OH/OD Isotopic Shift Factors of Isolated and H-Bonded Surface Silanol Groups

The OH/OD isotopic shift factors (i = nu(OH)/nu(OD)) of isolated silanols on SiO2 and [Si]BEA are between 1 3563 and 1.3568, values lower than the theoretical shift of 1.3744. However, i of the harmonic OH modes almost coincides with the theoretical value which indicates that the experimental deviations in this case are mainly due to anharmonicity. The anharmonicity slightly decreases when the OH groups participate in weak H-bonding with adsorbed CH4 or CO which should lead to an increase of i. However, contrary to these expectations, i additionally decreases. This is attributed to the lower acidity of the OD groups as compared to the respective OHs. The value of i is also lower for H-bonded silanols as compared to isolated SiOH groups. It is concluded that i depends on the extent of H-bonding which allows easy distinguishing between isolated and H-bonded surface hydroxyls.