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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CBrz has been produced by pyrolysis in a supersonic free-jet expansion and investigated by laser-induced fluorescence-excitation spectroscopy. Observations of the vibrational isotope effect for several bending progressions have led to a reassignment of the earlier gas-phase UZ' numbering and have consequently cleared up some earlier discrepancies. For the 79BrC81Br species, values of several vibrational frequencies (referred to the lowest vibrational level of each electronic state) are uy = 475 cm-l, u : = 185 cm-l, and u y = 599 cm-l; and the origin of the A 'B1 -X 'A1 electronic transition is placed at TOO = 15 091.5 cm-l. Partially resolved rotational sub-band structure yields so-me structural results. In particular, the BrCBr bond angle increases 21" upon excitation from the X to the A state. The electronic origins, TOO, and the argon matrix shifts of TOO, are shown to satisfy physically reasonable additivity relationships for CBrz, CC12, and CBrCl.

The recently developed scaled moment-of-inertia procedure for obtaining near-equilibrium bond distances has been applied to ethane. The resulting prototype carbon–carbon equilibrium bond length is found to be 1.522±0.002 Å, in good agreement with an earlier estimate based on electron diffraction results.

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