Molecular structures, vibrational frequencies and isotropic hyperfine coupling constants of FeF3 and MnF2: an ab initio molecular orbital study

New values for the equilibrium structures, harmonic vibrational frequencies and, for the first time to isotropic ESR hyperfine coupling constants A(is o) of the molecules FeF3 and MnF2 were obtained. Use was made of unrestrict ed quantum chemical ab initio methods (UHF, UMP2. UB3LYP) in combination wi th several all-electron basis sets as well as pseudo-potentials for iron an d manganese. All properties mentioned show a strong dependence on the basis set and method applied. Whereas for FeF3 UB3LYP calculations using the (9s 4p3d)/DZ basis set appear to be the best choice for the bond length and vib rational frequencies, all UHF and UMP2 calculations yielded ESR hyperfine c oupling constants in reasonable agreement with the experimental values avai lable. Based on the different physical and chemical nature of trivalent iro n and divalent manganese ions it turned out to be much more difficult to de termine these properties for MnF2 than for FeF3. (C) 1999 Elsevier Science B.V. All rights reserved.