ROTATIONAL SPECTRA AND STRUCTURE OF ETHYLENE-OZONE COMPLEX

Abstract

Microwave spectra of $CH_{2}=CH_{2}\ldots O_{23}, CD_{2}=CH_{2}\ldots O_{3}$ and $cis-CHD=CHD\ldots O_{3}$ were observed with a pulsed beam Fabry-Perot cavity Fourier Transform microwave spectrometer. Internal motions produced two components for each transition of the $CH_{2}=CH_{2}\ldots O_{3}$ and $CD_{2}=CH_{2}\ldots O_{3}$ complexes and only one component for each line of $cis-CHD=CHD\ldots O_{3}$. Nuclear spin statistics observed for $CH_{2}=CH_{2}\ldots O_{3}$ and $CD_{2}=CH_{2}\ldots O_{3}$ indicate the two states arise from internal rotation of ethylene about its C=C axis in the complex. The sets of lines for all three isotopes were independently fit to a Watson Hamiltonian. The rotational constants (in MHz) of $CH_{2}=CH_{2}\ldots O_{3}$ are $A=8246.841(2), B=2518.972(4)$ and $C=2044.248(5)$ for the ground state and $A=8241.897(4), B=2518.941(9)$ and $C=2044.287(11)$ for the excited state. Dipole moment measurements of $CH_{2}=CH_{2}\ldots O_{3}$ determined $\mu_{a}=0.017(1)$ and $\mu_{c}=0.466(2)$ Debye. The dipole and moment of inertia data show that the complex belongs to the $C_{s}$ point group. Least squares fits of the isotopic moment data find the distance between the two centers of mass of $CH_{2}=CH_{2}$ and $O_{3}$ to be 3.279 (27){\AA}. Four forms distinguished by tilts of the two planes containing the $CH_{2}=CH_{2}$ and $O_{3}$ subunits are consistent with the isotopic data. 1.3-dipolar cycloaddition theory and ab initio calculations are used with the structural results to argue that the $CH_{2}=CH_{2}\ldots O_{3}$ complex lies in a small minimum on the reaction coordinate prior to the transition state which produces ethylene primary ozonide $CH_{2}CH_{2}OOO$.