Spectroscopic data related to the structures of polyatomic molecules in the gas phase have been reviewed, critically evaluated, and compiled. All reported bond distances and angles have been classified as equilibrium (r 0)' average (r,), substitution (r.), or effective (ro) parameters, and have been given a quality rating which is a measure of the parameter uncertainty. The surveyed literature includes work from all of the areas of gas-phase spectroscopy from which precise quantitative struc• tural information can be derived. Introductory material includes definitions of the various types of parameters and a description of the evaluation procedure.

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Microwave spectra have been assigned for the ground state and three excited torsional states of species of cyclopropanecarboxaldehyde with the oxygen atom cis and trans to the cyclopropane ring. Dipole moments obtained by analysis of the Stark effect of the rotational transitions are as follows: cis species: μa = 2.02 ± 0.01 D, μb = 1.86 ± 0.01 D, μc = 0, μ = 2.74 ± 0.01 D; trans species: μa = 3.22 ± 0.01 D, μb = 0, μc = 0.49 ± 0.01 D, μ = 3.26 ± 0.01 D. From relative intensity measurements estimates of the torsional excitation energies of the two species and the cis–trans energy difference were obtained. By analysis of these energy values the first three coefficients in a Fourier series expansion of the torsional potential were deduced to be V1 = − 0.23 ± 0.12 kcal/mole, V2 = 4.39 ± 0.40 kcal/mole, and V3 = 0.28 ± 0.10 kcal/mole.

A refined procedure for obtaining the structure of free molecules from electron diffraction data is described which compensates for the interference arising from non-nuclear scattering. The procedure is applied to CCl, using somewhat more extensive rotating sector data than has hitherto been published for this molecule. Estimates are made for the first time in electron diffraction results of the effect of anharmonicity of vibration on the measurement of internuclear distance and of the effect of the failure of the Born approximation on the measurement of amplitudes of vibration. A method of estimating the reliability of the results is described.

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