VANDERWAALS COMPLEXES IN 1,3-DIPOLAR CYCLOADDITION REACTIONS - OZONE ETHYLENE

Microwave spectra of O3-CH2=CH2, O3-CD2=CH2, O3-trans-CHD=CHD, and O3-cis-CHD=CHD have been observed with a pulsed-beam Fabry-Perot cavity, Fourier transform microwave spectrometer. Internal motions in the van der Waals complex give two states for the normal, 1,1-dideuterated and trans-1,2-dideuterated isotopic forms. The c-type transitions of the two states for the isotopic species above, as well as the one observed isotopic form of O3-cis-CHD=CHD, independently fit to an asymmetric top Watson Hamiltonian. Stark effect measurements for O3-CH2=CH2 give mu-a = 0.017 (1) D and mu-c = 0.466 (2) D. The microwave data are only consistent with a structure having C(S) symmetry in which the nearly parallel planes of ethylene and ozone have a center of mass separation of R(cm) = 3.290 (3) angstrom. Ab initio calculations at the MP4 level indicate that the preferred geometry corresponds to small tilts of the ozone and ethylene planes, which place an exo-oriented pair of hydrogens toward the terminal oxygens of ozone. Both the theoretical and microwave results suggest the tunneling splitting arises at least in part from a 180-degrees rotation of ethylene about its C2 axis, which is perpendicular to the ethylene plane. 1,3-Dipolar cycloaddition theory, orbital symmetry rules, and ab initio calculations of the complex and transition states are used to argue that O3-CH2=CH2 lies in a shallow minimum on the reaction coordinate prior to the transition state in the reaction of ozone plus ethylene, which produces the primary product 1,2,3-trioxolane.