Influence of Vibrations on Molecular Structure Determinations. II. Average Structures Derived from Spectroscopic Data

Formulas are given which enable structural parameters for the average molecular configuration in the ground vibrational state to be calculated for some simple types of molecules. The data required are the observed effective moments of inertia and harmonic force constants. No knowledge of anharmonic constants is necessary. The average structural parameters have a well-defined physical meaning and are directly comparable with diffraction results. Polyatomic molecules for which explicit calculations are given are CO2, CS2, H2O, SO2, O3, NO2, CH4, HCN, and C2H2. It is found that the average bond lengths involving H are usually 0.003–0.005 Å longer than the corresponding D bond. For bonds involving heavier elements isotopic differences are smaller but nonetheless significant. Implications of the results for the general problem of structural determination are discussed.