Barrier to internal rotation In ethane

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,... 

A clear understanding of the origin of the barrier to internal rotation in ethane is a... 

Use a molecular-mechanics program to calculate the barrier to internal rotation in ethane. Use a molecular-mechanics program to find the geometries of the gauche and anti conformations of butane and the energy difference between them. 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

Depending on the relative orientation of the two methyl groups, the total electronic energy is different. This means that rotation about the carbon-carbon axis is not free , but involves an energy barrier. The maximum occurs for the eclipsed conformation. For a simple m.o. treatment of the barrier to internal rotation in the ethane molecule, see ref. 80. 

D. W. SMITH, A simple molecular orbital treatment of the barrier to internal rotation in the ethane molecule. J. Chem. Educ., 75, 907 (1998). 

The barrier to internal rotation in Si2He is 4.9 kJ mol i.e. less than half the barrier in ethane [4], The smaller barrier in disilane is, of course, consistent with a rationalization in terms of repulsion between C-H or Si-H bonds, but does not prove it. 

The inversion barrier of phosphine has not been determined experimentally. The best quantum chemical calculations yield a barrier of 141 kJ mol , i.e. more than five times higher than in ammonia [2]. Since the barrier to internal rotation in Si2H is smaller than in ethane, it may seem surprising that the inversion barrier of PH3 is higher than in NH3. The reason may be that the pyramidal structure of NH3 is significantly destabilized by repulsion between the H atoms. 

Thus the orbitals and r electrons lie in the outermost part of the valence shell of ethane. They should play a critical role in determining the chemical properties of the molecule. Some theories have ascribed the barrier to internal rotation to these orbitals. It should be noted that the existence of r electrons in ethane is not a novelty, and was first pointed out by Mullikcn in 1935. 

We can explore bond torsion in ethane to understand the barrier to internal rotation of one bond relative to another in saturated carbon chains, such as those found in lipids. The potential energy of... 

Historically, rotational barriers have played an important role in structural chemistry because the relationship between electronic and molecular structures is basic to barrier height and shape. The quantum mechanical nature of hindered rotation in molecules was recognized in the early years of quantum theory by Nielson, whose work provided the stepping stone for K. S. Pitzer s famous 1936-7 papers on internal rotation in ethane. E. B. Wilson, early on, realized that elucidation of rotational barrier origins would lead to an in-depth understanding of the electronic-molecular structure relationship. He and his students published many papers on this topic, ... 

The free rotor model is not adequate for most molecules as there is in general some potential barrier to internal rotation. As an example, consider ethyl fluoride. Ethyl fluoride, just like ethane, has a predominately threefold potential barrier as shown in Figure 7-3. The potential barrier can be expressed as the following function. 

Energy is required to convert from the staggered conformation to the eclipsed conformation, and for ethane, the energy required is about 12.0 kJ moP. Because of this energy requirement, there is a barrier to internal rotation, and thus the —CH3 groups in ethane do not rotate entirely freely about the C—C bond. However, the barrier to internal rotation is small enough that, at room... 

Beginning in 1937, I had been very much interested in the thermodynamic properties of various hydrocarbon molecules and hence of those substances in the ideal gas state. This arose out of work with Kemp in 1936 on the entropy of ethane ( 1) which led to the determination of the potential barrier restricting internal rotation. With the concept of restricted internal rotation and some advances in the pertinent statistical mechanics it became possible to calculate rather accurately the entropies of various light hydrocarbons (2). Fred Rossini and I collaborated in bringing together his heat of formation data and my entropy and enthalpy values to provide a complete coverage of the thermodynamics of these hydrocarbons in the ideal gas state O). As an aside I cite the recent paper of Scott ( ) who presents the best current results on this topic. 

The strikingly different characteristics of transition-metal hyperconjugative interactions are particularly apparent in their influence on internal rotation barriers. To illustrate, let us first consider ethane-like Os2H6, whose optimized staggered and eclipsed conformations (displaying conspicuous deviations from those of ethane) are shown in Fig. 4.81. 

---