Equilibrium structures and hyperfine parameters of some fluorinated hydrocarbon radical cations: a DFT B3LYP and MP2 study

Abstract

The fluorinated ethene cations (C 2F 4 +, C 2HF 3 +, and CF 2CH 2 +), 2-fluoropropene cation (CH 2CF–CH 3 +), hexafluoro-1,3-butadiene cations ( trans-C 4F 6 +and cis-C 4F 6 +), and perfluorocyclopropane cation ( c-C 3F 6 +) were studied by using the DFT B3LYP and MP2 methods. The equilibrium geometries of these cations were predicted by the B3LYP/6-311+G(d,p) calculations and compared with the geometries of the neutral molecules. The three fluorinated ethene cations are predicted to have planar geometries. The trans-C 4F 6 + and cis-C 4F 6 + cations are predicted to be both minimum-energy species having planar geometries. The CH 2CF–CH 3 + cation is predicted to have a C s geometry and the c-C 3F 6 +cation (a Jahn–Teller species) to have a C 2 v structure with one CCC angle larger than 60° in the 2A 1 ground state. At the B3LYP/6-311+G(d,p) geometries of these cations, isotropic hyperfine coupling constants on the F- and H-centers ( α(F) and α(H)) were calculated by using the MP2(full) and B3LYP methods in conjunction with the 6-31G(d,p) (B0), 6-31+G(d,p) (B0+), 6-311G(d,p) (B1), and 6-311+G(d,p) (B1+) basis sets. The MP2/B0 and MP2/B0+ α(F) values are in good or reasonable agreement with available experimental coupling data (the deviations are smaller than 20%), and the MP2/B1 and MP2/B1+calculations produced worse results (smaller α(F) values). The B3LYP/B0 and B3LYP/B0+ α(F) values are significantly smaller than the experimental values, and the B3LYP/B1 and B3LYP/B1+ α(F) values are only 34–53% of the experimental values. Accurate theoretical prediction for the α(F) values in these cations seems difficult.