Microwave Spectrum of Isobutylene. Dipole Moment, Internal Barrier, Equilibrium Conformation, and Structure

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Abstract
The microwave spectra of isobutylene and isobutylene-d-3 have been studied in the region 17–36 kMc. Observed rotational constants (Mc) for isobutylene are a0=9133.32, b0=8381.75, c0=4615.99; for sym-isobutylene-d-3, a0=9132.59, b0=7788.98, c0=4431.05; for asym-isobutylene-d-3, a0=8819.46, b0=7981.08, c0=4469.59. The most likely structure compatible with these data is rc–c=1.507 A, rc=c=1.34 A, rCH(methyl)=1.085 A, rCH(ethylenic)=1.086 A, <Me—C—Me=115.9°, <HCH(methyl)=108°, <HCH(ethylenic)=117.5°. Measurements of the Stark effect show that the dipole moment μ=0.503±0.009 D. Fine structure leads to a barrier height hindering internal rotation of the methyl groups of 2.21 kcal/mole. The equilibrium conformation is C2v with two methyl hydrogens in the plane of the carbon atoms and the CH bonds pointing away from the symmetry axis. It is concluded that although CC single bonds are shortened when adjacent to a double bond, the double bond length is probably not much affected by the presence of an adjacent CH3 group.