Interactions, intermolecular

It is believed that the particles interact with each other and, moreover, may be subject to external influence. Interatomic forces are represented in the form of the classical potential force (the gradient of the potential energy of the system). The interaction between atoms is described by means of van der Waals forces (intermolecular forces), mathematically expressed by the Lennard-Jones (LJ) potential [75]  

Where r.j is the distance between ithandyth particles normalized by typical length o, and 8 is the potential well depth, which governs the strength of interaction. It should be noted that the second term corresponds to the attractive force, which is caused by 

This potential gives a reasonable representation of intermolecular interactions in noble gases, such as Ar and Kr, and systems composed of almost spherical molecules, 

Gough et al calculated the C-C and C-H stretch polarizability derivatives for straight chain, branched and cyclic hydrocarbons. Their interpretation of the CH-stretching intensities has been based on AIM, which they use to compute atomic populations in the presence and absence of an applied field. Interestirjgly strain is associated with a decrease in dajdrcc and an increase in da/drcH- 

The exact atomic electrostatic potential was calculated and compared to a multipole expansion using spherical tensors. The authors prove that the convergence of this expansion is faster than previously assumed, even for com- 

Dehez et al. probed the accxnacy of a novel approach for evaluating induction energies. Fully distributed polarizabihty models obtained with AIM are used to generate large grids of induction energies much more rapidly than with a finite perturbation approach. Test calculations show that this method is able to provide compact, yet optimal distributed polarizability models. 

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Claverie P 1978 Elaboration of approximate formulas for the interactions between large molecules applications in organic chemistry Intermolecular Interactions From Diatomics to Biopolymers ed B Pullman (New York Wiley) p 69... 

Mirsky K 1978 The determination of the intermolecular interaction energy by empirical methods Computing in Crystaiiography ed R Schenk ef a/(Delft, The Netherlands Delft University) p 169... 

Tokmakoff A, Lang M J, Jordanides X J and Fleming G R 1998 The intermolecular interaction mechanisms in liquid CS2 at 295 and 165 K probed with two-dimensional Raman spectroscopy Chem. Phys. 233 231-42... 

Palacin S, Lesieur P, Stefanelli I and Barraud A 1988 Structural studies of intermolecular interactions in pure and diluted films of a redox-active phthalocyanine Thin Soiid Fiims 159 83-90... 

Halgren T A 1996b. Merck Molecular Force Field II MMEF94 van der Waals and Electrostatic Parameters for Intermolecular Interactions. Journal of Computational Chemistry 17 520-552. 

The tests in the two previous paragraphs are often used because they are easy to perform. They are, however, limited due to their neglect of intermolecular interactions. Testing the effect of intennolecular interactions requires much more intensive simulations. These would be simulations of the bulk materials, which include many polymer strands and often periodic boundary conditions. Such a bulk system can then be simulated with molecular dynamics, Monte Carlo, or simulated annealing methods to examine the tendency to form crystalline phases. 

The velocity of sound in liquid thiazole was also measured, and the adiabatic compressibility was determined (302) it was concluded that intermolecular interactions result from the electrical forces, originating in the heteratoms, between the molecules. 

The effect of temperature in Monte Carlo simulations is primarily to modulate the strength of intermolecular interactions, since temperature enters the simulation only through the Boltzmann factor exp(-AE/kT), where AErepresents a difference in potential... 

Many of the unusual properties of the perfluorinated inert fluids are the result of the extremely low intermolecular interactions. This is manifested in, for example, the very low surface tensions of the perfluorinated materials (on the order of 9-19 mN jm. = dyn/cm) at 25°C which enables these Hquids to wet any surface including polytetrafluoroethene. Their refractive indexes are lower than those of any other organic Hquids, as are theh acoustic velocities. They have isothermal compressibilities almost twice as high as water. Densities range from 1.7 to 1.9 g/cm (l )-... 

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

Release agents function by either lessening intermolecular interactions between the two surfaces in contact or preventing such close contact. Thus, they can be low surface-tension materials based on aUphatic hydrocarbon, fluorocarbon groups, or particulate soHds. The principal categories of material used are waxes, fatty acid metal soaps, other long-chain alkyl derivatives, polymers, and fluorinated compounds. 

Dilute Polymer Solutions. The measurement of dilute solution viscosities of polymers is widely used for polymer characterization. Very low concentrations reduce intermolecular interactions and allow measurement of polymer—solvent interactions. These measurements ate usually made in capillary viscometers, some of which have provisions for direct dilution of the polymer solution. The key viscosity parameter for polymer characterization is the limiting viscosity number or intrinsic viscosity, [Tj]. It is calculated by extrapolation of the viscosity number (reduced viscosity) or the logarithmic viscosity number (inherent viscosity) to zero concentration. 

For condensed species, additional broadening mechanisms from local field inhomogeneities come into play. Short-range intermolecular interactions, including solute-solvent effects in solutions, and matrix, lattice, and phonon effects in soHds, can broaden molecular transitions significantly. 

Starches. Starch (qv) granules must be cooked before they wiU release their water-soluble molecules. It is common to speak of solutions of polysaccharides, but in general, they do not form tme solutions because of their molecular sizes and intermolecular interactions rather they form molecular dispersions. The general rheological properties of polysaccharides like the starch polysaccharides are described below under the discussion of polysaccharides as water-soluble gums. Starch use permeates the entire economy because it (com starch in particular) is abundantly available and inexpensive. Another key factor to its widespread use is the fact that it occurs in the form of granules. 

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