Alkenes, chemisorbed structures

The next section will deal briefly with experimental techniques many of these have been introduced already, but the use of vibrational spectroscopy and of sum-frequency generation call for some further description. Section 4.4.1 describes the principal types of adsorbed hydrocarbon structure that have been found with alkenes and alkynes (aromatic hydrocarbons and cyclic Ce species will be considered in Chapters 10 and 12 respectively) Section 4.4.2 discusses the conditions under which the several chemisorbed forms of alkenes make their appearance. In Section 4.5 we look at detailed structural studies of a few adsorbed molecules, and Section 4.6 deals somewhat briefly with interconversions and decompositions of adsorbed alkenes, and structures of species formed. Finally there are sections on theoretical approaches (4.7), on the chemisorption of alkanes (4.8), and carbonaceous deposits that are the ultimate product of the decomposition process (4.9). 

The extent to which n or di-a structures are formed in organometallic complexes depends upon the nature of the metal, its oxidation state, and on the types of other ligands present. With chemisorbed structures, their occurrence (or the appearance of Types B and A spectra that are attributed to them) hinges on the kind of metal, and the size and composition of the site (number and CN of metal atoms, and presence of other adjacent species of the same or quite different type). In both cases, all these factors affect the number and occupancy of the valence orbitals available to take part in the bonding. Additionally the number and nature of the substituent groups about the double bond will influence the character of the 7t bond in the relevant alkene, and hence the way it interacts with the surface orbitals. 

Some of the evidence upon which is based the structure and geometry of chemisorbed alkenes is presented in Section II,B. Lacking direct information relatively involved arguments have been advanced to deduce these geometries. 

Apparently, the exchange patterns can be explained qualitatively by reference to either structure for the adsorbed olefin, the eclipsed 1,2-diadsorbed alkane or the olefin tt complex. This argument should, of course, refer to the transition state for the formation of chemisorbed olefin from monoadsorbed alkane, the critical step in the a,)3 exchange mechanism however the revised argument would be much the same. Nevertheless we are provided with two alternative descriptions of the chemisorbed alkene under conditions closely related to those employed in hydrogenation studies. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

One of the simplest transformations of a radical cation or anion is structural isomerization. A simple alkene such as c -2-butene is capable of donating an electron to SC(h ) on an illuminated SC, such as Ti02, ZnS, or CdS, to yield a chemisorbed radical cation which then imdergoes cis-trans isomerization. This radical cation takes back an electron from SC to give a mixture of cis- and trans-isomers [98-103]. The ratio of the two isomers is determined by the thermodynamic stability of their radical cations. 1,2-Diarylcyclopropanes also undergo cis-trans isomerization, using ZnO as a photocatalyst [Eq. (3)] [104] ... 

---