Rotamers of ethane

In practical evaluation of force-fields parameters the H... H and C... C non-bonded potentials were chosen first. Calculations of staggered and eclipsed rotamers of ethane, propane and n-butane then revealed the contribution of the non-bonded interactions to the total rotation barrier. For example, the van der Waals parameters proposed by Alhnger et al. in their first paper 40) on calculation of conformations were such that the repulsion between hydrogens on adjacent carbon atoms accounted for about 31% of the barrier in ethane. The remainder was accoimted for as a quantity which was added by considering the torsion interaction to be zero for all... 

Figure 1. Top row staggered conformation of ethane row B the three conformers of -butane (only carbon atoms are shown in this and subsequent drawings) row C the four orientations of vectors (grid coordinates a-d) around the tetrahedral central carbon atom of neopentane row D one of the chiral gauche rotamers, and the achiral anti rotamer of -butane row E chair-cyclohexane, and its Cartesian coordinates X, y, z.

Draw the staggered rotamer of 1-deuterioethane (one hydrogen atom of ethane is replaced with a deuterium) using D for the deuterium atom. 

The concept of atropisomerism developed to a considerable extent following other developments in chemistry, especially those in spectroscopy. Early work by Kohlrausch (4) and Mizushima (3), based on Raman spectra and dipole moment studies, established that rotational isomers—rotamers—must exist in 1,2-dichloroethane. Pitzer established that there are three energy minima when ethane is rotated about its C—C axis (6). Rotamers about single bonds have been found in a wide variety of organic compounds since then, mainly as a result of the application of vibrational spectroscopy to organic molecules (7). 

Figure 7. Modeling the C-C rotation in ethane, (a) Only the right hand tetrahedron moves (b) the cycle starts with the eclipsed Dih rotamer (c) one of the six chiramers (see text) (d) the Did staggered rotamer.

Ethane is the simplest hydrocarbon that can have distinct conformations. Two, the staggered conformation and the eclipsed conformation, deserve special attention and are illustrated in Figure 3.1. The C—H bonds in the staggered conformation are arranged so that each one bisects the angle made by a pair of C—H bonds on the adjacent carbon. In the eclipsed conformation each C—H bond is ahgned with a C—H bond on the adjacent carbon. The staggered and eclipsed conformations interconvert by rotation around the carbon-carbon bond. Different conformations of the same molecule are sometimes called conformers or rotamers. 

The configurational stability of primary Grignard reagents can be studied by an interpretation of exchange effects in the proton resonance spectra as a function of temperature (Whitesides et al., 1963). These changes in spectra are related to the high energy barrier in heavily substituted ethanes. The molecule studied is shown below in three rotamer forms of which only two are non-equivalent.)... 

The arrangement of bonds around a chiral carbon atom or a double bond is called configuration. When two structures differ by rotation around one or more single bonds, they are called conformers or rotamers, and usually they are more easily interconvertible than stereoisomers rotamers can, however, be stable compounds when considerable steric hindrance is present." In ethane viewed straight-on with superimposed carbon atoms, when the configurations of the two carbon atoms overlap, the confor-... 

Microwave spectra for EtPHj and its deuteriation products suggest a 45 55 mixture of gauche- and trans-rotamers at room temperature. In the 1 2 adducts of trimethylphosphine with PX3 or ASX3 (X = ClorBr), i.r, and Raman spectra showed that trans configurations were adopted for all these monomeric complexes,but that with l,2-bis(dimethylphosphino)ethane the complexes were forced into the cis configuration. 

If the steric bulk is further increased, stable rotamers can be isolated, when two adamantyl groups and two tert-butyl groups are attached to the ethane skeleton (X, X = tert-butyl, Y, Y = adamantyl) The sym.-tetra-tert-butylethane (X,X, Y,Y = tert-butyl) shows non-equivalent methyl groups which is attributed to an equilibrium of distorted conformers. The conformational analysis of tri-tert-butylethane has just recently been published Other examples are triisopropylmethane, and tetramethyl-pentane and -hexane With modern low-temperature NMR techniques even more complicated conformational equilibria can be assessed. As an example, the spectrum of 3-isopropyl-2,3,4-trimethylpentane (1,1,1-triisopropylethane) 30 is shown in Figure 8 for which a detailed analysis of the conformational equilibria was given. ... 

An extremely interesting example of Grignard reagent mediated alkane synthesis is the coupling reaction of l-adamantyl-l,l-dibromo-2,2-dimethylpropane with magnesium (equation 99) The highly hindered alkane product is novel in the sense that interconversion of rotamers across the ethane moiety does not take place under ambient conditions. 

F NMR chemical shifts are also very sensitive to temperature. Although their temperature dependence has been used extensively to evaluate free energies of activation for conversion of rotamers and ring conformers in substituted ethanes and other... 

---