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

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.