Rigid molecules

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. 

A polysaccharide such as xanthan gum is a chain of sugars. Some familiar polysaccharides are starch and cellulose. The backbone of xanthan gum is similar to cellulose, but the trisaccharide side chains of mannose and glucuronic acid make the molecule rigid, and allow it to form a right-handed helix. These features make it interact with... 

The NO2 molecule offers an example which illustrates this point. The spectrum of N02 molecules rigidly held on MgO at —196° is characterized by gxx = 2.005, gyv = 1.991, and gzz = 2.002 (29). If this molecule were rapidly tumbling, one would expect a value of Qa.v — 1 999. The spectrum of NO2 absorbed in a 13X molecular sieve indicates an isotropic gzv = 2.003 (.80), which is within experimental error of the predicted value for NO2 on MgO. The hyperfine constants confirm that NO2 is rapidly tumbling or undergoing a significant libration about some equilibrium position in the molecular sieve (81). 

Imagine what would happen if you started with a block of ice at a low temperature, say -10°C, and slowly increased its enthalpy by adding heat. The initial input of heat would cause the temperature of the ice to rise until it reached 0°C. Additional heat would then cause the ice to melt without raising its temperature as the added energy was expended in overcoming the forces that hold H20 molecules rigidly together in the ice crystal. The amount of heat necessary to melt a substance is called the enthalpy of fusion, or heat of fusion (AHfusjon), and has a value of 6.01 kj/mol for H20. 

What do a melting ice cube and the reaction of barium hydroxide octahydrate have in common The common feature of these and all other spontaneous processes that absorb heat is an increase in the amount of molecular disorder, or randomness, of the system. The eight water molecules rigidly held in the Ba(OH)2 8 H20 crystal break loose and become free to move about in the aqueous liquid product similarly, the rigidly held H20 molecules in the ice lose their crystalline ordering and move around more freely in liquid water. 

Along with the attraction of the hydrogen bond, the fact that these strong bonds separate the water molecules rigidly causes the solid density to be less than that of the liquid. In ice only 34% of the volume is occupied by water molecules, in contrast to 37% volume occupation in liquid water this explains the unusual property of a decrease in density on freezing, accounting for the fact that ice floats. 

For propane, there is a much better agreement among all the calculations, which considered flexible molecules. The results of Novak etal [14], obtained will both the zeolite lattice and the alkane molecules rigid, show the largest discrepancy. 

Here the signs of the strain terms S, are chosen to conform with the signs for the two coordinates and respectively. Consider a tetragonal perturbation initially (S = 0). A positive value for will make one of the three minima (j> = 0) lower than the other two in Fig. 2a. Unless the value of Sg is very small, this stabihzation of one minimum will make the molecule rigidly localized at an elongated geometry. 

In the second type of semiflexible polymer molecule, rigid units are interspersed with flexible ones. Some examples of molecules of this kind are given in Chapter 11. The freely jointed chain model might be a suitable model for such semiflexible polymers. If a persistently flexible molecule and a freely jointed molecule are characterized by the same values of L and Xp (or, equivalently, of bx and Nk), then the gross statistical measures of the coil dimensions of the two such isolated chains will be the same, despite the differences in the type of flexibility. 

IV. Nonlinear Polyatomic Molecule (Rigid Rotator, Harmonic Oscillator]... 

The model shows that the geometry of ethyne is linear. The triple bond makes the molecule rigid. 

The van der Waals forces in the strict sense, also called dispersive forces, are the attractive forces between two neutral, nonpolar molecules, for example anthracene molecules, which thus have no static dipole moments. Were the charge dishibution within the molecules rigid, then there would indeed be no interactions between them. However, due to their temporally fluctuating charge distributions, they also have fluctuating dipole moments and these can induce dipoles in other molecules, compare Fig. 2.3. This results in an attractive force, as we aheady calculated in the section on inductive forces. To distinguish the two cases (of a permanent dipole and fluctuating dipoles), these forces due to fluctuations are also termed dispersive forces. 

Consequently, molecular segments, or, better still, whole molecular chains, must be oriented in some way and then fixed in position to form fibers or filaments. In principle, two procedures are suitable for this purpose spinning fluid systems and splitting oriented films. The spin technologies and final properties of the fibers or filaments depend on whether flexible chain molecules, rigid chains, or emulsions are to be spun. Whether spinning can be carried out with melts or solutions is another consideration. 

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