Microwave Spectrum, Molecular Structure, Barrier to Internal Rotation, and Dipole Moment of Methylketene

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Abstract
The microwave spectra of the CD3 species and of seven monosubstituted isotopic species of CH3CH=C=O have been examined, and a complete structure of methylketene has been obtained by the substitution method. The structural parameters found are: C=O=1.171, C=C=1.306, C–C=1.518, C–H (vinyl)=1.083, and C–H (methyl)=1.083 or 1.11 Å (see text). The ketene group is linear, and the methyl group is staggered, ∠CCC=122.6°, ∠C(1)C(2)H(2)=113.7°, and ∠HCH=109.9° or 108.8°. The barrier splittings were used to determine the value of the barrier to hindered rotation and the angle between the methyl‐axis and the principal‐axis system for each of the monosubstituted species; V3=1177±20 cal/mole. The dipole moment of methylketene was measured on the 21,2→31,3transition, yielding μa=1.755, μb=0.35, μ=1.79 D.