Ab Initio Calculations of the Barriers to Internal Rotation of CH3CH3, CH3NH2, CH3OH, N2H4, H2O2, and NH2OH

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Abstract
The barriers to internal rotation of the molecules CH3CH3, CH3NH2, CH3OH, N2H4, H2O2, and NH2OH are calculated by the LCAO SCF MO method using a medium‐sized Gaussian orbital basis set. The calculated barriers in kilocalories/mole are CH3CH3 (2.88), CH3NH2 (2.02), CH3OH (1.59), N2H4 (11.05, 6.21), H2O2 (15.94), and NH2OH (9.90, 7.37). In addition, for CH3CH3 and H2O2 the basis set is augmented with p‐type orbitals on the hydrogens to investigate polarization effects. With p orbitals on the hydrogens, the ethane barrier was raised to 3.45 kcal/mole and for H2O2 a slight well (0.1 kcal/mole) not present in the smaller basis‐set calculation developed between the cis and trans configurations. For CH3NH2 the effect of the nonlinearity of the CH3 symmetry axis and the C–N bond was found to be significant. A population analysis indicates that, in general, as a hydrogen rotates to a position of nearest approach to a lone pair at the other end of the molecule, electron density is lost to the lone pair. Scaling the energy changes the barriers only a slight amount, but changes in the kinetic and potential energies are shown to be larger.