PNNL

Abstract

Continuing our recent endeavor, we systematically investigate in this work the origin of internal rotational barriers for small molecules using the new energy partition scheme proposed recently by one of the authors [S.B. Liu, J. Chem. Phys. 126, 244103 (2007)], where the total electronic energy is decomposed into three independent components, steric, electrostatic and fermionic quantum. In specific, we focus in this work on six carbon, nitrogen, and oxygen containing hydrides, CH¬3CH3, CH3NH2, CH3OH, NH2NH2, NH2OH, and H¬2O2, with only one rotatable dihedral angle ?H-X-Y-H (X,Y=C,N,O). Relative contribution of the different energy components to the total energy difference as a function of the internal dihedral rotation will be considered. Both optimized-geometry (adiabatic) and fixed-geometry (vertical) differences are examined, as are the results from the conventional energy partition and natural bond orbital analysis. A wealth of linear relationships among the total energy difference and energy component differences for different systems have been observed but no universal relationship applicable to all systems studied has been discovered, indicating that even for simple systems as such there exists no omnipresent, unique interpretation on the nature and origin of the internal rotation barrier. Different energy components must be employed for different systems in the rationalization and there are occasions where no single energy component can be invoked to justify the barrier height.