Core-level binding-energy shifts in small molecules

Abstract

Binding‐energy shifts of carbon and fluorine 1s levels in small (two to four carbon) fluorinated hydrocarbons are reported. Carbon, nitrogen, and oxygen 1s shifts for a number of other molecules are also reported. All samples were studied as gases: A total of 72 new shifts were measured. The results are analyzed using three electrostatic potential models. First the RPM theory, which includes final‐state relaxation, was used. It correctly predicts the N(1s) shifts in methylamines, for which relaxation shifts are dominant, showing the importance of considering relaxation when comparing unlike molecules. The GPM theory involves only ground‐state properties. It gives excellent results for carbon, oxygen, and fluorine shifts if only groups of similar molecules are compared. Finally, atomic charges are derived using the ACHARGE model. These charges agree very well with CNDO/2 charges. The ``charge alternation'' effect deduced in CNDO/2 calculations was observed— substitution of F for H on a carbon atom decreases the charge on the next carbon by about 0.02₅e in ethane, 0.04₅e in benzene, and 0.06e in ethylene, according to the ACHARGE analysis. Inductive charge transfer was found to be additive.