NMR spectrum of cyclohexyl iodide at

Conformational free-energy values for many substituent groups on cyclohexane rings have been determined by NMR methods some are recorded in Table 3.5. Conformational free energies measured at low temperatures are not believed to vary much from their values at room temperature. 

The various methods available for determining conformational free energies have been reviewed and compared. Several compilations of conformational free energies are available.  

A tert-butyl substituent experiences a strongly repulsive van der Waals interaction with y/i-axial hydrogens in the axial orientation, and cannot relieve the attendant strain by rotation about the bond to the ring. The conformational free energy of a tert-butyl group has been calculated by molecular mechanics to be 

4 kcal/mol. Experimental attempts to measure directly the energy difference between an axial and an equatorial teri-butyl group provide only a lower limit. The energy difference between an axial and an equatorial tert-butyl group is very similar to the energy difference between the chair and the twist conformations of cyclohexane, and therefore any equilibration measurement is complicated by the presence of nonchair contributions. Low-temperature infrared examination of trans-l,3-di-teri-butylcyclohexane indicates the presence of more than one conformation.  

When two or more substituents are present on a cyclohexane ring, the stereochemical considerations are similar, except that interactions between the substituents must also be included in the analysis. The dimethylcyclohexanes provide a straightforward example of disubstitution in six-membered rings in which qualitative considerations are supported by thermodynamic data. 

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Fig. 3.6. NMR spectrum of cyclohexyl iodide at —80°C. Only the lowest-field signals are shown (100-MHz spectrum). [Reproduced from J Am. Chem. Soc. 91 344 (1969) by permission of the American Chemical Society.]...

The case of cyclohexyl iodide provides an example of the use of NMR spectroscopy to determine the conformational equilibrium constant and the value of —AG°. At —80°C, the NMR spectrum of cyclohexyl iodide shows two distinct peaks in the area of the CHI signal as shown in Fig. The multiplet at higher field is a triplet of triplets with... 

An example of the use of NMR spectroscopy in determining the equilibrium constant between conformers, and therefore the value of -AG°, can be found in the case of cyclohexyl iodide. At —80°C, the NMR spectrum of cyclohexyl iodide is well resolved, and clearly indicates the presence of two conformations (Fig. 3.6). The region of the spectrum shown is for the proton on the carbon atom that bears the... 

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