Deuteroxyl groups

The value of the atom fraction H (of the OH + OD), initially, at equilibrium, and at time t, is denoted by x0, xm, and x, respectively. Allowance was made for the fact that the apparent optical densities of the hydroxyl and corresponding deuteroxyl groups are different by as much as 10%. The excess of D2 was at least 5-fold, so that the variation of the different types of hydroxyls could be evaluated separately (17). From Equation 1 the characteristic slope parameter is obtained (19) ... 

An impression of the quantity of these surface hydroxyls can be obtained from their FTIR spectra. In figure 9.36 a the FTIR-PAS spectrum for the 973 K deuterated modified silica is displayed. In the deuteroxyl region (2800 - 2500 cm 1) no absorption band is observed. All deuteroxyl groups have reacted. 

OD represents a deuteroxyl group, i.e. one oxygen and one deuterium atom. 

Physisorption may involve the amine-side as well as the silicon-side of the silane molecule. Silanol groups (on the surface as well as on the silane) therefore are very important in the reaction sequence. In this paragraph we aim to clarify the role of both surface and silane silanols. A clear distinction between both types is possible with the complementary use of solid-state NMR and FTIR, and with conversion of the surface hydroxyls to deuteroxyls (K) prior to modification with the silane. 

In order to be able to distinguish non-reacted surface silanols from silanols formed in the reaction, the silica surface was deuterated before modification. After thermal pretreatment, a maximal amount of surface hydroxyls is exchanged to deuteroxyls. If the silane is applied to this deuterated substrate, deuteroxyls found after reaction are non-reacted surface groups. Detected hydroxyls must have been formed in the course of the modification procedure. The deuteration was performed with D20 vapour, as discussed above. 

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