Small molecule mixtures

In this chapter, we start with a brief overview of the standard methods currently used for the NMR-spectroscopic identification of natural products and other types of organic small molecules, which is followed by a section dedicated to NMR spectroscopic characterization of small molecule mixtures and a discussion of approaches to increase NMR spectroscopic sensitivity. 

Comparing NMR spectroscopic characterization of proteins and small molecule mixtures... 

Figure 3.24C shows a type-IV P-T diagram that can occur with small molecule mixtures if the disparity in the size and/or the intermolecular... 

During the later stages one observes that the coarsening behavior changes gradually - an apparent temperature-dependent exponent x in the relation qm oc t x is found in the intermediate stage , see Fig. 8a, while the behavior in the transition stage [36] is similar to the behavior of small molecule mixtures... 

Finally, we will briefly discuss the properties of polymer blends under shear flow. In small molecule mixtures, shear flow is known to produce an anisotropy of critical fluctuations and anisotropic spinodal decomposition [244, 245], In polymer mixtures, the shear has the additional effect of orienting and stretching the coils, thus making the single-chain structure factor anisotropic. In the framework of the Rouse model these effects have been incorporated into the RPA description of polymer blends [246, 247]. Assuming a velocity field v = yyex, where x, y, z are cartesian coordinates, y the shear rate, and ex is a unit vector in x direction, the single chain structure factor becomes [246, 247]... 

Here we rather focus on effects of external surfaces (e.g. hard walls) on polymer blends. In general, one expects that the forces between the wall and monomers of type A will differ from those between the wall and monomers of type B, as it generally occurs at the surfaces of small-molecule mixtures as well [365]. For polymer mixtures that are partially compatible, the interactions in the bulk (as described by the Flory-Huggins x-parameter) must be relatively small, however, since the entropy of mixing is down by a factor of N (for simplicity, the following discussion is restricted to a symmetric mixture, Na = Nb = N). However, there is no reason that the difference of wall-A and wall-B forces is similarly small [125]. Thus one may expect rather pronounced surface enrichment effects in polymer mixtures [125], Indeed some experimental evidence for this prediction has been found [37, 38, 126, 127]. 

Mercier KA, Powers R (2005) Determining the optimal size of small molecule mixtures for high throughput NMR screening. J Biomol NMR 31(3) 243-258... 

Specifically, the styrene-ether small-molecule mixture was used as a mixed solvent in Eq. (2.4.12). The resulting diffusivity value of the mixed solvent was assumed to be valid for styrene in the subsequent calculation of the propagation rate coefficient. This is a valid assumption since ether and styrene have enough polarity in them. 

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