experimental measure of the difference in z ero-point energy between the O ... D-O-H and the O ... H-O-D species i s given by the intensity ratio 3/1 of corresponding lines when a sampl e of water 50% enriched in D is used.