Rotational barriers in ethane

This analysis provides a description of rotational barriers that is very different from the steric picture. Weinhold described it in the following way  

When bonding electrons delocalize, they can lead to alternative bonding patterns (resonance strnctnres). For 

This description is fully consistent with the jt-type character that is apparent in several ethane MOs, i.e. in its two HOMOs. 

CHJCH vs. - isoelectronic cousins with dramatically different hyperconjugation patterns The 

Note that the energy increase is relatively minor. Based on the steep increase in the energy of repulsive interactions (illustrated, for example, by the l/r repulsive part of Lennard-Jones potential), one would expect that, if C-H/C-Me interaction were dominated by sterics, such penalty would be greater. 

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Recall (Section 3 1) that the rotational barrier in ethane IS only 12 kJ/mol (3 kcal/mol)... 

The K constant is usually taken as 1.75 this value reproduces the rotational barrier in ethane. 

The nature of the rotational barrier in ethane is not easily explained. It is too high to be due to van der Waal s forces. It is considered to arise by interactions among the electron clouds of C—H bonds and quantum mechanical calculations show that the barrier should exist. 

Determination of the Rotational Barrier in Ethane by Vibrational Spectroscopy and Statistical Thermodynamics 166... 

Estimate the cost of nonbonded HH repulsion as a function of distance by plotting energy (vertical axis) vs. HH separation (horizontal axis) for methane+melham (two methanes approaching each other with CH bonds head on ). Next, measure the distance between the nearest hydrogens in eclipsed ethane. What is the HH repulsion energy in the methane dimer at this distance Multiplied by three, does this approximate the rotation barrier in ethane ... 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Now, there is no satisfactory theory of steric effects, although attempts to rationalize the barrier of ethane by quantum-mechanical calculations are appearing more frequently (dementi and Davis, 1966). Furthermore, simple group-additivity schemes of various kinds have had limited success, at best, e.g. for estimating rotational barriers in ethanes (Tang and Chen, 1962), correlating relative reactivities with Taft EB values (Wells, 1963), or evaluating asymmetric induction (Ugi, 1965 Ruch and Ugi, 1966). Semi-empirical calculations by equation (197) have... 

Peter R. Schreiner Teaching the Right Reasons Lessons from the Mistaken Origin of the Rotational Barrier in Ethane, Angew. Chem. 114(19), 3729-3731 (2002), Angew. Chem. Int. Ed. 41(19), 3579-3581 (2002)... 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational barrier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have... 

Now that we have these catalysts and have the ability to use them commercially, we would like to know how they work. When we look at energy calculations and realize that, to get 90% ee, we are talking about only a 2 kcal difference, and this is just about the same as the rotation barrier in ethane. Thus, the asymmetric bias may be caused by very subtle effects. 

However, it was not until the 1930s that the existence of a rotational barrier in ethane was established. Difficulties in the statistical mechanical treatment of the thermodynamic data for ethane led Kemp and Pitzer (1936, 1937) to propose the existence of a barrier of approximately 3 kcal mol . Spectroscopic techniques (IR, MW and ED) confirmed these findings, and on the basis of the combined results an estimated barrier of 2.90 0.03 kcal mol" was deduced (Weiss and Leroi, 1968 Hirota et al., 1979). 

The K constant is usually taken as 1.75, as this value reproduces the rotational barrier in ethane. An essentially identical approach has been used for periodic systems within the physics community, where it is here known as the tight binding model. Recent work in this area has used an approach to parameterize against density functional results, thereby providing a computationally very efficient model capable of yielding fairly accurate results. ... 

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