Effects and Intermolecular Interaction

Fundamental theories of transport properties for systems of finite concentration are still rather tentative (24). The difficulties are accentuated by the still uncertain effects of concentration on equilibrium properties such as coil dimensions and the distribution of molecular centers. Such problems are by no means limited to polymer solutions however. Even for the supposedly simpler case of hard sphere suspensions the theories of concentration dependence for the viscosity are far from settled (119,120). 

The Huggins constant k characterizes the first effects of interaction on the zero-shear viscosity  

Experimentally k is essentially independent of molecular weight for long chains, with values of roughly 0.30-0.40 in good solvents and 0.50-0.80 in theta solvents. 

Theories of k for random coil molecules are very difficult and still somewhat lacking in experimental confirmation (24,121). 

The culmination of this trend is illustrated in Fig. 5.2 by dynamic data on undiluted polystyrene of low molecular weight (124). Agreement with the Rouse model here is by no means as good as that seen in Fig. 5.1 with the Zimm model for a high molecular weight polystyrene at infinite dilution. Indeed, the value of Je° deduced from G (to) for the sample in Fig. 5.2 exceeds the value from 

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Torii H, Tasumi M. Raman noncoincidence effect and intermolecular interactions in liquid dimethyl sulfoxide simulations based on the transition dipole coupling mechanism and liquid structures derived by Monte Carlo method. Bull Chem Soc Jpn 1995 68 128-134. 

In the MAS NMR spectra of histidine-dipeptides, the C chemical shifts for and C span a large range (up to 13 ppm) and are highly influenced by the tautomer effect and intermolecular interactions <2005JA12544>. The imidazole ring chemical shifts of 50 (158.6, 123.8 (Vcf = 36.0), 131.0 ppm, Scheme 14) resemble more closely those of l-methyl-5-(/i-tolyl)-4-trifluoromethyl-l//-imidazole (137.6, 128.8 ( cF = 37.4Hz), 133.0ppm). Based on this observation, it was proposed that in the solid state 50 most likely assumes the tautomeric form 50a rather than 50b <2001JHC773>. 

The band positions associated with the various vibrations are primarily dependent on mass- and force-constant effects. They are sensitive to small, but highly specific perturbations, induced by effect of stereo chemistry, mechanical coupling, intramolecular electrical effects, and intermolecular interaction. Thus, the physical state of the compound, liquid, amorphous solid, crystalline solid (including polymorphism) may alter the frequency of absorption. When using band-by-band correlation with a standard compound for identification of an unknown, these factors should be considered. More detailed information appears in the textbooks on infrared spectroscopy (48-53). 

Haigh and Mallion used CCI4 as a solvent and measured either several samples of increasing dilution or - where this was not possible - very dilute solutions. Thus solvent effects and intermolecular interactions of solute molecules could be nearly eliminated. Normally these effects are the reason for a somewhat less satisfactory agreement between theory (which deals with isolated molecules) and experiment for hydrogen chemical shifts. Since the chemical shifts can be calculated up to such a precision the calculated chemical shift tensors, especially their out-of-plane components, can be used further to analyze some of the empirical ring current models in more detail (see also Ref. 56 for comparable studies). 

Finally, there are groups of liquid crystals where, at the current time, force fields are not particularly useful. These include most metal-containing liquid crystals. Some attempts have been made to generalise traditional force fields to allow them to cover more of the periodic table [40, 43]. However, many of these attempts are simple extensions of the force fields used for simple organic systems, and do not attempt to take into account the additional strong polarisation effects that occur in many metal-containing liquid crystals, and which strongly influence both molecular structure and intermolecular interactions. 

The general or universal effects in intermolecular interactions are determined by the electronic polarizability of solvent (refraction index n0) and the molecular polarity (which results from the reorientation of solvent dipoles in solution) described by dielectric constant z. These parameters describe collective effects in solvate s shell. In contrast, specific interactions are produced by one or few neighboring molecules, and are determined by the specific chemical properties of both the solute and the solvent. Specific effects can be due to hydrogen bonding, preferential solvation, acid-base chemistry, or charge transfer interactions. 

Tyrosine fluorescence emission in proteins and polypeptides usually has a maximum between 303 and 305 nm, the same as that for tyrosine in solution. Compared to the Stokes shift for tryptophan fluorescence, that for tyrosine appears to be relatively insensitive to the local environment, although neighboring residues do have a strong effect on the emission intensity. While it is possible for a tyrosine residue in a protein to have a higher quantum yield than that of model compounds in water, for example, if the phenol side chain is shielded from solvent and the local environment contains no proton acceptors, many intra- and intermolecular interactions result in a reduction of the quantum yield. As discussed below, this is evident from metal- and ionbinding data, from pH titration data, and from comparisons of the spectral characteristics of tyrosine in native and denatured proteins. 

Wolfsberg, M. Isotope effects on intermolecular interactions and isotopic vapor pressure differences. J. Chemie Physique 60, 15-22 (1963)... 

The magnitudes of the computed A and AG are relatively small for all four of the oxime isomerizations that have been considered. Thus the effects of intermolecular interactions with the surroundings—whether the pure liquid or solid phases or a solvent—may often determine which isomer is more stable in a condensed phase. (This point will be addressed again in Section VII.) For example, benzaldoxime is known to be in the anti form in an acidic environment but syn in a basic one . Temperature also plays a role in affecting K q. It should further be noted, as can be seen in Table 4, that even AE and AG in the neighborhood of just 3 kcalmol can produce Teq of two orders of magnitude. 

The effect of intermolecular interactions can be readily observed when comparing the absorption spectrum of a molecule in solution to that in the solid state. In solution, where the molecules can be considered as isolated, the spectra are characterized by sharp lines corresponding to absorption bands. However, in the solid, intermolecular interactions cause the formation of exciton bands and splitting of the levels. This phenomenon is often referred to as Davydov splitting. This splitting is thus a measure of the strength of the interactions and for MOMs it can amount to 0.2-0.3 eV. 

Highly fluorinated molecules have a nonpolar character and an extremely low polarizability, inducing only weak intra- and intermolecular interactions. As a consequence, perfluorocarbons behave almost like ideal liquids they are very compressible and have very high vapor pressure. For example, the physical properties of perfluoro-hexane, heptafluorohexane, and hexane are reported in Table 1.2 The effect of the polar character of the hemifluorinated compound (heptafluorohexane) on the dielectric constant value is remarkable. 

In addition to the above effects, the intermolecular interaction may affect polymer dynamics through the thermodynamic force. This force makes chains align parallel with each other, and retards the chain rotational diffusion. This slowing down in the isotropic solution is referred to as the pretransition effect. The thermodynamic force also governs the unique rheological behavior of liquid-crystalline solutions as will be explained in Sect. 9. For rodlike polymer solutions, Doi [100] treated the thermodynamic force effects by adding a self-consistent mean field or a molecular field Vscf (a) to the external field potential h in Eq. (40b). Using the second virial approximation (cf. Sect. 2), he formulated Vscf(a), as follows [4] ... 

Intra- and intermolecular interaction effects on spin-spin coupling constants... 

The TB MO calculation on the 15N chemical shift of polypyrrole in the solid state allows useful information to be extracted from the observed spectra, namely that the two peaks obtained are correctly assigned to the quinoid and aromatic structures.(l 1,38) ( The quinoid structure is closely to the electric conductivity.) A decrease in the band gap leads to a downfleld shift. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electric conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of NMR nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered. 

For gases, n = e 1 is an excellent approximation. The easiest approach to condensed phases maintains this approximation, where calculations of the molecular first-order and response properties are performed for the isolated molecule, while accounting for the effect of intermolecular interactions through the number density N = Aa/Vm, and therefore by taking appropriate values of Vm. This rough, often at best qualitative, approach is somewhat relaxed by employing expansions of the birefringence constant with the density, that is in inverse powers of Vm. This introduces the appropriate virial coefficients [15,16]... 

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