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Density functional theory (DFT) was used to investigate the acidity of the various silica alkyl sulfonic acids. In this regard, cluster models with various alkyl spacer lengths were selected to mimic the surface of silica gel. The effects of distance from the surface and the role of hydrogen bond (H–bond) on the ΔHacidityvalues of these catalysts were investigated. DFT calculations revealed that a notable gap of ΔHacidityvalues exists between the structures considering lateral hydrogen bonding with the surface of the silicaHB structureand the structures with omitted surface interactions (non-HB structures). Natural bonding orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses were carried out to obtain detailed information about the nature of the H–bonds. [ABSTRACT FROM AUTHOR]