Microwave Spectrum, Structure, and Dipole Moment of Propane

The microwave spectrum of ordinary propane and five isotopic species have been measured and analyzed. The rotational constants of C3H8 are 29 207.36, 8446.07, and 7458.98 Mc, and the dipole moment is 0.083±0.001D. In its equilibrium configuration the molecule has C2v symmetry, with both CH3 groups staggered with respect to the CH2 group. The complete structure has been determined by the substitution method. Important parameters are: r(CC)=1.526±0.002 A, ≰CCC=112.4°±0.2°; in the CH2 group, r(CH)=1.096±0.002 A, ≰HCH=106.1°±0.2°; in the CH3 groups, r(CH)=1.091±0.010 A, ≰HCH=107.7°±1.0°. The influence of zero-point effects on the structure determination is discussed; these are found to be particularly bad for the CH3 group, probably because of its large vibrational amplitude.