THE INVESTIGATION OF FLUORINATED PROPENES ON THE CU(111) SURFACE

We have examined the effect of fluorine on the bonding of selectively fluorinated propenes (CH3CH=CH2, CFH2CH=CH2, CF3CH=CH2, CF3CH=CF2 and CF3CF=CF2) to the Cu(111) surface using temperature programmed desorpt ion (TPD) and high resolution electron energy loss spectroscopy (HREEL S). The fluorinated propenes adsorb reversibly at low temperatures, de sorbing during heating at 120-140 K. TPD measurements indicated that t he desorption activation energy increased with increasing fluorination of the propene molecule. Specifically, the desorption activation ener gy increased from 5.9 +/- 0.9 and 5.5 +/- 0.6 kcal/mol for propene and 3-fluoropropene respectively to 8.1 +/- 0.9 kcal/mol for trifluoropro pene. Further fluorination of propene did not increase the desorption activation energy (E(d) = 9.0 +/- 1.5 kcal/mol for 1,1,3,3,3-pentafluo ropropene and E(d) = 8.0 +/- 0.4 kcal/mol for hexafluoropropene). HREE LS was used to monitor changes in the vibrational spectrum of the fluo rinated propenes upon adsorption. Specifically, the nu(C=C) mode was m onitored to probe the extent of backbonding that occurs between the mo lecule and metal. A significant, downward shift in the frequency of th e nu(C=C) mode of the propenes upon adsorption would indicate backbond ing to the metal surface by the adsorbate. However, no changes in the vibrational spectra of any of the fluorinated propenes were detected u pon adsorption, indicating no significant backbonding. Fluorine is fou nd to have a weak effect on the adsorbate-metal bond strength but the vibrational spectra do not indicate that there is substantial backbond ing.