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 desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The fluorinated propenes adsorb reversibly at low temperatures, desorbing during heating at 120-140 K. TPD measurements indicated that the desorption activation energy increased with increasing fluorination of the propene molecule. Specifically, the desorption activation energy 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 trifluoropropene. 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-pentafluoropropene and E(d) = 8.0 +/- 0.4 kcal/mol for hexafluoropropene). HREELS was used to monitor changes in the vibrational spectrum of the fluorinated propenes upon adsorption. Specifically, the nu(C=C) mode was monitored to probe the extent of backbonding that occurs between the molecule and metal. A significant, downward shift in the frequency of the nu(C=C) mode of the propenes upon adsorption would indicate backbonding to the metal surface by the adsorbate. However, no changes in the vibrational spectra of any of the fluorinated propenes were detected upon adsorption, indicating no significant backbonding. Fluorine is found to have a weak effect on the adsorbate-metal bond strength but the vibrational spectra do not indicate that there is substantial backbonding.