Exploring the origin of the internal rotational barrier for molecules with one rotatable dihedral angle

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 ͑2007͔͒, where the total electronic energy is decomposed into three independent components, steric, electrostatic, and fermionic quantum. Specifically, we focus in this work on six carbon, nitrogen, and oxygen containing hydrides, CH 3 CH 3 , CH 3 NH 2 , CH 3 OH, NH 2 NH 2 , NH 2 OH, and H 2 O 2 , with only one rotatable dihedral angle ЄH-X -Y -H ͑X , Y =C,N,O͒. The relative contributions of the different energy components to the total energy difference as a function of the internal dihedral rotation will be considered. Both optimized-geometry ͑adiabatic͒ and fixed-geometry ͑vertical͒ differences are examined, as are the results from the conventional energy partition and natural bond orbital analysis. A wealth of strong 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 for both cases has been discovered, indicating that even for simple systems such as these, there exists no omnipresent, unique interpretation on the nature and origin of the internal rotation barrier. Different energy components can be employed for different systems in the rationalization of the barrier height. Confirming that the two differences, adiabatic and vertical, are disparate in nature, we find that for the vertical case there is a unique linear relationship applicable to all the six molecules between the total energy difference and the sum of the kinetic and electrostatic energy differences. For the adiabatic case, it is the total potential energy difference that has been found to correlate well with the total energy difference except for ethane whose rotation barrier is dominated by the quantum effect.

Origin of molecular conformational stability: Perspectives from molecular orbital interactions and density functional reactivity theory

“…This result, meanwhile, confirms the validity of the Walsh rule. Also, this result is consistent with our other recent studies, [10][11][12][13][14][15][16][17][18][19][20][21][22][23] where we found that the electrostatic interaction is the major determinant in many chemical effects and processes such as bond rotation, and chemical reactions.…”

“…Notice that there is also a kinetic counterpart of the dynamic correlation effect, [29][30][31][32][33][34][35][36] This DFT approach to partition energies has been applied to a number of systems, such as conformational changes of small molecules, S N 2 reactions, chained and branched alkanes, and other systems. [10][11][12][13][14][15][16][17][18][19][20][21][22] Reasonably, good trends and linear relationships between theoretical and experimental scales of the steric effect have recently been observed by Taft at both group and entire molecular levels.…”

“…To that end, the total energy difference is partitioned in a couple of different ways in terms of different energetic contributions accounting for different physiochemical effects, such as kinetic, exchange-correlation, electrostatic, steric, and fermionic quantum. 9 The new analysis has recently been applied to a number of systems, [10][11][12][13][14][15][16][17][18][19][20][21][22] such as bond rotation barriers, cis-effect, anomeric effect, SN 2 reaction barriers, and water clusters. A clear picture has seemingly merged from these different studies, showing that for all systems considered thus far, the electrostatic interaction plays the predominant role, whereas other effects such as those from the steric and exchange-correlation interactions play minor but indispensable roles.…”

Computational Study of Chemical Reactivity Using Information-Theoretic Quantities from Density Functional Reactivity Theory for Electrophilic Aromatic Substitution Reactions

“…[19][20][21][22][23][24][25][26][27][28][29][30][31] In this work, we will examine the role of these quantities and relationships for a number of electrophilic aromatic substitution reactions. In particular, we are interested to see (i) what the performance of applying Hirshfeld charge and information gain in predicting the reaction barrier height of these reactions is; (2) whether the electrostatic component is still the dominant contributor to the barrier height; and (3) if there exist similar strong linear correlations from the information-theoretic quantities for these systems with transition state involved.…”

Exploring the G3 method in the study of rotational barrier of some simple molecules

“…Some of these molecules are still seen through a classic model of lone pair and bond pair interactions. Therefore, accurate results are necessary [19] and are currently of great interest in the elucidation of the nature of the rotational barrier considering that many analysis of the rotational barriers are based on the decomposition of the electronic energy in orbitals, attractive, repulsive and quantum contributions, and steric effects [20][21][22][23][24].…”