Ab initio studies of structural features not easily amenable to experiment : Part 13. Structural consequences of methyl hyperconjugation in some CH3C(=X)Y systems

The molecular structures of several conformations of acetaldehyde, acetic acid and isobutene were completely refined by standard single determinant ab initio procedures on the 4–21G level. Calculated and experimental parameters are in close agreement in all the cases in which the experimental values can be considered as reliably established. In some examples, however, some essential structural features that should be found in CH₃C(=X)Y systems have not yet been resolved by any experimental technique. The structural consequences for methyl hyperconjugation in the various systems are discussed. The study demonstrates that high quality ab initio geometries can provide information which is complementary to experimental results. The calculations are particularly useful in discussing discrepancies between microwave and electron diffraction structures and in interpreting microwave substitution structure uncertainties which are often difficult to determine accurately.