Equilibria and Thermodynamic Functions

Conformational Equilibria and Related Topics.—Conformational Equilibria and Thermodynamic Functions. The electron-difiraction technique is often used to study conformational equilibria, as discussed in a recent review paper. Shortly after Hassel and Ottar had shown that cyclohexane exists in the chair form, the study of fra s-l,2-dibromocyclohexane showed that two conformers may coexist in the gas phase. In many cases the ratio 

Bastiansen, H. M. Seip, and J. E. Boggs, in Perspectives in Structural Chemistry , ed. J. D. Dunitz and J. A. Ibers, Wiley, New York, London, and Sydney, 1971, Vol. 4. O. Hassel and B. Ottar, Arch. Math. Naturvuiensk., 1942, BXLV, No. 10. 

F nre 10 Theoretical and experimental r(r)/r curves for, A-lTaxt chloro-cyclohexane 

The weight ratio (or ratios) may be refined, but it is oft i easier to refine structural parameters for various fixed ratios and compare the resulting sum of squared residuals. The accuracy of the ratio determination may be estimated by the factor test (c/. see p. 54). In practice it is necessary to assume that some of the parameters are equal in both (or all) conformers. A typical example is the investigation of CH2=CH-S-CH . Two conformers were found, one syn form with a planar skeleton, and a second form which seems to have a non-planar skeleton. The bond lengths and bond angles were assumed to be independent of the torsional angle about the C-S- bond, though it is rather likely that some parameters e.g. 

When the fraction of each conformer has been obtained, the differences in free energy are easily calculated. For two conformers we have for the molecular ratio 

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