Kinetic energy of structural protons in silica xerogels

The kinetic energies of the protons in the silanol groups (Si-OH units) of silica xerogels were deduced by ab initio calculations using the basis set mp2/6-311G**. The silanol groups were simulated using the Si(OH) ₄ unit. The calculated result of the H-kinetic energy was found to be 150 meV, which is ∼ 50% smaller than a recently reported experimental value for porous silica xerogels. For comparison, the same calculations of the proton kinetic energies of other H-containing molecules such as H ₂O and CH ₄ (being also ∼ 150 meV) were found to be in excellent agreement with measurements. Possible reasons for the huge deviations in the case of the silanols are discussed.