Molecular organization rotation

Low frequency dielectric studies on smectic C, F and I phases are complicated by the intrinsic biaxiality of these phases. It is possible to use dielectric measurements to determine the tilt angle in SmC materials [27], but of more interest is the direct determination of the three principal components of the dielectric tensor, since such measurements can give additional information on the local molecular organization from Eq. (29). The orientation of the principal axes for tilted smectic phases is not determined by symmetry, except that one principal axis coincides with the C2 rotation axis perpendicular to the tilt-plane. It is assumed that a further principal axis lies along the tilt direction, and this appears to be justified by experiment the orientation of the axes are indicated in Fig. 9. 

Since the first publications on this subject in 1963, NMR in liquid crystalline systems has been a wide and active field of research in many branches of organic and physical chemistry. In fact, NMR spectroscopy has revealed a powerful means of probing molecular structure, anisotropic magnetic parameters and dynamic behaviour of solute molecules dissolved in liquid crystals. Moreover, this technique has been successfully employed to investigate properties of mesophases themselves, such as their orientational ordering, translational and rotational diffusion and their effects on nuclear relaxation, and molecular organization in different liquid crystalline phases. 

A comparison of rotational and translational diffusion results obtained in l-octyl-3-imidazolium tetrafluoroborate, [omim][BF4], and in 1-propanol and isopropyl benzene has been given for TEMPONE. Measurements at different temperatures and concentrations indicate that rotational motion can be described by isotropic Brownian diffusion only for the classical organic solvents used, but not for the IL. Simulation of the EPR spectra fit with the assumption of different rotational motion around the different molecular axes. Rotational diffusion coefficients >rot follow the Debye-Stokes-Einstein law in all three solvents, whereas the translational diffusion coefficients do not follow the linear Stokes-Einstein relation D ot versus Tlr ). The activation energy for rotational motions Ea,rot in [omim][BF4] is higher than the corresponding activation energies in the organic solvents. 

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... 

Double bonds and their influence on molecular shape are vitally important for living organisms. For instance, they enable you to read these words. Vision depends on the shape of the molecule retinal in the retina of the eye. cis-Retinal is held rigid by its double bonds (41). When light enters the eye, it excites an electron out of the iT-bond marked by the arrow. The double bond is now weaker, and the molecule is free to rotate about the remaining o-bond. When the excited electron falls back, the molecule has rotated about the double... 

Given the specific, internuclear dipole-dipole contribution terms, p,y, or the cross-relaxation terms, determined by the methods just described, internuclear distances, r , can be calculated according to Eq. 30, assuming isotropic motion in the extreme narrowing region. The values for T<.(y) can be readily estimated from carbon-13 or deuterium spin-lattice relaxation-times. For most organic molecules in solution, carbon-13 / , values conveniently provide the motional information necessary, and, hence, the type of relaxation model to be used, for a pertinent description of molecular reorientations. A prerequisite to this treatment is the assumption that interproton vectors and C- H vectors are characterized by the same rotational correlation-time. For rotational isotropic motion, internuclear distances can be compared according to... 

Many fluorophores are sensitive to changes in the hydropho-bicity of the immediate environment. Therefore, bringing these fluorophores into a different environment may also produce a change in FRET, when a second fluorophore is affected by the emission change of the first. Fluorophores like Nile Red with changes of up to 100 nm when transferred from water to an aprotic organic solvent are principally suitable for such an approach [71], Molecular rotors have the characteristic of having a quantum yield that depends on the viscosity. Such dyes are formed by an electron donor unit and an electron acceptor unit that can rotate relative to each other upon photoexcitation with a behavior that depends on the viscosity of the environment. These dyes have been included in FRET probes for viscosity studies [53],... 

In conclusion, for condensed phases molecular rotations have quite a short lifetime, because of collisions. The eventual oscillations induced by the electric field are then dissipated in the liquid state leading to vibration. At collision densities corresponding to liquids the frequency of the collisions become comparable with the frequency of a single rotation, and because the probability of a change in rotational state on collision is high, the time a molecule exists in a given state is small. It is, therefore, obvious that the electric field cannot induce organization in condensed phases such as in the liquid state. 

---