Acyclic Alkenes An Update since Part

Further pertinent nonspectroscopic determinations of the structure of ethene on single-crystal metal surfaces have recently been published. Using photoelectron diffraction Bradshaw, Woodruff, and co-workers (363) have 

Another system that has received recent intensive study by both diffraction and spectroscopic methods is that of ethene on Pt(lll). In recent papers, Somorjai et al. have investigated the adsorption of ethene by diffuse LEED to give the unexpected result that the di-cr species [which on Pt(lll) gives an extreme type I spectrum] is adsorbed across threefold sites with 

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Investigating the same C2H4/Pt(lll) system, McDougall and Yates (373) determined the temperature dependence of the di-cr to ethylidyne conversion and then ethylidyne decomposition, giving the approximate energies of activation of 17.5 and 29 kcal/mol, respectively. The same authors investigated the slow hydrogenation of the ethylidyne species (presumably at ambient temperature) and showed that this accelerated at a pressure of 

A recent study of ethene adsorbed on Fe(100) by Hung and Bernasek (48) showed adsorption as a di-cr species (type I spectrum) at 100 K, but with a soft-mode component at ca. 2720 cm-1 in addition to the stronger 2985-cm 1 absorption in the vCH2 region. When the temperature had been raised to 253 K, the spectrum largely changed to that probably indicative of a mixture of CH and a(CCH) species (see also Section IV.D). At 523 K, only adsorbed carbon remained on the surface. 

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