Symmetry and Time Scale

2-Butanol is asymmetric, so the two hydrogens of the CH2 group are diastereotopic. Shouldn t the three hydrogens of the CH3 group at Cl (or C4) be diastereotopic also It depends. In particular, it depends on the timescaleof our observation of the molecule. 

If we lower the temperature or greatly increase our speed of observation, rotation will appear to be slow, and the hydrogens of the CH3 groups will be different. In practice this is difficult. However, computational methods typically produce static structures. Look carefully at the output of a computed structure of even a simple asymmetric molecule using molecular mechanics or quantum mechanics. In the particular case of 2-butanol, there are three different C-H bond lengths calculated for both of the methyl groups. 

Typically, if a flexible molecule can achieve a reasonable conformation that contains a symmetry element, the molecule will behave as if it has that symmetry element. The classic example is an amine with three different substituents. The pyramidal form is chiral, but the two enantiomers interconvert rapidly by pyramidal inversion (Eq. 6.3). That rapid inversion leads to an effectively achiral system is appreciated when we consider that the transition state for inversion is a planar, achiral structure. 

Time scale is important for all stereochemical concepts. Even our most cherished stereochemical concept, the stereogenic tetracoordinate carbon, is undone if we are at high enough temperatures and long enough time scales that inversion of the center is possible through 

There are many chiral molecules for which enantiomeric forms can be interconverted by a rotation about a single bond. The enantiomeric conformations of gauche butane provide an example, where rapid rotation interconverts the two under most conditions. If the rotation that interconverts a pair of such enantiomers is slow at ambient temperature, however, the two enantiomers can be separated and used. Recall from our first introduction of isomer terminology (Section 6.1) that stereoisomers that can be interconverted by rotation about single bonds, and for which the barrier to rotation about the bond is so large that the stereoisomers do not interconvert readily at room temperature and can be separated, are called atropisomers. One example is the binaphthol derivative shown in the margin. It is a more sterically crowded derivative of the biphenyl compound discussed previously as an example of a chiral molecule with no chiral center . A second example is frans-cyclooctene, where the hydrocarbon chain must loop over either face of the double bond (Eq. 6.4). This creates a chiral structure, and the enantiomers interconvert by moving the loop to the other side of the double bond. 

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The first illustration is provided by ferroelectrics belonging to the family of pyridinium salts. Complex interplay between the contributions of van der Waals, Coulomb, dipolar and hydrogen-bonding interactions are expected because of the hybrid nature of the compound. The majority of reported NMR experiments are proton second-moment and relaxation studies on polycrystalline samples. The most sophisticated NMR methods with regard to resolution, symmetry and time-scale interpretations applied to the historical problem of assigning a pure order disorder or displacive mechanism to a ferroelectric phase transition will provide the second example with the study of squaric acids and perovskites compounds like BaTi03. 

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