Intramolecular electrostatic interactions

The intermolecular electrostatic interactions are found in bimolecular reactions of a charged reactant approaching a molecule with strong dipolar bonds or even charges (e.g., in enzyme-catalyzed reactions, where they are used not only to properly position a substrate in the active site of an enzyme but also to lower the activation energy barrier for the subsequent chemical transformation of a substrate). 

The intramolecular electrostatic interactions play a very important role in the control of the conformation of a molecule and consequently control its chemical behavior. These interactions will be discussed first. 

In 1953, Corey [1] studied the conformational equilibrium of a-halocyclohexanones (a-bromo- and a-chlorocyclohexanones) since the C=0 and the C-X (X = halogen) bonds are both strongly polarized, mutually repulsive, and next to each other. The conformer having the halogen atom equatorially oriented should be destabilized due to dipolar interactions between the C-X and the C=0 dipoles which are almost coplanar and equatorially oriented, whereas the conformer having the halogen atom 

Miljkovic, Electrostatic and Stereoelectronic Ejfects in Carbohydrate Chemistry, 

Compound Position of carbonyl absorption, cm Frequencies shift due to a-halogen, cm  

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Wang L-S, Ding C-F, Wang X-B and Nicholas J B 1998 Probing the potential barriers in intramolecular electrostatic interactions in free doubly charged anions Phys. Rev.Lett. at press... 

The structures of these molecules show the effects of intramolecular electrostatic interactions. Two examples are the lone pair—lone pair repulsion that is an important determinant of hydroxylamine and oxime conformations, and the intramolecular hydrogen bonding in hydroxamic acids that promotes the near-planarities of their —C(=0)—NO frameworks. 

The viscosity of xanthan solutions is also distinct from that of flexible polyelectrolyte solutions which generally shows a strong Cs dependence [141]. In this connection, we refer to Sho et al. [142] and Liu et al. [143], who measured the intrinsic viscosity and radius of gyration of Na salt xanthan at infinite dilution which were quite insensitive to Cs ( > 0.005 mol/1). Their finding can be attributed to the xanthan double helix which is so stiff that its conformation is hardly perturbed by the intramolecular electrostatic interactions. In fact, it has been shown that the electrostatic persistence length contributes only 10% to the total persistence length even at as low a Cs as 0.005 mol/1 [142]. Therefore, the difference in viscosity behavior between xanthan and flexible polyelectrolyte... 

However, Eq. (2.21) is not very convenient in the context of intramolecular electrostatic interactions. In a protein, for instance, how can one derive the electrostatic interactions between spatially adjacent amide groups (which have large local electrical moments) In principle, one could attempt to define moment expansions for functional groups that recur with high frequency in molecules, but such an approach poses several difficulties. First, there is no good experimental way in which to measure (or even define) such local moments, making parameterization difficult at best. Furthermore, such an approach would be computationally quite intensive, as evaluation of the moment potentials is tedious. Finally, the convergence of Eq. (2.20) at short distances can be quite slow with respect to the point of truncation in the electrical moments. 

Charges can be obtained at different level of moments such as monopole (s = 1), dipole (s = 3) and quadrupole (s = 9). Torsion energy barriers for the HS-SH molecule calculated by several methods can be seen in Fig. 9 [90]. For the PCM model of this molecule the number of expansion centers is six (c = 6) beside the atomic centers, one center per S-H bond is further included. It can be seen that the PCM result is very close to the CMMM one and the PCM charges can be used for calculating intramolecular electrostatic interactions as well. 

The viscosity of a linear polyelectrolyte solution depends on the conformation of the molecules, which in turn is affected by intramolecular electrostatic interactions between charged segments located along the polymer backbone, but the interactions in systems of charged polyelectrolytes are still far from being understood. The study on the solution property of cyclic polyelectrolyte is of interest, since the chain expansion of a cyclic... 

Treatment of 3 -tosyloxy-5a-cholestan-6j8-ol (363) with sodium azide gave the 3a-azido-derivative (364), which was oxidized to the corresponding 6-ketone (365). The 3/8-azido-5/3-cholestan-6-one (366) was prepared by a similar sequence. Equilibration experiments of (365) and (366) showed the influence of intramolecular electrostatic interaction between the azido- and keto-groups. ... 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

More recently, in addition to random ionomers, telechelic ionomers in which ionic groups are located only at the chain end(s) became available and were used for the study of polyelectrolyte behavior [26-29]. Discussion was made from the point of view that the behavior of telechelic ionomers in nonaqueous solutions is basically similar to that of polyelectrolytes in aqueous/nonaqueous solutions (including random ionomers in nonaqueous solutions). Also, the study of fundamental aspects of polyelectrolytes was made possible because of the simplicity of the structure of telechelic ionomers. For example, telechelic ionomers with only one ionic group at the chain end can be used to study the role of intermolecular interactions, since there is no intramolecular electrostatic interaction available for this type of ionomer [27]. Due to space limitations, this chapter will only cover polyelectrolyte behavior of random ionomers in polar solvents. Some results on telechelic ionomers can be found elsewhere [26-29]. 

Several additional HR-C-based peptides have been designed to take advantage of intramolecular electrostatic interactions that stabilize a helical conformation, including CP32M (35), S29EK (36), SC34EK (37), and T-2635 (38) [137-143],... 

The molecular mechanics program MM2 and its recent successor model electrostatic interaction with bond dipoles, supplemented as necessary with monopoles (for ions) and atomic dipoles (for lone pairs). The MM3 parameterization places increased emphasis on compatibility of intra- and intermolecular van der Waals and electrostatic parameters. This compatibility is needed to allow concurrent molecular structure and intermolecular structure optimization, such as finding optimum molecular conformation in a crystal, for instance. The reader should consult other reviews - for detailed discussions of intramolecular electrostatic interaction in various types of molecules. 

Noy. D., Fiedor, L., Hartwich, G., Scheer, H., Scherz, A. (1998). Metal-substituted bacte-riochlorophylls. 2. Changes in redox potentials and electronic transition energies are dominated by intramolecular electrostatic interactions, J. Anh Chem. Soc 120 3684. 

Methoxycyclohexanone is another example of the intramolecular electrostatic interaction control of the conformation of a molecule. It was found that 4-methoxycyclohexanone favors, in a number of solvents, the conformation in which the strongly electronegative C4 methoxy group is axially oriented due to the presence of the strongly polarized Cl carbonyl oxygen bond [2, 3], as shown in Fig. 1.2 and Table 1.2. The axial conformer 9 is favored over the equatorial conformer 3 by 0.4 kcal/mol. 

Section 7.3.1 describes the experimental apparatus for collecting and analyzing fluorescence lifetime data vs temperature. Section 7.3.2 presents the results of lifetime measurements of polyproline peptides and MD simulations to (a) relate fluorescence quenching rates to specific conformational fluctuations, and (b) calculate the implications of intramolecular electrostatic interactions for the quenching mechanism. 

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