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The infrared absorption spectra of polycrystalline films of C2H2, C2HD, and C2D2 have been examined at 63°K in the frequency range between 4500 and 450 cm—1. The spectra of the solid state and the gas phase of the various acetylenes are compared and the appropriate vibrational assignments are made. Several absorptions attributable to combinations of lattice modes and molecular fundamentals were found. The observed multiplet structure of the fundamental vibrations indicates that in the low-temperature modification of C2H2 and C2D2 the molecules are located at sites of C2h symmetry. Spectra of various solid solutions composed of mixtures of isotopic species were obtained and their significance discussed. The effect of isotopic substitution on the splittings in the ν5 region led to the conclusion that the dipole—dipole coupling model is sufficient to explain the observed spectra.

Gas-phase infrared spectra of all eight partially deuterated methyl halides have been obtained; samples were of high isotopic purity. Molecular asymmetry, though quite small, was found to yield significant shifts in the perpendicular structure for the two central Q branches. This was most helpful in the choice of band centers. Observed bands were assigned, including the 72 fundamentals and also 35 overtones and combinations. Although most earlier work was done with samples containing two or more isotopic species, agreement with the present results is quite good in many cases. Evidence for Fermi resonance was found, including two examples in which it occurred below 2000 cm−1. Coriolis coupling between fundamentals was detected in several cases for the fluorides, including a strong resonance between a perpendicular and a parallel vibration. Application of the product and sum rules yielded satisfactory agreement. In a few cases substitution of a heavier atom, such as deuterium for hydrogen, was found to result in an increase for one of the vibration frequencies. However, all such instances involved perpendicular bands in which the choice of origin was quite difficult, and the increases were rather small. Anharmonicity corrections for the partially deuterated fundamentals were computed with the aid of a simple expression, using the corrections employed by others for the C3v molecules. The fundamentals investigated here were compared with values calculated from three different harmonic potential functions for CH3X and CD3X molecules; anharmonic corrections were subtracted from the calculated values. The force field of Aldous and Mills yielded substantially better agreement than an earlier one which was also of the general quadratic type. For the chlorides, bromides, and iodides a Urey—Bradley field was also found to give good agreement with the observed frequencies. In the fluorides the deviations for the Urey—Bradley field were larger than for either of the other two potential functions.

The microwave spectra of four isotopic species of cyclopropene, CH2(CH)2, CH2CDCH, CH2(CD)2, and CDH(CH)2 were assigned and three rotational constants for each isotopic species were determined. From these data the following structural parameters were evaluated: C–C 1.515 A, C=C 1.300 A, C–H (methylene) 1.087±0.004 A, C–H (vinyl) 1.070 A, H–C–H angle 114°42′±10′ and C=C–H angle 149°55′. The uncertainties in the methylene values are the magnitude of a rotation-vibration interaction correction. The dipole moment of cyclopropene was found to be 0.455±0.01 debye unit.