Energy rotation barrier, ethane

Several of the more common MO methods were mentioned in Chapter 1, where it was noted that most of the methods were very good at predicting minimum-energy geometries. As a result, there have been numerous calculations of the rotational energy barrier in ethane and related molecules that have matched the experimentally determined barriers quite well. Rather than elaborating on all the conformational equilibria that have been treated by MO methods, the emphasis in this section will be on the unique information available through MO calculations that is absent in molecular mechanics calculations. 

A staggered conformer is more stable, and therefore lower in energy, than an eclipsed conformer. Thus, rotation about a C—C bond is not completely free since an energy barrier must be overcome when rotation occurs (Figure 3.6). However, the energy barrier in ethane is small enough (2.9 kcal/mol or 12 kJ/mol) to allow continuous rotation. 

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about... 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

This barrier (torsional strain) causes the potential energy of the ethane molecule to rise to a maximum when rotation brings the hydrogen atoms into an eclipsed conformation. 

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

The four-electron destabilization rationale The rotation barrier of ethane is sometimes explained in terms of the mnemonic energy-level-splitting diagram shown in Fig. 3.58. The figure purports to depict how two filled MOs of ethane ( and 4>+) evolve perturbatively from two... 

Actually this energy barrier creates hindrance in free rotation in the molecule. Therefore, strictly speaking there is not free rotation in ethane. But since this value is small we, may neglect it and regard that there is free rotation about C—C single bond in ethane. 

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

It follows that the preferred conformation of ethane is a staggered one but, since the energy barrier to rotation is relatively small, at room temperature there will be free rotation about the C-C bond. 

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. 

We saw in Chapter 7 that rotation about the C-N bond in an amide is relatively slow at room temperature—the NMR spectrum of DMF clearly shows two methyl signals (p. 165). In Chapter 13 you learned that the rate of a chemical process is associated with an energy barrier (this holds both for reactions and simple bond rotations) the lower the rate, the higher the barrier. The energy barrier to the rotation about the C-N bond in an amide is usually about 80 kj mol-1, translating into a rate of about 0,1 s-1 at 20 °C. Rotation about single bonds is much faster than this at room temperature, but there is nonetheless a barrier to rotation in ethane, for example, of about 12 kj mol-1. 

Why should there be an energy barrier in the rotation about a single bond In order to answer this question, we should start with the simplest C-C bond possible—the one in ethane. Ethane has two extreme conformations called the staggered and eclipsed conformations. Three different views of these arc shown below. 

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