Knowledge Bank

The microwave spectrum of propionaldehyde $(CH3CH2CHO)$ has been examined in the region 8,000--35,000 mc. Rotational transitions have been assigned for two different conformations. The moments of inertia of one of these agree quite well with those predicted for a cis conformation, in which all heavy atoms lie in a plane. Here $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/2(Ia+Ib-I0)=3.093$ amu {\AA}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ is observed compared with 3.075 amu {\AA}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ calculated for four out of plane hydrogen atoms. For the second conformation, a-type R branch lines were assigned. Additional aspects of this problem will be discussed.