Affinity energy distribution

Gritti, F. et al. Determination of single component isotherms and affinity energy distribution by chromatography J. Chromatogr. A. 2003, 988, 185-203. 

Figure 3.30 Affinity energy distributions calculated with one hundred millions iterations from the experimental isotherm adsorption data measured by FA on six commercial Qg-bonded stationary phases, with methanol/water (30/70, v/v) as the mobile phase. T = 296 K. Reproduced with permission from F. Gritii, G. Guiochon, Anal. Chem., 75 (2003) 5726, Figure 4b). 2003, American Chemical Society.

These include multipole moments, molecular polarizabilities, ionization potentials, electron affinities, charge distributions, scattering potentials, spectroscopic transitions, geometries and energies of transition states, and the relative populations of various conformations of molecules. Some of these properties are directly related to molecular reactivity (e.g., charge distribution, molecular polarizabilities, scattering potentials), and they can be implemented in QSAR studies. Quantum mechanical methods can therefore be used to obtain reactivity characteristics in order to relate molecular structure to the observed biological activity (183, 230). 

When the surface of the stationary phase is heterogeneous (see Chapter 3, Section 3.3), the molecules adsorb on the surface with different affinity energies. With the stochastic model, one can take into accoimt rather flexibly the adsorption energy distribution of the various sites. 

A solution, which requires an advanced technology, is to improve experimental conditions so that the energy distributions approximate 5-functions. Leffert et al. (1972) describe a high resolution apparatus with a time-of-flight selected primary beam and a narrow perpendicular crossed beam. The first results show sharp onsets at threshold, indicating a small energy spread. The N02 electron affinity has been determined by this technique (Leffert et al., 1973). 

Figure 7 is an example of an affinity distribution, which is a measure of the number of binding sites (N) having a particular binding affinity (log K). The x-axis is usually plotted in log K format to make this axis proportional to the binding energy (AG), and for this reason, affinity distributions are also called site-energy distributions. 

In Eqs. (56), is the adsorption affinity at infinite temperature e and a are the mean and square root of variance of energy, respectively and 5 is the heterogeneity parameter related to the spread of the energy distribution. 

For a proton transfer reaction to proceed, the proton affinity (PA) (Table 5) of the analyte molecule M must exceed that of the reagent gas thus, the reaction between NH4 and pyridine will yield the protonated pyridine, while no proton-transfer reaction will occur between NH4 and water. The reaction products show little internal energy (depending on the PA difference between anal)d e and reagent gas) and a narrow internal energy distribution. As a result, generally little fragmentation is observed. 

Molecular properties molecular energies, heats of formation, ionization energies, electron affinities, charge distributions, electronic spectra (color), isopotential maps, volume, conformational freedom, dipole moments, molecular polarizabilities ... 

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