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 H2O and CH4 (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.