Theoretical Investigations on Fluorine‐Substituted Ethylene Dications C₂H_nF_4‐n²⁺(n = 0–4)

The optimized geometries and energies of fluorine‐substituted ethylene dications C₂H_nF_4‐n²⁺ (n = 0–4) have been investigated by means of ab initio methods. At the MP3/6‐31G**//6‐31G* + zero‐point energy level of theory, the results predict that C₂F₄²⁺ and C₂HF₃²⁺ are planar, while C₂H₄²⁺, C₂H₃F²⁺ and 1,1—C₂H₂F₂²⁺ prefer a perpendicular geometry. For 1,2—C₂H₂F₂²⁺ an energy difference of only 0.3 kcal/mol is found between the (trans) planar and perpendicular structure. The stabilizations attributed to hyperconjugation, fluorine lone‐pair donation, and (CF) double‐bond conjugation are discussed. A comparison is made for the CC and CF stretching frequencies determined at 6‐31G*//6‐31G* between the neutral and dicationic species. The theoretically determined ionization energies for the vertical process N⁺ → N²⁺ at the MP3/6‐31G*//3‐21G level are compared with experimental Q_min values.