Origins of Rotational Barriers in Hydrogen Peroxide and Hydrazine.

Compared with their isoelectronic system ethane, both hydrogen peroxide and hydrazine exhibit a double well torsional energy curve where skew conformers are favored over trans conformers and cis conformers are energy-maximum states. Clearly, the involvement of the lone oxygen and nitrogen pairs, or more specifically, the enhanced stabilizing n→σ* negative hyperconjugation effect and destabilizing repulsion among lone pairs, complicates the conformational analysis. In this work, the modern ab initio valence bond (VB) method is employed to quantitatively investigate the torsional energy curves of hydrogen peroxide and hydrazine in terms of hyperconjugative stabilization, steric repulsion, and structural and electronic relaxations. It is found that if the hyperconjugation effect is completely quenched, the trans conformers will be favored, while the cis conformers are the only transition state pertaining to the torsional motion in the potential energy surfaces of H2O2 and N2H4. Although usually the steric effect includes the contributions from the electronic and geometric changes, our energy decomposition analysis shows that even the steric effect favors the skew conformers, while the electronic and geometric changes stabilize the trans conformers. Thus, we conclude that both the hyperconjugative and steric interactions lower the energy of skew conformers and eventually form low barriers from skew to trans conformers and high barriers from skew to cis conformers in both H2O2 and N2H4. Comparison between the VB and the natural bond orbital (NBO) results show similarities and discrepancies between the two methods.