Group-contribution method

A group-contribution method in chemistry is a technique to estimate and predict thermodynamic and other properties from molecular structures.

A group-contribution method uses the principle that some simple aspects of the structures of chemical components are always the same in many different molecules.

The vast majority of organic components, for example, are built of carbon, hydrogen, oxygen, nitrogen, halogens (not including astatine), and maybe sulfur or phosphorus.

The next slightly more complex building blocks of components are functional groups, which are themselves built from few atoms and bonds.

The simplest form of a group-contribution method is the determination of a component property by summing up the group contributions

This is often done for the critical temperature, where the Guldberg rule implies that Tc is 3/2 of the normal boiling point, and the group contributions are used to give a more precise value: This approach often gives better results than pure additive equations because the relation with a known property introduces some knowledge about the molecule.

Commonly used additional properties are the molecular weight, the number of atoms, chain length, and ring sizes and counts.

[3] If the majority of group-contribution methods give results in gas phase, recently, a new such method[4] was created for estimating the standard Gibbs free energy of formation (ΔfG′°) and reaction (ΔrG′°) in biochemical systems: aqueous solution, temperature of 25 °C and pH = 7 (biochemical conditions).

The given pure component and mixture properties are then assigned to the groups by statistical correlations like e. g. (multi-)linear regression.

It can be used to estimate critical temperature, critical pressure, critical volume, standard ideal gas enthalpy of formation, standard ideal gas Gibbs energy of formation, ideal gas heat capacity, enthalpy of vaporization, enthalpy of fusion, normal boiling point, freezing point, and liquid viscosity.