Barriers to rotation adjacent to double bonds. 4. Effect of basis set on structures, and of electron correlation on relative energies

The optimized geometries for the rotamers of propanal, 2‐butanone, isobutyraldehyde, methyl isopropyl ketone, and isobutyric acid obtained using the 3–21G and 6–31G* basis sets are compared, and systematic changes are noted. The relative 6–31G* energies using the 3–21G and 6–31G* geometries are generally the same within 0.1 kcal/mol. The effect of electron correlation on the relative energies is generally small. These and related data show that 6–31G* relative energies obtained using 3–21G geometries are generally satisfactory when studying rotation about CC bonds. However, this is not the case for CO bonds. The calculated relative energies of isomeric compounds are reproduced only with the full MP4 correction for electron correlation.

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