PNNL

Abstract

Based on an alternative energy partition scheme where density-based quantification of the steric effect was proposed [S.B. Liu, J. Chem. Phys. 126, 244103 (2007)], the origin of the internal rotation barrier between the eclipsed and staggered conformers of ethane and n-butane is systematically investigated in this work. The new definition is repulsive, exclusive, and extensive, and is intrinsically related to Bader’s atoms in molecules approach. Two kinds of differences, adiabatic (with optimal structure) and vertical (with fixed geometry), are considered in this work. We find that in the adiabatic case the eclipsed conformer possesses a larger steric repulsion than the staggered conformer for both molecules, but in the vertical cases the staggered conformer retains a larger steric repulsion. For ethane, a strong correlation between the total energy difference and the fermionic quantum energy difference is discovered. This linear relationship, however, does not hold for n-butane, whose behaviors in energy component differences are found to be more complicated. The impact of basis set and density functional choices on energy components from the new energy partition scheme has been investigated, as has its comparison with another definition of the steric effect in the literature in terms of the natural bond orbital analysis through the Pauli Exclusion Principle. Profiles of conceptual DFT reactivity indices as a function of dihedral angle changes have also been examined. Put together, these results suggest that the new energy partition scheme provides insights from a different perspective of internal rotation barriers.