The use of molecular graphics and quantum mechanically derived electrostatic potentials to visualize Brönsted acid...

The use of the quantum mechanically derived electrostatic potential is introduced to visualize the variation in acidity across a series of cluster molecules corresponding to models of terminal and bridged hydroxyl groups in zeolites. The electrostatic potential is color coded onto the electron density surfaces of the molecules involved, and the variation in acid strength can be followed from the color scheme used. This simple visualization scheme is shown to mirror the experimentally observed acidity variation of terminal hydroxyls, bridged gallium hydroxyls, and bridged aluminium hydroxyls found for zeolites. From the value of the electrostatic potential near the acidic hydrogen we estimate that the acidity of terminal hydroxyls is similar to acetic acid while the bridged aluminium form is the same as sulphuric acid.