Cyclohexane twist-boat - chair energy difference

The Inversion barrier In cyclohexane has received much attention throughout the history of conformational analysis. The enthalpy of activation, as found by Is usually 40 - 50 kJmol depending on solvent and temperature range. 

The results are remarkably alike, the only significant difference being the near disappearance of E( )) in PEP401. This is one example of the impossibility of ascribing conformational differences to any one term of a potential energy function it all depends on the underlying model and its parametrisation. The total energy difference of 20 - 25 kJmol corresponds to what is found by other 

Some years ago, when PEF400 was new, it was applied to cyclodecane. Five conformations were selected. Three of them have been found in crystal structures Nos. 1, 2 and 4 in Table 10-3 No. 3 

Results of the use of three of the newly optimised sets are shown in Tables 10-3 and 10-4. Inspection of the valence and torsional 

The two conformers of symmetry. Nos. 2 and 4 of Table 10-3, have conformations like those found in the solid state 

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Table 10-2 Cyclohexane twist-boat - chair energy difference— PEP303 PEP304 PEP401...

This twisting gives rise to a slightly different conformation of cyclohexane called the twist-boat conformation, which, although not as low in energy as the chair form, is lower in energy (by 4 kjmol-1)... 

Allinger (p. 305) found the energy difference between cis- and /wwj-l,3-di-/er/-butylcyclohexane to be 5.9 kcal/mole, and considers that this value represents the energy difference between the chair and twist-boat forms of cyclohexane. Defend Allinger s position. 

For substituted cyclohexanes, two conformational properties are of fundamental importance. A force field should be able to predict both the correct conformation of the ring system and the position (axial or equatorial) of a substituent. Fig. 7 shows the ability of the different force fields to predict the energy difference between the twist-boat and chair conformation of cyclohexane [44]. As can be seen in the figure most of the force fields reproduce this well. However, the energy difference is overestimated by several of the force fields, in particular by CVFF and UFF1.1. 

Figure 7 Calculated energy differences in kcal/mol between the twist-boat and chair conformers of cyclohexane. The dashed line indicates the experimental value.

Squillacote M, Sheridan RS, Chapman OL, Anet FAL. Spectroscopic detection of the twist-boat conformation of cyclohexane. A direct measurement of the free energy difference between the chair and the twist-boat. J Am Chem Soc 1975 97 3244—3246. 

The demonstration by electron diffraction that the chair conformation of cyclohexane is preferred to the boat (actually a twisted boat) conformation is one of the classic achievements of the method. Hexasilacyclohexane has similar conformations, but the energy difference between them is much smaller, less than one third as much. Now the structure of an intermediate compound, 1,3,5-trisilacyclohexane, has been determined. The energy gap between the chair and twisted boat conformations is calculated (B3LYP/6-31G ) to be 9.2 kJ mol , similar to that for hexasilacyclohexane (8.0 kJ moF ), but much less than for cyclohexane (27 kJ mol ). The refined Si-C distance, 187.2( 1) pm, and SiCSi and... 

Because the boat conformation of cyclohexane is 29 kj mole" less stable than the chair conformation, only a tiny fraction of cyclohexane molecules exist in a boat conformation. The steric energy of the boat conformation can be lowered by rotation around the C-2 to C 3 and C-5 to C-6 bonds, which decreases the hydrogen—hydrogen eclipsing interactions. The result is a twist boat conformation. It is about 22 kJ mole less stable than the chair conformation. This energy difference means that only about 0.01% of the cyclohexane molecules are in the twist boat conformation at room temperature. 

At this point we should reiterate that the relative positions of atoms in many structures are continuously changing. The term different conformations of a molecule is used if two different three-dimensional arrangements of the atoms in a molecule are rapidly interconvertible, as is the case if free rotations about sigma bonds are possible. If rotation is not possible, we speak of different configurations, which represent isomers that can be separated. Obviously, the conformation(s) with the lowest energy [the most stable form(s)] is(are) the one(s) in which a molecule will preferentially exist. In the case of six-membered rings such as cyclohexane, three stable conformations (i.e., the chair, twist, and boat form) exist (Fig. 2.9). 

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

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