1.2- Dichloroethane conformers

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cyclohexane (e=2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (flfl=3.65) and using the IPCM model at the B3LYP level. The 6-31+G(d) basis set was used for all jobs. We also ran gas phase calculations for both conformations at the same model chemistries, and an IPCM calculation for the trans conformation (SCRF=Dipole calculations are not necessary for the trans conformation since it has no dipole moment). 

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cydohexane ( =2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (oo=3.65) and using the IPCM model at the B3LYP level. The... 

Figure 7.13 reprinted with permission from Jorgensen W L, R C Binning Jr and B Bigot. Structures md Properties of Organic Liquids u-Butane and 1,2-Dichloroethane and Their Conformational Equilibria. The Journal of the American Chemical Society 103 4393-4399. 1981 American Chemical Society. 

IR spectroscopy is an inherently faster method than NMR and an IR spectrum is a superposition of the spectra of the various conformations rather than an average of them When 1 2 dichloroethane is cooled below its freezing point the crystalline matenal gives an IR spectrum consistent with a single species that has a center of symmetry At room temperature the IR spec trum of liquid 1 2 dichloroethane retains the peaks present in the solid but includes new peaks as well Explain these observations... 

The principle of the method is that once a substance crystallizes, the interconversion of the various conformers stops and so the crystals correspond to one discrete conformation or another. This has established, that 1,2-dichloroethane crystallises exclusively in the anti form, while ethylene chlorohydrin crystallizes in a gauche form because of intra molecular hydrogen bonding. But when the molecule crystallizes in two or more conformations, we say that it shows polymorphism. 

Now, consider what happens if the two fluorines are replaced by two chlorines. In this case, the dominant interactions will be acci-° ci because the C—Cl bond is both a better donor and a better acceptor than the C-H bond. Hence, it is predicted that 1,2-dichloroethane will have a preferred trans conformation which combines in an anti orientation the best donor and best acceptor bonds. 

Figure 3. Different conformations with different symmetries in ethane, H3C-CH3 (left) and 1,2-dichloroethane, CIH2C-CH2CI (right). ...

Fig. 10 Ratio of UV absorbances vs solvent polarity. Squares represent solvents which bring about a folded or partially folded conformation. Triangles represent denaturating solvents (CHCI3, CH2CI2,1,2-dichloroethane). The overall trend of increased folding nature in more polar solvents is evident...

In fact, for random-coil conformations, b in Equation 41 is explicitly related to the exponent in the Mark-Houwink equation by 3b = a 1. In this way values of M may be determined directly from measurements of D° provided KD and b are known. Although both these parameters could in principle be calculated from a detailed knowledge of the geometry of the solute, it is usual to regard them as experimentally determinable parameters. A similar relationship has also been found to hold true for the polypeptide poly(y-benzyl L-glutamate) (PBLG) dissolved both in 1,2-dichloroethane and in dichloroacetic acid. These results are shown in Figures 6 and 7, respectively. 

Figure 6. Graph of D° vs. molecular weight for poly(y-benzyl-L-glutamate) (PBLG) in 1,2-dichloroethane. PBLG is in a helical conformation.

The authors prefered the latter explanation. They argue that the difference in the dielectric constant of the medium, when a hydrocarbon rich mixture is compared with that rich in sulfur dioxide, affects the ratio of gauche to trans conformations of the polymer chain. Such effects were observed in other systems, e.g. in 1,2 dichloroethane (38). Hence,... 

The purpose of this research was to compare the effect on the conformational equilibrium for the hydroxylated compounds listed in Table II of changing the solvent from dimethyl sulfoxide, which is expected to minimize intramolecular hydrogen bonding, to 1,2-dichloroethane which should promote such bonds. These solvation effects on conformational equilibria were then to be compared with those of water which can serve as a hydrogen donor and hydrogen acceptor in hydrogen bond formation. As will be seen, the conformational equilibria generally appear similar for water and dimethyl sulfoxide but often different from those in 1,2-dichloroethane. 

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