Knowledge Bank

Low resolution microwave spectra of ethyl trifluoroacetate display three a-type band series characterized by $B+C$ values of $1842\pm 2$ MHz, $1947\pm 2$ MHz, and $1910\pm 6$ MHz associated with three rotameric forms designated extended, compact, and intermediate, respectively. Ethyl cyanoformate and ethyl chloroformate also display band spectra of three conformations with $B+C$ values of $2752\pm 2$ MHz, $2954\pm 3$ KHz, and $2870\pm 2$ MHz for the cyanoformate and $2819\pm 3$ MHz ($2762\pm 3$ MHz, $\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}Cl$), $3061\pm 3MHz$ ($2989\pm 3$ MHz, $\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}Cl$), and $2954\pm 5$ MHz ($2885\pm 17$ MHz, $\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}Cl$) for the chloroformate. For each compound the rotational constants of the extended species are consistent with a cis-trans[$\tau (O=COC=0\circ$, $\tau (COCC)=180\circ$] heavy atom planar structure, the compact species with a cis-gauche[$\tau (O=COC)=0\circ$, $\tau (COCC)\sim 90\circ$] structure, and the intermediate species with a gauche-gauche[$\tau (O=COC)\sim 50\circ$, $\mathrm{<mrow\; data-mjx-texclass="ORD">\circ </mrow>}(COCC)\sim 21\circ$ structure. The cis-trans and cis-gauche structures have been identified in ethyl formate by Riveros and Wilson. (J. Chem. Phys. 42, 4605 (1967)). From the temperature dependence of the band Intensities, the relative energies of the extended, compact, and intermediate species are $0,0,1\pm 0.2$, and $2.0\pm 0.2$ kcal/mole for ethyl trifluoroacetate; $0,0.1\pm 0.2$, and $1.4\pm 0.4$ kcal/mole for ethyl cyanoformate; and $0,0\pm 0.1$, and $1.2\pm 0.5$ kcal/mole for ethyl chloroformate.