Knowledge Bank

A complete structure has been found for the nonpolar molecule, trans-1,2-dichloroethylene, from the analysis of the rotational structure in the high-resolution ($0.002cm-1$) infrared spectra of four isotopomers at this molecule. In each case the band that was analyzed was the C-type band due to out-of-plane CH or CD flapping. The spectral resolution was sufficient to distinguish series due $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl37Cl$ isotopic species in natural abundance. Rotational constants were found for the $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl2$ species of the normal from and for the $d2\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl2,d2\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl37$ Cl and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C2-\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl2$ species. Since each different atom was isotopically substituted, the set of rotational constants was sufficient to provide a substitution$(rs)$ the structure. The geometric parameters are $rCH=1.078\backslash AA,rCC=1.304\backslash AA,rCC=1.744\backslash AA,\alpha CCH=125.5\circ$ and $\alpha CCCI=119.4\circ$. Significant adjustments in structural parameters occur in going from the cis to the trans isomer. The principal changes are for the CCCl bond angle to decrease and for the CCH bond angle to increase. These changes will be considered with regard to the ``cis effect,” the surprisingly lower energy of the polar, cis isomer.