Theoretical Investigations on Fluorine‐Substituted Ethylene Dications C 2 H n F 4‐ n 2+ ( n = 0–4)

The optimized geometries and energies of fluorine‐substituted ethylene dications C 2 H n F 4‐ n 2+ ( 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 C 2 F 4 2+ and C 2 HF 3 2+ are planar, while C 2 H 4 2+ , C 2 H 3 F 2+ and 1,1—C 2 H 2 F 2 2+ prefer a perpendicular geometry. For 1,2—C 2 H 2 F 2 2+ 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 (CF) double‐bond conjugation are discussed. A comparison is made for the CC and CF stretching frequencies determined at 6‐31G*//6‐31G* between the neutral and dicationic species. The theoretically determined ionization energies for the vertical process N + → N 2+ at the MP3/6‐31G*//3‐21G level are compared with experimental Q min values.