Dipole moment and conformation

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

Values of molar Kerr constants and dipole moments of nitrogen azoles and their complexes with phenols have been obtained. " These complexes are formed by an intermolecular hydrogen bond between the pyridine-type nitrogen of the azole and the phenolic proton. " The use of dipole moments in conformational studies has shown that A-aryl- and C-aryl- and A-furyl- and C-furyl imidazoles (and benzimidazoles) are nonplanar, but l-(a-furyl)-4,5-diphenylimidazoles do have a planar bicyclic fragment. The dipole moments and conformations of azolides (A-acylazoles) have been studied. In the 1-arylimidazoles the dipole is toward the aryl group. In 4,5-di-t-butylimidazole the molecule is essentially planar, but the C-4—C-5 bond is slightly stretched. Among other imidazole derivatives which have been studied by X-ray are histidine hydrochloride, 4-acetyl-amino - 2 - bromo - 5 - isopropyl -1 - methylimidazole, 4- acetyl - 5 - methyl - 2 -phenylimidazole, and imidazole-4-acetic acid hydrochloride. 

The conformational characteristics of PVF are the subject of several studies (33,48). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. 

DIPOLE MOMENT AND CONFORMATIONAL ENERGY DATA OF MODEL DIAMIDES In the most general case for a diamide, 2-trans the two linkages lie in two planes pi and P2 (Figure 2). 

An appreciable amount of information concerning the conformational preferences of substituted heterocycles has accrued, largely through dipole moment and NMR studies. However, the earliest appreciation of this topic apparently arose out of the extension of studies of restricted rotation in biphenyls to heterocyclic analogues. 

Conformational populations of cyanomethylphosphine oxides (136) have been estimated from dipole moments and indicate a preference for the tra/15-conformation. The moments of the o-, m- and p-chloro- and tolyl-derivatives of triaryl phosphites (137, Y = ) and triaryl phosphates (137 Y = O) indicate that the oxygen atom in the latter series causes the aryl rings to rotate further away from a position in which their planes all meet along the molecular symmetry axis. Conformational studies have also been carried out on the dioxaphosphorinanes. The moments of the isomeric series (138a) and (138b) were in the ranges 3.7—4.2 and 5.4—5.5 D respectively. ... 

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... 

If a substance that can form a transmembrane channel exists in several conformations with different dipole moments, and only one of these forms is permeable for ions, then this form can be favoured by applying an electric potential difference across the membrane. The conductivity of the membrane then suddenly increases. Such a dependence of the conductivity of the membrane on the membrane potential is characteristic for the membranes of excitable cells. 

St.-Amant, A., W. D. Cornell, P. Kollman, and T. A. Halgren. 1995. Calculation of Molecular Geometries, Relative Conformation Energies, Dipole Moments, and Molecular Electrostatic Potential Fitted Charges of Small Organic Molecules of Biochemical Interest by Density Functional Theory. J. Comp. Chem. 16, 1483. 

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... 

I. Hargittai (Budapest) for electron diffraction. The CNDO/2 method proved able to reproduce accurately experimental data, mainly preferred conformations, rotational barriers, dipole moments, and other monoelectronic properties. This was the case for F2HP BH3 (3) (microwave study by Pasinski and Kuczkowski (4)), H3B CO (5), (CH3)H2P BH3 (6) and (CH3)3P BH3 (7) (microwave study by Bryan and... 

The dipole moment of 2,2 -bipyridine in benzene or carbon tetrachloride has been reported as less than 0.68, 0.91, 0.69, and 0.61 d. Because the conformation with the two nitrogen atoms transoid to each other should have a zero dipole moment and the cisoid configuration a value of 3.8 D, the consensus is that the molecule is in the transoid conformation and is approximately planar in solution with an angle of about 20° between... 

Both the measured dipole moments and H NMR spectra of 1,2-dithiane, its 4,4,5,5-tetradeutero analog, 3,3,6,6-tetramethyl-l,2-dithiane, and the m//ra r-isomers of 3,6-dimethyl-l,2-dithiane provided unequivocal evidence for the chair conformation adopted by the saturated six-membered ring, supported further by X-ray solid-state structures. Other conformers, for example the twist conformer, were not detected . 

Data on the apparent dipole moment and H NMR spectra of thianthrene-5-oxide 24 under different conditions of solvent and temperature support the rapid conformational equilibrium of 24, which flaps between two limit boat conformations with the sulfoxide group in a pseudoaxial or pseudoequatorial position, respectively. Variable-temperature... 

Energy barriers for internal rotation have been derived, especially during the 1950s, by analyzing (68M12 68M13) microwave spectra of molecules. The method works with molecules with a permanent dipole moment and in the gas phase. Limitations are dictated by the molecular size. The barriers are obtained from rotational energy levels of the molecule as a whole, perturbed by the internal rotor. When different conformers are present in the sample and their interconversion is slower than microwave absorption (barriers smaller than 20 kJ mol can be measured), the spectrum is just a superposition of the lines of the separate species which can be qualitatively and quantitatively determined. 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

A RIS model with neighbor dependence is used to calculate mean-square dipole moments and their temperature coefficients for PDMS chains over a wide range of molecular weight. Chain conformational energies required in the calculations are obtained from a previous analysis of the random-coil dimensions of PDMS chains in the limit of large x (S 116). 

The conformational energy E(op) associated with gauche states about CH2-CH2 bonds in polyethylene sulfide) (PSE) is estimated from the RIS analysis of experimental dipole moments and their temperature coefficients corresponding to the alternating copolymer of pentamethylene sulfide and ethylene sulfide (PXS) as well as to 1,2-bis(butylthio)ethane. 

Inductive effects on dipole moments and the effects of intervening atoms on electrostatic interaction energies are represented by polarizability centers In conjunction with bond centered dipoles. Solvation energies are estimated by means of a continuum dlpole-quadrupole electrostatic model. Calculated energies of a number of conformations of meso and racemic 2,4-dichloropentane and the iso, syndio, and hetero forms of 2,4,6-triehloroheptane give satisfactory representations of isomer and conformer populations. Electrostatic effects are found to be quite important. 

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