Isomerism rotational

Rotational isomers can be conveniently represented by so-called projection formulae in which the two bonds (or groups of atoms) at the two ends are projected onto a plane which is perpendicular to the central bond. This plane is denoted by a circle whose center coincides with the projection of the rotation axis. The bonds in front of this plane are drawn as originating from the center, while the bonds behind this plane, i.e., the bonds from the other end of the rotation axis, are drawn as originating from the perimeter of the circle. 

Two important cases in rotational isomerism are distinguished by considering the nature of the central bond. When it is a double bond, rotation of one form into another is hindered by a high potential 

The symmetry of the cis isomer is characterized by two mutually perpendicular mirror planes generating also a two-fold rotational axis. This symmetry class is labeled mm. An equivalent notation is C2V as 

Two important cases in rotational isomerism may be distinguished by considering the nature of the central bond. When it is a double bond, rotation of one form into another is hindered by a very high potential barrier. This barrier may be so high that the two rotational isomers will be stable enough to make their physical separation possible. An example is 1,2-dichloroethylene (3-4). 

The symmetry of the cis isomer is characterized by two mutually perpendicular mirror planes generating also a twofold rotational axis. This symmetry class is labeled mm. An equivalent notation is C2,., as will be seen in the next section. The trans isomer has one twofold rotation axis with a perpendicular symmetry plane. Its symmetry class is 2/m (C )- 

The arrangement of a single chain without regard to its neighbour rotational isomerism. 

The arrangement of chains with respect to each other orientation and crystallinity. 

Since the energy barriers between the conformers at room temperature are small compared to the product kT, the respective molecules exist as an equilibrium mixture of different confonners. 

Different conformations of a molecule are usually represented by different spectroscopic signals, so that the respective concentrations can easily be derived from the intensities of the respective signals. The temperature dependence of the equilibrium constant of the conformational equilibrium affords the enthalpy and entropy of rotation about the single bond, as shown in this text. 

Assuming that AH and AS are constant within a limited temperature range, these quantities are accessible if K can be detennined at least at two temperature values. 

If In K is plotted against l/T, AH and AS may be obtained from the slope and the intercept of the straight line with the ordinate 

The exact determination of AH and AS by this general approach requires knowledge of the temperature dependence of Ag ,uae/A .ans as well as that of rtgauche/atrans the latter value can only be determined indirectly (de Meijere et al., 1974 Braun and Liittke, 1975). 

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Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

Sen S, Cohen J M, McCoy J D and Curro J G 1994 The structure of a rotational isomeric state alkane melt near a hard wall J Chem. Phys. 101 9010... 

Rehahn M, Mattice W L and Suter U W 1997 Rotational isomeric state models in macromolecular systems Adv. Polym. Sc/. 131/132... 

The rotational isomeric state (RIS) model assumes that conformational angles can take only certain values. It can be used to generate trial conformations, for which energies can be computed using molecular mechanics. This assumption is physically reasonable while allowing statistical averages to be computed easily. This model is used to derive simple analytic equations that predict polymer properties based on a few values, such as the preferred angle... 

RIS (rotational isomeric state) a polymer simulation technique... 

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... 

Selected physical properties of chloroprene are Hsted in Table 1. When pure, the monomer is a colorless, mobile Hquid with slight odor, but the presence of small traces of dimer usually give a much stronger, distinctive odor similar to terpenes and inhibited monomer may be colored from the stabilizers used. Ir and Raman spectroscopy of chloroprene (4) have been used to estimate vibrational characteristics and rotational isomerization. 

Furan-2-carbonyl chloride, 5-alkyl-3,4-dichloro-synthesis, 4, 690 Furancarboxamides rotational isomerism, 4, 543 Furan-2-carboxylic acid, 5-acetylamino-ethyl ester reactions, 4, 647 Furan-2-carboxylic acid, amino-properties, 4, 708 Furan-2-carboxylic acid, 5-bromo-nitration, 4, 603, 711 Furan-2-carboxylic acid, 3-methyl-methyl ester bromination, 4, 604 Furan-2-carboxylic acid, 5-methyl-nitration, 4, 602... 

The IR spectra of several enaminoketones have been reported, and a study of these spectra has shown strong coupling between the C=0 and C=C stretching vibrations and band splitting due to rotational isomerism (7SciJJb). 

Moller,M. Cross Polarization — Magie Angle Sample Spinning NMR Studies. With Respect to the Rotational Isomeric States of Saturated Chain Molecules. Vol. 66, pp. 59 — 80. 

With Respect to the Rotational Isomeric States of Saturated Chain Molecules... 

Rotational Isomeric States in Medium Size Cycloalkanes.67... 

The purpose of this article is to discuss conformational shift variations in some selected solid state spectra on the basis of discrete rotational isomeric states and to compare them with molecular packing effects. 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

Fig. 18. Staggered rotational isomeric states of the CH—CH bond of threodiisotactic poly(l,2-dimethyltetramethylene)...

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

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