Effects on Conformation

Because the precise energies of charge-transfer interactions are sensitive to small structural modifications, purely intuitive predictions often turn out to be wrong. In tetrafluorohydrazine, for instance, hyperconjugation of the type wN — should 

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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. 

Jorgensen W L 1983. Theoretical Studies of Medium Effects on Conformational Equilibria, jouniM Physical Chemistry 87 5304-5314. 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

Foresman et al.175 applied the DFT(B3LYP)/SCRF calculations to obtain the polar solvent effect on conformational equilibria in furfuraldehyde and on the C-C rotational barrier of (2-nitrovinyl)amine. The authors demonstrated that the poor results obtained using either spherical or ellipsoidal cavities can be significantly improved upon performing the SCRF calculations for the cavity of molecular shape. 

W. L. Jorgensen, Theoretical studies of medium effects on conformational equilibria,... 

The number and type of cross terms vary among different force fields. Thus, AMBER2 contains no cross terms, MM23 uses stretch-bend interactions only and MM34 uses stretch-bend, bend-bend and stretch-torsion interactions. Cross terms are essential for an accurate reproduction of vibrational spectra and for a good treatment of strained molecular systems, but have only a small effect on conformational energies. 

The importance of the C-3 functionality and its attendant effect on conformation is illustrated by the discovery of the oxazolidinedione vinzoli-dine (107). This group of compounds represents a serendipitous observation of an orally active vinca alkaloid, but unfortunately the clinical application of this compound was not fully realized. 

R. U. Lemieux, A. A. Pavia, J. C. Martin, and K. A. Watanabe, Solvation effects on conformational equilibria. Studies related to die conformational properties of 2-methoxytetrahydropyran and related methyl glycopyranosides, Can. J. Chem., A1 (1969) 4427-4439. 

JCP(70)1898>. Analysis of bonding and antibonding interactions of substituents with these j-type bonds can be used to predict their effect on conformation <83U72>. 

There is a strong correlation between substituent effects on conformational preferences in tetrahydropyrans and Claisen transition states. 

In order to include an group in a data set in which the substituents exhibit an steric effect it is necessary to know 0 so that the appropriate values of the delocalized electrical effect parameter, ctd and the steric parameter can be used. Values of 0 are usually unavailable. A method has been developed to circumvent this difficulty for chemical reactivities. The procedure is 1) A basis set, which contains only those members of the data set that show minimal dependence of their steric effect on conformation, is correlated with the LDS equation,... 

Similar aromatic ring-substitution effects on conformational equilibria to those shown by 216 are shown by the dibenzo[a,/]quinolizidines (221). Examples are provided by 29 and 30,51 described in Section II,B,1. In these systems Jgem and angular proton shifts are sensitive to aromatic ring-orbital-nitrogen lone-pair overlap (Sections II,B,1 and 2). 

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. 

Solvent polarity has a negligible effect on conformational equilibrium in the aldopentopyranose tetrabenzoates, but a regular effect is observed for methyl glycoside derivatives. 

Fig. 26 Cartoon illustration of the temperature effect on conformational changes of DNA in solutions (up) and in gels (down). The green pendants represent cross-linker molecules named EGDE. Reproduced with permission from [110]...

Theories of conformations of polyelectrolytes fall into two groups. In the first group [32-34] the chain is assumed to be a flexible chain and the consequence of electrostatic interaction is calculated. In the second category [35-42], the chain is assumed to be a stiff chain and calculations are performed to obtain the effect of the electrostatic interaction between charges on the chain backbone. To date, there is no satisfactory theory in the literature to describe the electrostatic effect on conformations of polyelectrolyte chains with arbitrary intrinsic stiffness. In the following we briefly outline the developments for both groups of theories. 

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