Gauche conformation solvent effects

What is the reason for this apparent discrepancy It is a solvent effect. In aqueous solution (Figure 9.6), the OH group at the anomeric C atom of the glucose becomes so voluminous due to hydration that it strives for the position in which the steric interactions are as weak as possible. Thus, it moves into the equatorial position—with a AG° value of approximately -1.6 kcal/mol—to avoid a gauche interaction with the six-membered ring skeleton. (Remember that axially oriented substituents on the chair conformer of cyclohexane are subject to two gauche interactions with the two next-to-nearest C. —C bonds. They therefore have a... 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

If interactions of an electrostatic nature have some effect on the relative stability of trans and gauche rotational isomers, it seems reasonable to expect an effect of solvent, though perhaps small, on the factor a. Such effects on the conformations of small molecules are well known [see Mizushima (14,188) and Wada (259)]. The relatively high value of a for poly(methyl methacrylate), as discussed in paragraph (iv) above, may be due to electrostatic interactions, and it is therefore appropriate to search for a solvent effect here. No such effect was found by Marchal and Lapp (175) in their measurements of the apparent dipole moment of the polymer in various solvents as compared with that of the model monomeric compound, methyl isobutyrate. They concluded that to within experimental error (about 3%) the unperturbed dimensions were independent of the solvent. More recently a very small solvent effect on the dipole moment of the isotactic polymer has been reported by Salovey (223a). 

The increase in gauche conformations in polar solvents caused by the gauche effect is also seen with w 5o-2,4-pentane diol (Table 4-4). The proportion of TT diads with this compound decreases with increasing solvent polarity, and the proportions of TG and G T diads increase. It is noteworthy that the proportion of pure gauche diads is practically zero in all solvents. 

It is of interest that A//° (gas phase) for dibromoethane is greater than that for butane (3.7-4.1 kJ mol ) even though bromine and methyl are usually considered to be of comparable size. This suggests that a factor other than steric repulsion affects the situation the obvious candidate is dipole-dipole repulsion in the gauche conformers of the dibromide. This contributor to the overall repulsion should be diminished in more polar solvents, both because of an increase in the effective dielectric constant and a concomitant decrease in coulom-bic repulsion, and because of more effective solvation of the higher-dipole confotmer in more polar solvents. Indeed, A// for 1,2-dibromoethane diminishes to 3.6 kJ mol in the pure liquid (dielectric constant e = 4.8) and to 2.8 kJ mol in acetonitrile (e = 37.5). (It must be noted, however, that because of differential volatility of the gauche and anti conformers of (nonpolar) butane, the conformational enthalpy difference A//° between these conformers also diminishes in the liquid phase, to 2.3-2.4 kJ mol .) We indicated earlier that ethane has a threefold potential (called V3). In the 1,2-dihaloethane, there is a superposed onefold potential (Vi) since the optimum (anti) orientation of the C-X dipoles is achieved only once in the course of a 360° rotation about the C-C bond. 

In contrast, Nagy pointed out [13] that the zwitterionic P-alanine ( H3N-CH2-CH2-C00 ), which forms an intramolecular hydrogen bond in its favorable NCCC gauche conformation in aqueous solution (note the difference with GABA upon one less CH2 group in the linker), transforms to the intramolecularly hydrogen-bonded neutral species in chloroform with an anti -COOH group. This is an important consequence of the solvent effect on the amino acid tautomeric forms in different media. 

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