Negative enhancement

As the Gibbs standard free energy for the peptide-nonpolar ligand interaction decreases, and AGassoci becomes increasingly negative, enhanced retention will occur. The relationship between the relative retention of a specific peptide in a RPC separation process carried out at constant pressure, P, and constant molar volume, V, can thus be expressed in terms of the following well-known, fundamental thermodynamic relationship ... 

We studied these phenomena experimentally in a wetted wall column and two stirred cell reactors and evaluated the results with both a penetration and a film model description of simultaneous mass transfer accompanied by complex liquid-phase reactions [5,6], The experimental results agree well with the calculations and the existence of the third regime with its desorption against overall driving force is demonstrated in practice (forced desorption or negative enhancement factor). 

Consequently the enhancement factor of CO2 yields negative values. Previously we defined this phenomenon as forced desorption or negative enhancement factor [2,6]. No experimental evidence of this phenomenon has been available until now. 

Figure 8.14. The three-spin steady-state effect. The negative enhancement at C arises from an indirect effect via spin B when spin A is saturated. The altered C-spin populations also contribute to the enhanced NOE at B (cf. Fig. 8.12).

A similar series of experiments have been performed on sp hybridized atoms,but all enhancements were found to be negative. However it was found that in Cl2C =C HCl, the is more negatively enhanced that the doublet, which does not appear to be in accordance with the results of sp hybridized atoms.Since the dipolar correlation time for both the carbon atoms could be expected to be similar, it is argued that the increased scalar interaction with... 

Fig. 2 (a) Population distribution at thermal equilibrium for a general three-spin system, (b) Saturation of the allowed EPR transitions for one of the dipolar coupled electrons (cuoei) leads to negative enhancement, (c) Saturation of the transition corresponding to the second electron (cuoe2) leads to positive enhancement. Mei, Me2, and are the spin states of electron 1,2, and nucleus. Reproduced with permission from [13]... 

Figure 2 shows the thermal equilibrium spin population for a three-spin system. Generally there is no degeneracy present. When an appropriate polarizing agent is used, the energy levels IIV > and IV > or IVI > and IIII > become degenerated (Fig. 2a). Irradiation of EPR transition and CE transitions leads to positive (Fig. 2b) or negative enhancement (Fig. 2c) of the nuclear polarization.

Emulsions consist of hydrophilic and hydrophobic ingredients. Therefore emulsions are not physically stable and the phases may separate into a water layer and a fatty layer. The physical stability of emulsions can be increased by decreasing the size of the droplets of the inner phase, by increasing the viscosity of the outer phase and first of all by decreasing the surface tension between the aqueous and the lipophilic phase. The presence of an active substance can influence the stability of an emulsion negatively. Enhancing the viscosity of the aqueous phase may increase the physical stability of an emulsion. Thereby sometimes the percentage of fatty phase can be decreased. An example is the emulsimi base in Table 12.7, where carbomer has been used for the viscosity enhancement. 

Fig. 8 Experimental DNP enhancement profiles for the SE and the CE mechanisms showing the positions of positive and negative enhancement and their dependence on the microwave irradiation frequency (or, rather the magnetic field for a given microwave frequency) as well as on the electron and nuclear Larmor frequencies coos and too/, respectively, (a) A tj/pical SE enhancement profile obtained with 40 mM trityl. (b) A typical CE enhancement profile obtained with 10 mM TOTAPOL (20 mM electrons). The EPR spectrum of each radical is shown on top. The lines connecting the data points are to guide the eye. Reprinted with permission from [206]. Copyright 2012, American Institute of Physics...

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