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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.

The microwave spectra of seven isotopic species of propylene have been studied in order to obtain an accurate molecular structure. The complete rs (substitution) structure has been calculated. The more important parameters are: r(C=C)=1.336±0.004 A,r(C–C)=1.501±0.004 A, ≰CCC=124.3∘±0.3∘. The structure is compared with those of related molecules. It is concluded that no difference can be detected in the double-bond lengths in ethylene, propylene, and the vinyl halides. The CC single-bond length in propylene is indistinguishable from that in acetaldehyde and other acetyl compounds, and is 0.025 A shorter than the CC distance in saturated hydrocarbons. In the =CH2 group in propylene, the CH bond trans to the methyl group appears slightly shorter than the cis CH bond; a similar effect occurs in the vinyl halides.

The microwave spectra of gaseous CsOH and CsOD have been studied in a high-temperature spectrometer. The spectrum indicates a linear or near-linear molecule with a large-amplitude, low-frequency bending vibration. A large number of excited states involving the bending mode and the Cs–O stretching mode have been identified. The rotational constant Bv shows an unusual variation as the bending mode is excited; the cause is not yet understood. The Cs–O bond length is found to be 2.40±0.01 Å and the O–H distance is probably about 0.97 Å. The electric dipole moment is 7.1±0.5 D. Relative intensity measurements indicate a Cs–O stretching frequency of 400±80 cm−1 and a bending frequency in the neighborhood of 300 cm−1. All of the evidence supports a highly ionic cesium—oxygen bond.

A new formulation of the vibration–rotation interactions in linear triatomic molecules is developed for the purpose of explaining the anomalies found in the microwave spectra of CsOH and RbOH. This formulation, which explicitly takes into account the curvilinear nature of the motions, leads to a some-what different partitioning of the interaction constant α2 into harmonic and anharmonic contributions. The harmonic contribution resulting from direct averaging of the moment of inertia over the bending mode is found to dominate the α2 constant in most linear molecules, in contrast to the traditional treatment, which leads to a large anharmonic contribution. In the alkali hydroxides the harmonic and anharmonic parts are comparable in magnitude but of opposite sign; the resulting cancellation is responsible for the unusual vibrational dependence of Bυ in these molecules. The deuterium isotope shifts of Bυ are in excellent agreement with the predictions of this treatment. All of the microwave and infrared data on CsOH and RbOH are consistent with a reasonable force field, although uncertainties in the harmonic force constants prevent a quantitative determination of anharmonic constants. In the absence of contrary evidence, we conclude that the alkali hydroxides very probably have a linear equilibrium configuration. The equilibrium bond lengths in CsOH are estimated to be: rCsO. = 2.391 Å, rOH = 0.960 Å; in RbOH: rRbO = 2.301 Å, rOH = 0.957 Å.