Conformation, molecular gauche

Construct a molecular model corresponding to the Fischer projection of rneso 2 3 dibro mobutane Convert this molecular model to a staggered conformation in which the bromines are anti to one another Are the methyl groups anti or gauche to one another in this staggered con formation" ... 

Construct molecular models of the gauche and anti conformations of 1 2 ethanediol and explore the possibility of intramolecular hydrogen bond formation in each one... 

A cross-linked polymer has a density of 0.94 g cm" at 25°C and a molecular weight between crosslinks of 28,000. The conformation of one bond in the middle of the molecule changes from trans to gauche, and the molecule opens up by 120°. In w-butane, the trans to gauche transformation requires about 3.3 kJ mol". Estimate a value for AH of stretching based on this model, and use the law of cosines to estimate the magnitude of the opening up that results. 

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

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