However, the calculated adduct formation energy between pyridine and pyrosulfate of -39.7 kcal mol -1 , while significantly more exothermic than pyridine binding to a Zr-Lewis site (-32.3 kcal mol -1 ), is weaker than expected for a strong Lewis acid. For example, the very strong tricoordinate aluminum Lewis site on Al2O3, which activates C-H bonds by a heterolytic mechanism, 40 forms an adduct with pyridine that has a calculated adduct formation energy of up to -50 kcal mol -1 . Two explanations can account for this discrepancy. First, adduct formation between pyridine and pyrosulfate should not be viewed as classical adsorption onto a "bare" Lewis site on an oxide. The calculated adduct formation energy is a composite the strong NàS interaction (exothermic) and the breaking of the S-O bond in the pyrosulfate (endothermic). This reduces the overall adduct formation energy compared to the more standard adsorption energy onto a bare Lewis site because bonds are generally not broken in the latter case. Second, sterics probably reduces the overall binding energy in py*SZO compared to the more sterically accessible oxygen in TEPO or the C-H bond in Ph3CH. This is reflected in the sum of the O-S-O bond angles (SOSO). In py*SZO the SOSO is 348.4 o , which is far larger than SOSO for TEPO*SO3 (332.85 o ) or Ph3CH*SO3 (330.8 o ).