The effect of fluorine on the electronic spectra and ionization potentials of molecules

Abstract

The effect of substituents on the spectra and ionization potentials of electrons in chromophoric groups can in a general way be divided into two major contributions, namely, ( a ) inductive effects which act mainly by changing the binding of electrons in the ground state, and ( b ) resonance effects which stabilize the molecular ion. When the substituent is a fluorine atom these effects are large but of opposite sign. The magnitude of the inductive effect is shown to be in some cases as much as several electron volts by considering sets of molecules where resonance stabilization of the molecular ion is negligible, for example, hydrocarbons and their perfluoroderivatives, NH 3 and NF 3 , the perfluoromethyl halides, etc. In other classes of molecules, such as the trifluoromethyl radicals, the fluorinated ethylenes and aromatics, large resonance stabilization of the molecular ion occurs along with the inductive effect and as these are of opposite sign, relatively small changes of ionization potential result from the substitution. The study of the dependence of increases of ionization potential due to inductive effects on the distance of the substituted fluorine from a chromophore shows how this is reduced as the separation increases. Many of the longer wavelength bands of various chromophores show shifts on substitution which are in the opposite direction to the changes which occur in the ionization potential. It is suggested that these can in many cases be attributed to the different spatial dependence of resonance and inductive effects. The interplay of this spatial variation and the dimensions of the upper orbital, especially when this has a large overlap with the substituting group, can produce energy changes in the excited orbitals which make the bands appear to behave anomolously on substitution. New values for the ionization potentials of the fluorinated ethylenes, aromatics and alkyl halides are given.