Calculation of Energy Levels for Internal Torsion and Over-All Rotation. II. CH3CHO Type Molecules; Acetaldehyde Spectra

Methods are described for the calculation of energy levels for the over-all rotation and internal torsion of molecules with a plane of symmetry and one symmetric internal rotor. Typical examples are CH3CHO, CH3SiH2D, and CH3OH. Analysis of the microwave spectra of CH3CHO, CD3CDO, CD3CHO, CH2DCHO, CHD2CHO, C13H3CHO, CH3C13HO, C13H3C13HO, and CH3CHO18 yields V3=1162, 1151, 1143±30 cal, and V6<100 cal, respectively, for the first three isotopic species, where the barrier to internal rotation is V=V3(1—cos3α)/2+V6(1—cos6α)/2. The aldehyde oxygen was found to be opposite one of the methyl hydrogens in the equilibrium orientation of the methyl group. The measured moments of inertia of the nine isotopic species lead to a structure with the parameters Methyl group: C–H=1.086 A C–D=1.088 H–C–H=108°16′ D–C–D=108°32′ C–C=1.501 Aldehyde: C–H=1.114 C=0=1.216 C–C–H=117°29′ C–C–O=123°55′ The Stark effect gave a dipole moment of 2.69 debyes.