Density functional study of Fe-2-N-2

The interaction of two iron atoms with molecular nitrogen was studied by means of density functional techniques. Calculations were of the all-electron type, and both conventional local and gradient-dependent approximate (GDA) models were used. A ground state (GS) of linear structure was found for Fe-2-N-2 with 2S + 1 = 7;whereas a distorted tetrahedral structure, being also a septuplet, was located at 4.0 and 14.3 kcal/mol above the GS, at the local and GDA levels of theory, respectively. The N-N bond is moderately perturbed in the GS, but it is strongly activated in the tetrahedral mode: It has bond orders of 2.6 and 1.5, vibrational frequencies of 2148 and 1496 cm(-1) and equilibrium bond lengths of 1.14 and 1.24 Angstrom, for the linear and tetrahedral geometries, respectively. These values are 3.0, 2359 cm(-1), and 1.095 Angstrom, for free N-2. At GDA level of theory, the Fe-2-N-2 binding energy is 15 kcal/mol, which is bigger than that of Fe-N-2 (9 kcal/mol). The pi-back donation, in the linear GS, is of 0.31 electrons, but the total charge transfer, from Fe-2 to N-2, is only 0.05 units. This is relevant in comparison with the tetrahedral mode, where the Fe-2 to N-2 total charge transfer is of 0.45 electrons, yielding a stronger activated N-2 moiety. (C) 1996 John Wiley & Sons, Inc.