Microwave Spectrum of Propionaldehyde

Abstract

The microwave spectrum of propionaldehyde, CH₃CH₂CHO, has been investigated in the frequency region 8–38 Gc/sec. The existence of two stable rotamers has been demonstrated. The cis rotamer, in which the four heavy atoms are coplanar, is more stable by 900±100 cal/mole. An approximate treatment of the coupling of the —CH₃ and C–C torsional motions in the cis rotamer is presented. Using this treatment the barrier to internal rotation in the cis rotamer is found to be 2280±110 cal/mole. The dihedral angle in the higher energy, gauche rotamer is about 131°. Evidence is given for coupling of the C–C torsional momentum and over‐all rotational angular momentum in the gauche rotamer. Other aspects of the spectrum are also discussed.