Microwave spectrum, structure, and barrier to internal rotation of acetone

Abstract

The microwave spectrum of (CH 3) 2CO has been reinvestigated and the spectra of two additional isotopic species, ( 13CH 3) 2CO and (CH 3) 3C 18O, have been observed and assigned. These data together with data previously reported for (CD 3) 2CO are used to calculate a structure based on the assumption of a symmetrical methyl group. The r s structural parameters obtained are: CC = 1.507 Å CCC = 117° 12′ CO = 1.222 Å HCH = 108° 46′ CH = 1.085 Å 2θ = 119° 54′ where 2θ is the angle between the symmetry axes of the methyl groups as determined by analyses of multiplet splittings arising from coupling of internal and over-all rotation. These same analyses yield barriers to internal rotation of 778, 777, and 726 cal/mole for the species (CH 3) 2CO, ( 13CH 3) 2CO, and (CD 3) 2CO, respectively. Evidence for the general occurrence of “tilted” or “cocked” methyl groups in molecules with methyl groups attached to doubly bonded carbon atoms is presented and discussed.