Natural steric analysis of internal rotation barriers

Abstract

ABSTRACT:We apply natural bond orbital NBO steric analysis introduced in a . previous article to obtain the steric exchange contribution to the internal rotation barriers Ž . of butane, ethane, and other related molecules CH NH , CH OH, NH OH . The expected exchange repulsion between the two methyl group C-H bonds within van der Waals contact in butane is shown to be the major contributor to the syn barrier and provides a method for calculating allowed ranges of torsional angles in macromolecules. However, the exchange-energy difference between the staggered and eclipsed forms predict a counterintuitive eclipsed minimum for ethane, methylamine, and methanol. We show that the full SCF barrier in such apolar molecules can be reasonably approximated as a sum of this steric term plus the hyperconjugative terms as previously evaluated.

Preparation of a Novel Nano-scale Lead (II) Zig-Zag Metal–Organic Coordination Polymer with Ultrasonic Assistance: Synthesis, Crystal Structure, Thermal Properties, and NBO Analysis of [Pb(μ-2-pinh)N3 H2O]n

“…However, NBO 5.0 is much more detailed when describing the electronic structure. For example, it can illustrate the localized NBO/NLMO contributions to the steric exchange energy [51]. …”

Conformational properties of 1-cyano-1-silacyclohexane, C5H10SiHCN: Gas electron diffraction, low-temperature NMR and quantum chemical calculations

“…The total steric energy E (ST) from the NBO analysis was calculated as described in Refs. [38][39][40] The Natural Coulomb…”

“…67 NBO analyses of the SCF wave function 41, 68-70 and calculations of the total steric energy [38][39][40] were performed using the NBO 6.0 program 71,72 at the M06-2X/6-311++G** level of theory.…”

Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

“…Steric Interactions The steric exchange energy is an important part of the total Lewis energy of a molecule and is very useful for the explanation of the torsional barrier formation in the case of ortho-and meta-substituted molecules. 5,28 We investigated the change in the total NBO steric exchange energy as well as the pairwise exchange energy 29 in going from the pseudo-trans to the pseudo-cis conformation. In the case of the pseudo-cis conformer, the total NBO exchange energy is 784.38 kcal/ mol against the 784.42 kcal/ mol in the pseudo-trans conformation, while the pairwise exchange energy in the pseudo-cis conformation is 224.28 kcal/ mol against the 224.49 kcal/ mol of the pseudo-trans conformer.…”