Chemisorbed molecules, orientation

Stdhr J and Jaeger R 1982 Adsorption-edge resonances, core-hole screening and orientation of chemisorbed molecules CO, NO and N2 on Ni(IOO) Rhys. Rev. B 26 4111-31... 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

XANES, which can be used to determine molecular structure and orientation of chemisorbed molecules on well-characterised single-crystal surfaces and is able to discriminate between the same atoms in different bonding situations, has been used to examine the supramolec-ular organisation adopted by the dye Reactive Red 3 1 physisorbed and chemisorbed on to cotton and cellophane substrate materials [315]. A distinct difference in the nature of the dye/cotton interaction was observed for different preparative methods. The mode by which... 

After dissociation, the two atoms will be chemisorbed on the surface. For H2 on metals, the critical reaction barrier occurs when the molecule is oriented with the H-H axis parallel to the surface (the broadside orientation). For molecules oriented end-on to the surface, the reaction barrier is very much higher (several eV), because in this geometry the final state of one of the atoms lies in the gas-phase. In such circumstances, the dissociation probability will be less than 1 because molecules oriented end-on will not dissociate. A favourable dissociation trajectory will be one in which the bond axis remains approximately parallel to the surface during traversal of the barrier. 

A variation of XANES or NEXAFS has been used to determine the structure of molecules chemisorbed on surfaces. In this approach photoemitted electrons excite molecular orbitals in the chemisorbed molecules. By varying the polarization of the incident photons, molecular orientation can be determined from selection rules for excitation. The bond lengths can be determined from a quasi-empirical correlation between bond-length and the shift in the molecular orbital excitation energy. This technique has been used to study the chemisorption of several hydrocarbon molecules on different metal surfaces./17/... 

Non-linear optical techniques, such as second harmonic generation (SHG), have recently been used as surface probes. Bulk materials with inversion symmetry do not generate second harmonic signals, while surfaces and interfaces cannot have inversion symmetry, so the total SHG signal will come from the surface region for many systems. The components of the non-linear polarizability tensor have been used to determine the orientation of chemisorbed molecules. 

In order to deduce the orientation of the chemisorbed molecules, the metal-surface dipole selection mle needs to be invoked this states that only vibrations with components perpendicular to the metal surface are HREELS-active. That is, if benzene... 

Itaya s group presented images of benzene, naphthalene and anthracene on Cudll), and naphthalene and anthracene on Rh(l 11)/ Wandlowski and coworkers monitored adsorption of uracil on gold surfaces They reported imaging chemisorbed molecules as well as physisorbed molecules and determined their adlattice structures. They also made a correlation between the structure and lateral interaction forces of adsorbed molecules. They showed that application of sufficiently positive electrode potentials results in uracil deprotonation, leading to different surface structure and geometric orientation. 

Angular resolved XANES has been used to determine the orientation of chemisorbed molecules on surfaces. The absorption spectra at the carbon K-edge of CO on Ni(lOO) have been used to show that the CO molecule stands upright on the surface The sulfur L2,3 edge and the carbon K-edge of thiophene molecules on the platinum Pt(l 11) surface has been used to investigate thermal decomposition of the molecules on the surface... 

Stohr J, Baberschke K, Jaeger R, Treichler R, Brennan S (1981) Orientation of chemisorbed molecules from surface-absorption fine-structure measurements CO and NO on Ni(100). Phys Rev Lett 47 381-384... 

Techniques that are sensitive to the orientation of the chemisorbed molecules (NEXAFS, for example) clearly show that the C==0 bond, when chemisorbed on the nickel (100) surface, is perpendicular to that surface. 

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

Fig. 18. Infrared spectra of C -H vibrations of different coverages of CH30 on Cu(lOO) at 100K, showing the symmetric (at 2800cm ) and two asymmetric stretch modes. Inset shows a tilted chemisorbed methoxide molecule and the orientations of the dynamical dipole moments ji. (Reproduced by permission from...

Parson s monolayer model, Fawcett s three-state model, and Guidelli s model, (a) Allowed orientations of water molecules with H bonds directed toward the vertices of the unit cell, (b) Rotations required to form H bonding between two water molecules of the monolayer, (c) H bonding of a chemisorbed water molecule in the monolayer with two bulk water molecules. (Reprinted from R. 

If the pressure exceeds about 10 mm., a third layer is adsorbed. At least part of this is chemisorbed as molecules with a bond strength of about 2.0 ev. Even at room temperature these molecules may change their orientation with respect to the substrate. O2 and O4 molecules are observed frequently, O3 and Oe less frequently. Some of these molecules have vibrational and rotational degrees of freedom. These molecules do not migrate over many atom diameters or lattice sites before they disappear in the substrate or evaporate. There are probably physisorbed molecules in the third layer which are held with smaller energies and which migrate over the surface even at 300°K. 

---