Stereoisomers energy differences

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

As m the 1 4 dimethylcyclohexanes the 6 kJ/mol (1 5 kcal/mol) energy difference between the more stable (trans) and the less stable (cis) stereoisomer is attributed to the strain associated with the presence of an axial methyl group m the cis isomer... 

Molecular mechanics strain energies have another use They can also be used to com pare the energies of models that share the same molecular formula that is models that are either stereoisomers or different conformations of a single molecule (allowed com parisons are shown here)... 

Note that the 1,2-diequatorial substituted examples in Fig 7.10(c and d) are individual stereoisomers. The corresponding cis-species (Fig. 7.11b) is not another conformation, but another stereo isomer. The experimentally by calorimetry determined energy difference between the isomers is 6.5 kJ mol" . 

As mentioned above, the calculations performed for styrene as a substrate suggests that the enantioselectivity can be directly correlated with the relative thermodynamic stabilities of the r 3-allylic complexes. Indeed, the exo stereoisomer, precursor of the enantiomeric product found in excess experimentally, becomes favoured with respect to the endo one upon t 3-coordination, and remains thermodynamically more stable until product release. However, the observed energy differences in the relative stabilities of the different allylic forms (1-2 kcal/mol) are certainly at the limit of accuracy of density functional calculations. 

Stereoselectivity arises from small differences in the activation energies of competing reactions leading to different stereoisomers. These differences originate... 

Irrespective of the mechanism of resolution, HPLC CSPs work by providing a chiral environment for analyte stereoisomers to interact with. Resolution relies upon the formation of reversible, transient diastereomers on the CSP that have different free energies of interaction and therefore stability. The stereoisomer forming the most stable diastereomer with the CSP will be the most retained and vice versa. Free energy differences are typically small in such systems but may be large enough to produce useable resolutions provided the column efficiency is sufficient [41]. If column efficiency is insufficient to allow complete separation of the stereoisomers, inaccurate integrations can result in erroneous... 

Stereoselectivity axiom 2 If two or more stereoisomers of different free energy can enter into or result from similar elementary processes, the rate of one will be preferred. 

Case 1. The 7t-barrier is larger than the steric barrier (Fig. 8). The two minima (which are generally split into enantiomeric forms) define stereoisomers of different energies which can be separated if the barrier to rotation is high enough. Steric effects will be reflected in the populations of the different isomers and in the angle of twist between the two planes. Steric effects will... 

Stereoisomers in general, and conformational isomers in particular, are characterized not only by the energy barrier separating them, but also by their free energy difference AG°, which is related to the conformational equilibrium constant K (or conformational ratio) by the equation ... 

The strain energy values that SpartanBuild calculates have another use besides structure refinement. They also can be used to compare the energies of models that share the same molecular formula—that is, stereoisomers or conformational isomers. Allowed comparisons are shown below. Strain energy differences between these pairs of molecules correspond closely to differences in heat of formation and to differences in free energy. SpartanBuild reports strain energies in kcal/mol (1 kcal/mol = 4.184 kj/mol) in the lower left-hand corner of the SpartanBuild window. 

Isomerization of ll-cw-retinal to the all-trans diastereomer is sterically favored because of the sterical repulsion between the 13-methyl and 10-H substituents in a planar polyene. In solutions of retinal or retinyl ester in which all stereoisomers are equilibrated by the reversible addition of iodine or trifluo-roacetic acid one finds only 0.1% of the W-cis diastereomer. The energy difference between all-trans- and ll-cis- retinal is 4.1 kcal/mol. The crystal structure of 11-cw-retinal shows a highly twisted polyene at C12-C13. The methyl group... 

The practical details of free energy difference calculations depend on the particular system and the particular mutation that is studied. For example, determining the potential of mean force between two small molecules in solvent clearly demands a simulation strategy different from that applied to the calculation of the free energy difference between two stereoisomers of an enzyme-bound inhibitor. 

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