Rotational energy barriers cyclohexane

Stereochemistry. Cyclohexane can exist ia two molecular conformations the chair and boat forms. Conversion from one conformation to the other iavolves rotations about carbon—carbon single bonds. Energy barriers associated with this type of rotation are low and transition from one form to the other is rapid. The predominant stereochemistry of cyclohexane has no influence ia its use as a raw material for nylon manufacture or as a solvent. 

Semi-empirical models do not provide good descriptions of the energy barrier to ring inversion in cyclohexane. The MNDO model underestimates the barrier by a factor of three, and the AMI and PM3 models by almost a factor of two. This behavior is consistent with previous experience in dealing with single-bond rotation barriers. 

In all cases the conclusions reported refer to solutes at infinite dilution, usually in carbon tetrachloride (CT) or benzene (B)—but sometimes in dioxan or cyclohexane (the solvent is indicated when it seems likely to affect seriously the dissolved species). The majority of the examples are flexible molecules in which the energy barriers to internal rotations about bonds are not too high for this reason the interpretations in Table 23 are stated either in terms of mixtures, or of effective or equivalent conformation (meaning by these adjectives the specifiable models which permit a priori calculation of the observed mK, /ireSuitant> etc.) which may often, of course, approximate to the mean or predominant structures about which rotational oscillations are occurring with temperature dependent amplitudes. 

Van t Hoff appreciated that free rotation occurs about single bonds. The consequent different arrangements of atoms in space were termed the conformations of the compound by Walter Norman Haworth (1883-1950) in 1929. In 1936 K. S. Pitzer pointed out that there would be an energy barrier to rotation about the carbon-carbon bond in ethane caused by repulsion between the hydrogen atoms, but it was studies on cyclohexane and its derivatives which emphasised the importance of conformational analysis. 

This chapter assesses the ability of molecular mechanics and quantum chemical models to properly assign preferred conformation, and to account quantitatively for differences in conformer energy as well as for barriers to rotation and inversion. The chapter ends with a discussion of ring inversion in cyclohexane. 

Momany and Rone compared the CHARMm, TRIPOS, DREIDING, and MM2 force fields using a subset of conformational energy data (15) from the data set in Ref. 14. The MM2 and TRIPOS 5.2 results were also taken from this earlier study. In addition, 15 experimental rotational barriers were used to test the CHARMm, DREIDING, and TRIPOS force fields, and the CHARMm and DREIDING force fields were tested by means of an additional set of seven rotational barriers. In the subset of 15 items of conformational energy data, nine were for various cyclohexanes. In that study, the DREIDING and TRIPOS... 

The effect of introducing sp -hybridized atoms into open-chain molecules has been discussed previously, and it has been noted that the torsional barriers in 1-alkenes and in aldehydes and ketones are smaller than those in alkanes. Similar properties carry over to incorporation of sp centers in six-membered rings. Whereas the free energy of activation for ring inversion in cyclohexane is 10.3 kcal/mol, the barrier is reduced to 7.7 kcal/mol in methylenecyclohexane, and to 4.9 kcal/mol in cyclohexanone." The decrease in activation energy is related to the lower torsional barriers for rotation about sp -sp bonds, and to the decreased steric requirements of a carbonyl or methylene group. 

However, energies are just as crucia 1, and these are sometimes harder to come by. For hydrocarbons and simpler organics, there is a large database of heats of formation, and, hence, strain energies, and these are valuable in parameterization. Other energies include rotation barriers and conformational differences. A competent force field should reproduce the butane torsional profile of Figure 2.6, and should obtain the A values for many cyclohexane substituents. Due to the similarity to real molecular vibrational modes, IR vibrations should be a valuable source for a force field, but in practice few modern force fields use them in their parameterization. 

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