A computational examination of helical perfluoroalkane conformations and implications for the adsorption of perfluoroalkanes on platinum(111)surfaces

摘要

The two lowest-energy conformers of perfluorohexane and perfluoroheptane were investigated at the Hartree-Fock RHF/6-31G~* level and the structures,properties,and vibrational frequencies determined.The lowest-energy conformers of perfluorohexane were found to differ by 8.600 kJ mol~(-1)(2.055 kcal mol~(-1))at this level of theory,while the lowest-energy conformers of perfluoroheptane differed by 8.662 kJ mol~(-1)(2.070 kcal mol~(-1)).In addition,a transition state for the interconversion between the two lowest-energy conformers of perfluorohexane was investigated at the RHF/6-31G~* level,and was found to lie 13.247 kJ mol~(-1)(3.166 kcal mol~(-1))above the energy of the lowest-energy conformer,while a transition state for the equivalent interconversion between the perfluoroheptane conformers was 15.279 kJ mol~(-1)(3.652 kcal mol~(-1))above the energy of the lowest-energy con-former.These results have been used to interpret previously published thermal desorption and infrared reflection-absorption spectroscopic data for the monolayer and sub-monolayer adsorption of perfluorocarbon molecules on a single-crystal Pt(111)surface under ultra-high vacuum conditions.