Molecules rigid-polar

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

Below in Sections III—V we consider linear rigid polar molecules (rotators), while in Sections VI and VII a dipole is assumed to be nonrigid. 

The librational fraction is discussed in the context of the concepts of the water structure The hat potential models the defects of the water (ice) structure and rigid polar molecules reorient relatively freely in these defects. In the case of water the lifetime Tor of this fraction (on the order of 10 13 s) is several times greater than that of the H-bond. 

A. Exact Expressions for e for Rigid Polar Molecules in Terms of the Pair... 

Actually, such bowl phases are still to be found. However, polar achiral phases have been observed in the so-called polyphilic compounds [8]. The rod-like molecules of these compounds consist of distinctly different chemical parts, a hydrophilic rigid core (a biphenyl moiety) and hydrophobic perlluoroalkyl- and alkyl-chains at opposite edges. Such molecules form polar blocks that, in turn, form a polar phase manifesting pyroelectric and piezoelectric properties with a field-induced hysteresis characteristic of ferroelectric phases. 

BRATOS - The polarizability in any event plays a major role in this process. If the theory is built starting from protons and oxygen nuclei or starting from our rigid molecules the polarization effects would be described in different terms. Nevertheless they must necessarily be considered whatever the description one may choose ... 

Vibrational effects on optical rotation may be predicted by zero-point vibrational correction as developed by Ruud et al. however, solvent effects remain an important issue to be addressed and may be considered one of the most important sources of deviations between experimental and calculated data. In solutions, solute-solvent interactions play an important role in conformer populations of flexible molecules. In the case of rigid molecules, solvent polarization effects may be the leading cause of deviations. To date, the use of the polarizable continuum model (PCM) seems to be the best alternative to predicting solvent effects on [q ]d nevertheless, it should be used with caution because the results for carbon tetrachloride, benzene, and chloroform showed poor agreement with observed data. ... 

R = (A7)n=o/(A/)n=9o a value of -2 is obtained with rigid molecules. For polar, flexible molecules this ratio is a complicated function of/V and is far from —2. 

The simulations to investigate electro-osmosis were carried out using the molecular dynamics method of Murad and Powles [22] described earher. For nonionic polar fluids the solvent molecule was modeled as a rigid homo-nuclear diatomic with charges q and —q on the two active LJ sites. The solute molecules were modeled as spherical LJ particles [26], as were the molecules that constituted the single molecular layer membrane. The effect of uniform external fields with directions either perpendicular to the membrane or along the diagonal direction (i.e. Ex = Ey = E ) was monitored. The simulation system is shown in Fig. 2. The density profiles, mean squared displacement, and movement of the solvent molecules across the membrane were examined, with and without an external held, to establish whether electro-osmosis can take place in polar systems. The results clearly estab-hshed that electro-osmosis can indeed take place in such solutions. 

Molecules do not consist of rigid arrays of point charges, and on application of an external electrostatic field the electrons and protons will rearrange themselves until the interaction energy is a minimum. In classical electrostatics, where we deal with macroscopic samples, the phenomenon is referred to as the induced polarization. I dealt with this in Chapter 15, when we discussed the Onsager model of solvation. The nuclei and the electrons will tend to move in opposite directions when a field is applied, and so the electric dipole moment will change. Again, in classical electrostatics we study the induced dipole moment per unit volume. 

In practice a polar molecule is not a rigid dipole but suffers some distortion in the field. In this case we shall assume that in the step (a) not only is the axis of every dipole held in a fixed position, but also the distortion of the molecule is held at a fixed value. During this step (a) the total polarization P of the dielectric will remain constant. 

Fig. 2a-f. Mesogenic molecules with differing degrees of polar and sterical asymmetry a symmetric molecule with rigid core and two hydrocarbon tails b terminally polar molecule (arrow indicates the permanent dipole) c swallow-tailed (hiforked) molecule d hanana shaped molecule e terminally fluorinated molecule f polyphilic molecule (hatched areas correspond to the fluorinated fragment)... 

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