Sterimol parameters are built upon the Corey-Pauling-Koltun atomic models, which take into consideration the Van der Waals radii of each atom in the molecule. Unlike most other steric parameters such as A-value, Taft parameters and Tolman cone angle, which group all the spatial information into a single cumulative value, Sterimol parameters consist of three sub-parameters: one length parameter (L), and two width parameters (B, B). The three parameters add together to profile the 3-dimentional spatial information of a molecule.
In order to define the Sterimol parameters of a molecule, an axis needs to be defined at first. Since Sterimol parameters are usually applied for describing the bulkiness of a certain substituent which is attached to the substrate, the default choice of the axis is the one that passes through the atoms which link the substrate and substituent together. This axis is defined as the X-axis.
Once the X-axis has been defined, the Sterimol parameters can be assigned. Take the 1,2-dimethylpropyl group as an example (Figure 1). The length parameter (L) refers to the farthest extension of the substituents in the direction parallel to the X axis (shown in Figure 1, left). The width parameters can be assigned from the point of view which is perpendicular to the X axis. The width parameter B refers to the minimal profile width of the substituents on the linking atom from the X axis, while parameter B refers to the maximal width from the same axis (shown in Figure 1, right).
Sterimol B–B parameters were initially used for obtaining the maximal width. However, in his second generation Sterimol approach, Verloop pointed out that due to their directional dependence on Sterimol B, discrepancies arose when computing those three parameters in cases where B can point to multiple directions. Since Sterimol B and B hardly contributed significantly to any regression functions obtained, and Sterimol B was practically equal to B, the parameters B–B were omitted.
Sterimol B parameter demonstrates the steric effects imposed by branching at the linking atom of a substituent. The more branches the linking atom bears, the larger Sterimol B value the substituent has. On the other hand, Sterimol B parameter is more susceptible to the steric effects of the substituent's terminus. In general, Sterimol B represents vicinal steric effects of the substituent, while Sterimol B represents remote steric effects.
Several open-source programs have already included the feature of calculating Sterimol parameters, such as Morfeus, Kallisto, and dbstep.