Group Contribution Revisited: The Enthalpy of Formation of Organic Compounds with “Chemical Accuracy” accomplished

The Group Contribution (GC) approach is meanwhile old and was developed for many molecular properties. These quick and easy to use methods, in particular when a Graphical Interface is provided, and very beneficial to chemical process developers. One of these properties is the heat of formation ΔH_f or organic molecules. To appropriately describe chemical transformations and equilibria, the accuracy of the heat of formation must have chemical accuracy, i.e. 1 kcal/mol or better. Moreover, a method must be reliable which means there should be no or really very few outliers. So not only the absolute averaged deviation of the model values should be small, but each individual value should be within chemical accuracy. Up till recently GC methods for the heat of formation of organic molecules did not achieve this.

We have revised the GC approach for ΔH_f and have been able to achieve chemical accuracy. This was the result of a number of specific actions, mostly different from other implementations. First of all, in the procedure to determine the numerical values for the Group Contribution parameters we almost exclusively used reliable and consistent experimental data selected after consulting top-experts on experimental thermodynamics, which is crucial because of the 1 kcal/mol requirement. Secondly, the parameters were determined step-by-step, in fact by hand, so we could identify specific deviations which could, in a series of cases, be attributed to nearest or next-nearest neighbour interactions which were well understood based on concrete physico-chemical background. This resulted in an absolute minimum number (also compared to previous methods) of GC parameters which avoids overfitting and therefore improves predictability. A further aspect is that it is crucial to define the relevant size of the chemical Groups, rather than using the smallest possible entities. This enabled very good results which could not be obtained otherwise.

Finally, it is important to realize and to make explicit that certain effects, i.e. certain molecules, cannot be treated by a simple linear additive method as the GC method. Suggestions how to treat such molecules will be presented. More specifically, high level quantum calculations reveal useful data to close the gap.