Hydrogen bonding, adsorbate-adsorbent intramolecular

Figure 5.1 Silica surfaces contain high concentrations of hydrogen-bonded adsorbed water, siloxane (Si-O-Si) groupings and intramolecular hydrogen bonds via silanol (SiOH) groupings.

A secondary amide group is nearly planar and has a hindered rotation around the C-N bond which exists predominantly in the transoid conformation. This provides a dipole which is accessible for inter- and intramolecular hydrogen donor and acceptor interactions and dipole stacking. The silanol groups (Si-OH) of the adsorbent are ideal partners not only for hydrogen bonding but also, due to their acidity, for electrostatic interactions. 

Synthesis of the methoxy compounds (Scheme 6) was str ieht-forward with the two isomers being best separated by column chromatography at the hydroxy stage. (The cis or axial isomer is intramolecularly hydrogen bonded and therefore less strongly adsorbed.)... 

The trans-cis retention sequence depends on the number of adsorbed groups. The conformational factors are more favorable for the /ran -isomer under one-point adsorption and for the cM-isomer under two-point adsorption. The established relations of Rp(trans) > f F(cij) and / f(cu) > EF(trans) have practical importance if the adsorption of the groups X and Y can be reliably predicted for other newly synthesized compounds of the type II without intramolecular hydrogen bonds or with bonds of the types OH... N or OH... O. 

We consider two cases (see Fig. A.13). First, the metal has a work function that is between electron affinity (the energy of the o -level) and the ionization potential (the energy of the o-level) of the molecule. Upon adsorption, the levels broaden. However, the occupation of the adsorbate levels remains as in the free molecule. This situation represents a rather extreme case in which the intramolecular bond of the adsorbate molecule stays about as strong as in the gas phase. The other extreme occurs if both the a-level and the o -1evel fall below the Fermi level of the metal. Because the antibonding G -level is filled with electrons from the metal, the intramolecular bond breaks. This is the case for hydrogen adsorption on many metals. Thus, a low work function of the metal and a high electron affinity of the adsorbed molecule are favorable for dissociative adsorption. 

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