Structure of the Isobutane Molecule; Change of Dipole Moment on Isotopic Substitution

The microwave spectra of 5 isotopic species of isobutane have been measured and analyzed. The isobutane molecule is shown to belong to point group C3v. The structural parameters determined by the substitution method are r(CC)=1.525 A, ≰CCC=111.15°, r(CH)=1.108 A (tertiary), r(CH)=1.09–1.10 A (methyl), ≰HCH=107.5°—109.5°. The structure is compared with that of other hydrocarbons, and it is suggested that a small systematic difference exists between CC distances determined by microwave and electron diffraction methods. Accurate Stark measurements show that the dipole moment of (CH3)3CD is 6.5%±0.9% higher than that of (CH3)3CH.