Tilted adsorption

A molecule may also lose mirror symmetry if the presence of the surface induces non-equivalence of otherwise identical groups. This is shown for the theoretical example of methane in tilted adsorption geometry (Fig. 8). Another example is the amino acid glycine, hi tilted geometry, it will become chiral on the surface [22], This surface-induced handedness, in turn, gives rise to four diastereomeric configurations for alanine on a surface (Fig. 9). 

This would imply that after reaching certain optimum modifier coverage, a further increase in the modifier concentration would result in a gradual decline in the enantios-electivity. In a tilted adsorption mode, the catalyst surface accommodates more modifier onto it and less substrate and as a result, a lower reaction rate is obtained. This results in a maximum in enantioselectivity as well as reaction rate, when the modifier concentration is increased. 

Adsorption of the reactant (A) follows a classical multisite adsorption behavior (step 1) with Am denoting the adsorbed substrate and m the number of sites required for adsorption. Adsorption of the modifier is described by steps 2 and 3 where Mp denotes the parallel adsorption mode of the (—)-cinchonidine involved in the enantiodifferentiation with the adsorbed reactant Am, while Mq denotes the tilted adsorption mode of the (—)-cinchonidine, which appears on the catalyst surface as a spectator. 

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

Figure Bl.22.10. Carbon K-edge near-edge x-ray absorption (NEXAFS) speetra as a fiinotion of photon ineidenee angle from a submonolayer of vinyl moieties adsorbed on Ni(lOO) (prepared by dosing 0.2 1 of ethylene on that surfaee at 180 K). Several eleetronie transitions are identified in these speetra, to both the pi (284 and 286 eV) and the sigma (>292 eV) imoeeupied levels of the moleeule. The relative variations in the intensities of those peaks with ineidenee angle ean be easily eonverted into adsorption geometry data the vinyl plane was found in this ease to be at a tilt angle of about 65° from the surfaee [71], Similar geometrieal detenninations using NEXAFS have been earried out for a number of simple adsorbate systems over the past few deeades.

The c(4 x 2)-2CO structure observed20 at Ni(lll) at room temperature has CO occupying both fee and hep threefold hollow adsorption sites with a surface coverage of 0.5 ML. So as to maximise the 0-0 distance, the molecular axis is tilted away from the surface normal towards atop positions. Corrugation of the adlayer is attributed to a CO-induced buckling of the surface nickel atoms, which is manifested by height differences between adjacent CO molecules (Figure 8.6). 

Schematic diagrams for adsorption geometries of (c) and (e) are shown in (d) and (f), respectively a linear atop and a tilted off-site CO are implicated. The black (red) circles represent carbon (oxygen) atoms and the large gray circles are silver atoms. The sizes of the circles are scaled to the atomic covalent radii. (Reprinted with permission from Ref. [25]. Copyright 2001, The American Physical Society.)...

The influence of the organocation structure on the exchange adsorption becomes evident from the data in table V. 4,4 Bipyridinium cations adsorb two times more energetically (AH s 2j2 kJ Eq ) than do 2,2 bipyridinium cations (AH° = 11 f5 Eq ). The former adapt a planar orientation (dnm = f 26 nm) in contrast to the inclined position of the latter ( qq = 1.4 nm), despite the fact that sufficient surface is available for adsorption in a flat configuration. Smaller enthalpy terms are consistent with smaller electrostatic interaction energies. The reason for the tilting is unknown however. 

Perform calculations similar to Exercise 4, but for the adsorption of hydroxyl groups, OH, on Pt(lll). What tilt angle does the OH bond form with the surface normal in its preferred adsorption configuration What numerical evidence can you provide that your calculations adequately explored the possible tilt angles ... 

In order to elucidate the results of the CO TPD experiment, the detailed structure of the oxygen-modified Mo(l 12) surfaces and the adsorption sites of CO on these surfaces have been considered. Zaera et al. (14) investigated the CO adsorption on the Mo(l 10) surface by high-resolution electron-energy-loss spectroscopy (HREELS) and found vatop sites. Francy et al. (75) also found a 2100 cm loss for CO on W(IOO) and assigned it to atop CO. Recently, He et al. (16) indicated by infrared reflection-absorption spectroscopy that at low exposures CO is likely bound to the substrate with the C-0 axis tilted with respect to the surface normal. They, however, have also shown that CO molecules adsorbed on O-modified Mo(l 10) exhibi Vc-o 2062 and 1983 cm L characteristic to CO adsorbed on atop sites. Thus it is supposed that CO adsorbs on top of the first layer Mo atoms. 

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy was reported in [44]. The result suggests that both pyrazine and pyridine were strongly adsorbed onto the substrates. It also implies that pyridine was adsorbed perpendicularly onto the substrate, while pyrazine adsorbed onto the substrate in a slightly tilted vertical configuration. 

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