Dipole moment conformation, effect

Compare energies for both diaxial and diequatorial chair conformers of trans-2-fluorocyclohexanol (X = OH). Which conformer is preferred Does the preferred conformer minimize steric repulsion Is it reasonable to attribute the conformational preference solely to steric effects Explain. Examine dipole moments for the two conformers. Does the preferred conformer minimize electrostatic repulsion (or maximize electrostatic attraction) Is it reasonable to attribute the conformational preference solely to electrostatic effects Explain. 

Dipole Moments. The dipole moments of quinoxaline, several derivatives, some pyrazine analogs, and comparable homocyclic compounds have been measured in an attempt to rationalize the values in terms of mesomeric effects and/or conformational isomerism. ... 

Dipole Homents and Kerr Effect. - The structures of two conformers of ethyldifluorophosphane have been determined using a combination of dipole moments and microwave spectroscopy.227 Dipole moments have been used to study the different tuist angles of allenic phosphonates (81),228 and also to deduce the stereochemistry of 1,3,2,5-dioxaboraphosphorinane.22 ... 

Combined dipole moment and Kerr effect studies are regularly used by Russian workers for the conformational analysis of phosphorus heterocyc1es.135 230 In a study of the interaction of phenol with phosphoryl groups the Kerr effect was used to evaluate not only the extent of hydrogen bonding but also the influence of changes in polarity and polarisation upon stability constants.231 In a similar study the orientation of the aryl groups of 1,3,5-triazaphosphorines (82) were shown to be less coplanar than biphenyl in the gas phase. 2 3 2... 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Experimental and theoretical results are presented for four nonlinear electrooptic and dielectric effects, as they pertain to flexible polymers. They are the Kerr effect, electric field induced light scattering, dielectric saturation and electric field induced second harmonic generation. We show the relationship between the dipole moment, polarizability, hyperpolarizability, the conformation of the polymer and these electrooptic and dielectric effects. We find that these effects are very sensitive to the details of polymer structure such as the rotational isomeric states, tacticity, and in the case of a copolymer, the comonomer composition. 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

We have shown in this paper the relationships between the fundamental electrical parameters, such as the dipole moment, polarizability and hyperpolarizability, and the conformations of flexible polymers which are manifested in a number of their electrooptic and dielectric properties. These include the Kerr effect, dielectric polarization and saturation, electric field induced light scattering and second harmonic generation. Our experimental and theoretical studies of the Kerr effect show that it is very useful for the characterization of polymer microstructure. Our theoretical studies of the NLDE, EFLS and EFSHG also show that these effects are potentially useful, but there are very few experimental results reported in the literature with which to test the calculations. More experimental studies are needed to further our understanding of the nonlinear electrooptic and dielectric properties of flexible polymers. 

Since the parameters used in molecular mechanics contain all of the electronic interaction information to cause a molecule to behave in the way that it does, proper parameters are important for accurate results. MM3(2000), with the included calculation for induced dipole interactions, should model more accurately the polarization of bonds in molecules. Since the polarization of a molecular bond does not abruptly stop at the end of the bond, induced polarization models the pull of electrons throughout the molecule. This changes the calculation of the molecular dipole moment, by including more polarization within the molecule and allowing the effects of polarization to take place in multiple bonds. This should increase the accuracy with which MM3(2000) can reproduce the structures and energies of large molecules where polarization plays a role in structural conformation. 

The energetically favorable conformations of model alkyl nitronate (88) and related compounds (89 to 91) (283) were determined by the B3LYP/6-31G(d) method (see Chart 3.9). It appeared that the marked conformations with the minimized dipole moments are energetically most favorable for compounds (88-91) (the use of the dipole-dipole interaction model in conformational analysis was also described in the publication (284)). Calculations including n-n interactions (see Fig. 3.1) did not change this situation because the contribution of this effect is equal in magnitude for both limiting conformations shown in Chart 3.9. 

One is the steric effect. As in the case of amides, the E conformation is disfavored by the steric effect when R is equal to or larger than a methyl group. The second factor is the dipole moment. The E conformation is much more polar than the Z conformation. Because of these factors, esters usually assume the Z conformation and no E conformation is observed. To observe the latter, a special probe is needed. From the foregoing discussion, it is apparent that if R is hydrogen and R a large group, the E conformation might be present. Thus the E form of tm-butyl formate was detected by H NMR spectroscopy at low temperature (84). The barrier was found to be ca. 11 kcal/mol (85). 

The rotational spectrum of 1,2-dithiin was measured using a pulsed-beam microwave spectrometer in the 8-18 GHz range <1996JSP(180)139> by Stark effect measurements, the electric dipole moment was also determined (/ta = 1.85 D). The molecule proved to be of C2 symmetry with a twisted conformation about the S-S bond and a C-S-S-C dihedral angle of 53.9... 

Dioxin prefers the half-chair conformer 44 the corresponding boat and planar conformers proved to be less stable by 10.6 and 12.3 kcalmoP, respectively. If 4//-1,3-dioxin is alkyl-substituted at position 2, the same level of theory indicated the equatorial conformer 45-eq to be more stable than its axial counterpart, 45-ax (Me, 2.95 kcal moP Et, 2.89 kcal mol iso-Vi, 2.97 kcal moP neo-Vc,n, 2.16kcal moP and SiMc3, 4.45 kcal moP ) since the dipole moments of the two conformers are nearly equivalent, the position of the conformational equilibria of 2-alkyl-substituted 4//-1,3-dioxins is influenced by both steric effects (synclinal and H,H-, H,0-nonbonding interactions due to short C-O bonds) and a number of stereoelectronic orbital interactions. 

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