Abraham equation

from Wikipedia, the free encyclopedia

The Abraham equation is a special form of the linear solvation energy relationship (LSER). In these, solvatochromatic parameters were originally used to describe the dependence of the UV / Vis absorption bands of a substance on the polarity of the solvent. Another example of LSERs is the Hammet relation , which characterizes the quantitative relationship between the structure of chemical reactants and their reactivity.

Michael Abraham developed a form of LSER, the solution properties P of substances, such as water solubility and distributions between two phases (e.g. octanol-water partition coefficient ), with system-specific parameters (also phase parameters, referred to as e, s, a, b, v and regression constant c ) and molecular properties (descriptors; E , S , A , B , V ) linked. The molecular properties describe intermolecular interactions of neutral molecules such as van der Waals interactions and hydrogen bonds . The electrostatic interactions are not taken into account.

Using multilinear regressions , the system parameters for an Abraham equation can be calculated from a sufficiently large training set of experimentally determined solution properties. In this way, a prediction equation is obtained with which the solution parameters for chemicals with known molecular descriptors can be theoretically estimated.

the equation

To obtain an Abraham equation, a set of chemicals is necessary whose solution properties (e.g. the octanol / water partition coefficient log K ow ) and molecular descriptors are known. The molecular descriptors can be obtained from appropriate databases, calculated quantum chemically or partially determined experimentally. The system parameters are determined from the so-called training set with multilinear regression and the Abraham equation results in the general form:

The system parameters are completely independent of the chemicals under consideration and describe the system. The molecular descriptors are defined as follows:

E = excess molar refraction. It is theadditional molar refractive index at 20 ° Ccompared to a hypothetical pure alkane of the same size. E is therefore zero for alkanes as well as for branched aliphatic alkanes and noble gases. The associated substrate-solvent interaction is mediated by n and pi electrons. E is based on the strength of induced dipoles and thus corresponds to the dispersion interaction ( London interaction ) as part of the Van der Waals interaction.
S = effective (dis) polarity and polarizability . It was originally introduced by Kamlet (1977) as a combined descriptor that characterizes the frequency shift induced by a solvent. S is based on substrate-solvent- dipole-dipole or dipole-induced dipole interactions and thus corresponds to the Debye and Keesom contributions to the Van der Waals interaction. The determination is based on quantum chemical calculations, as described by Arey 2005 or experimentally by gas-liquid chromatography on polar stationary phases.
A = acidity , total effective strength of all hydrogen bond (H) donors in the molecule. The acidity A assumes the value zero for substances that lack any H-donor capacity (e.g. alkanes). Monofunctional substances with a very high hydrogen donor strength are assigned the value A = 1 (e.g. pentachlorophenol ).
B = basicity , effective total strength of all hydrogen bond (H) acceptors in the molecule. The basicity B assumes the value zero for substances that lack any hydrogen donor capacity (e.g. alkanes).
Both A and B can be determined experimentally via HPLC or by theoretical methods (e.g. 3D supermolecular approach)
V = characteristic McGowan volume (unit: m³ / 100 mol). This is determined from a simple fragment model or from group additive methods. V was originally introducedfor liquid-liquid systems and replaced for gas-liquid systems by the logarithmic hexadecane- air distribution coefficient at 20 ° C L (unit m³ / m³).

Individual evidence

  1. ^ MH Abraham, GS Whiting, RM Doherty, W. ShuelyJ 1990. Hydrogen bonding .13. A New Method for the Characterization of GLC Stationary Phases - the Laffort Data Set. J. Chem. Soc. Perkin Trans. 2: 8, 1451-1460.
  2. ↑ a b c J. S. Arey, WH Green, PM Gschwend: The electrostatic origin of Abraham's solute polarity parameter in J. Phys. Chem. B 109 (2005) 7564-7573.
  3. ↑ AD Gunatilleka, CF Poole, 1999. Models for estimating the non-specific aquatic toxicity of organic compounds. Anal. Commun. 36: 235-242.
  4. ↑ MJ Kamlet, JL Abboud, RW Taft, The solvatochromic comparison method. 6. The π * scale of solvent polarities. J. Am. Chem. Soc. 1977, 99, 6027-6038.
  5. ^ MH Abraham, JC McGowan 1987. The Use of Characteristic Volumes to Measure Cavity Terms in Reversed Phase Liquid Chromatography. Chromatographia 23: 4, 243-246.