Solvent effects conformations

The axial-equatorial conformational equilibria for 2-fluoro- and 2-chlorotetrahydropyran have been investigated with several MO calculations, including calculations at the MP2/6-31G level. The MP2/6-31G calculations give values of 3.47 and 2.84kcal/mol, respectively, for the energy favoring the axial conformer. Solvent effects were also explored computationally and show the usual trend of reduced stability for the axial conformation as solvent polarity increases. 

Peptides containing three or more amino-acid residues have been demonstrated to be better models for protein studies than the amino-acids themselves. This is because the effect of peptide-bond formation on chemical shifts has been included, while other factors such as peptide conformation, solvent effects, ionization of terminal groups, and vicinal electrostatic and steric interactions have remained... 

A structure-activity relationship has been developed for formamides which activate allyltrichlorosilane in the allylation of benzaldehyde. In the case of secondary formamides (79), the frani-conformer trans-19) predominates, but the cis- (cis-19) is suggested to be the reactive conformation. Solvent effects are also explored. 

Yun-Yu S, W Lu and W F van Gunsteren 1988. On the Approximation of Solvent Effects on Conformation and Dynamics of Cyclosporin A by Stochastic Dynamics Simulation Teclmiqi Molecular Simulation 1 369-383. 

It is quite common to do the conformation search with a very fast method and to then optimize a collection of the lowest-energy conformers with a more accurate method. In some cases, single geometry calculations with more accurate methods are also performed. Solvent effects may also be important as discussed in Chapter 24. 

A prototype of such phenomena can be seen in even the simplest carboxylic acid, acetic acid (CH3CHOOH). Acidity is determined by the energy or free energy difference between the dissociated and nondissociated forms, whose energetics usually depend significantly on their conformation, e.g., the syn/anti conformational change of the carboxyl-ate group in the compound substantially affects the acid-base equilibrium. The coupled conformation and solvent effects on acidity is treated in Ref. 20. 

The followmg types of studies will not be presented individually but may have contnbuted supportmg data to coverage by compound type conformational analyses [23 24, 25, 26 27], fluoropolymers [28, 29, 30 31, 32], solid-state NMR [ii], and solvent effects [34 35, 36, 37] Many excellent articles with m-depth NMR interpretation of one specific compound or of a small, structurally related group of compounds can be found in the chemical hterature A few of these, not incorporated elsewhere in this secUon are referenced here carbonyl fluondes [JS 39 40], fluoropropanes [41 42, 43], fluorocyclopropanes [44, 45 46], fluorobutanes [47], perfluorocyclobutanone [48], fluorohexanes [49], and vinyl fluondes [50, 51 52, 53, 54]... 

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

Table 11 summarizes the relative conformation stabilities of various sulfmyl carbanions, based on the H/D exchange rates of the corresponding sulfmyl compounds 36-39. The results are in good agreement with the order of stabilities obtained from the MO calculations using the 3-21G basis set. This is remarkable, since the calculation did not take into consideration the solvent effect, despite the strong unsymmetrical solvation on the a-sulfmyl carbanion. 

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

Jorgensen et al. [84] studied how solvent effects could influence the course of Diels-Alder reactions catalyzed by copper(II)-bisoxazoline. They assumed that the use of polar solvents (generally nitroalkanes) improved the activity and selectivity of the cationic copper-Lewis acid used in the hetero Diels-Alder reaction of alkylglyoxylates with dienes (Scheme 31, reaction 1). The explanation, close to that given by Evans regarding the crucial role of the counterion, is a stabilization of the dissociated ion, leading to a more defined complex conformation. They also used this reaction for the synthesis of a precursor for highly valuable sesquiterpene lactones with an enantiomeric excess superior to 99%. 

As can be seen from the histogram in Figure l-l(b), the loose conformation is preferred over the tight one, a result only possible with inclusion of solvent effects. Ab-initio calculations of those conformers show that, without the inclusion of solvent effects, the tight conformer is preferred by 7.4 kcal/mol, while the inclusion of solvent effects (with polarizable continuum model, PCM) shifts the preference towards the loose conformer, which becomes more stable than the tight one by 0.1 kcal/mol. 

Conformational Equilibria. The solvent effect on the conformational equilibria represents a typical problem studied using the DFT/SCRF methods. The presence of the environment may affect the free energy of a given conformer, its equilibrium conformation or even destabilize a particular conformation. The DFT/SCRF calculations have been applied to study such effects using various KS methods as well as different techniques for calculating [Pg.112]

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