Internal rotation in molecules with two internal rotors: Microwave spectrum of acetone

Abstract

The theory of internal rotation in molecules is extended to the case of two internal symmetric rotors attached to a molecule with C2ν symmetry. The calculation of the internal rotational energy levels and of the interaction with the over-all rotational energy levels is discussed both for the high barrier and the low barrier approximation. The symmetry of the Hamiltonian for a molecule with the acetone structure is considered and the selection rules and relative intensities of the various fine structure components are derived. The microwave spectra of acetone and of completely deuterated acetone have been measured and assigned. From the rotational constants, the molecular structure has been partially determined. The most probable values of the structural parameters are r0(C = 0) =1.215 A (assumed), r0(C - C) = 1.515±0.005 A, r0(C - H) = 1.086±0.010 A, ∠CCC = 116°14′±1°, and ∠HCC = 110°16′±1°. The angle between the axes of the methyl groups is approximately 3° larger than the ∠ CCC. The dipole moment has been determined to be 2.90 D by a measurement of the Stark effect on the microwave transitions. Each of the rotational transitions is split into a number of components by the internal rotation. From an analysis of these patterns the barrier height to internal rotation is found to be 274 cm-1 for (CH3)2CO and 258 cm-1 for (CD3)2CO. The difference in the two calculated barrier heights and the errors in the method are discussed.