Andreas Fiedler

The energetical and structural properties of FeO+. An application of multireference perturbation theory

Abstract The FeO + cation has been examined by using the following ab initio methods: (i) the single determinant oriented quadratic configuration interaction (QCISD(T)) and coupled-cluster CCSD(T) approaches, (ii) the multiconfiguration wavefunction-based complete active space SCF/multireference CI (CAS-MRCI) method and complete active space/perturbation theory (CASPT2). The effects of different basis sets and the inclusion of core electrons in the correlation treatment have been evaluated. The experimental bond energy is well reproduced by the CASPT2 calculations employing a large basis set. The D e values calculated by the QCISD(T) method are almost identical to the CAS-MRCISD values, which in turn are slightly lower than those obtained with CASPT2. The computer-time requirements for the different methods are compared.

Reversible β-hydrogen transfer between Fe(C2H5)+ and HFe(C2H4)+: case of two-state reactivity?

Abstract The iron ethyl cation, Fe(C 5 H 5 ) + , and its tautomer, the ethene complex of the iron hydride cation HFe(C 2 H 4 ) + , have been examined computationally using a hybrid of density functional theory and the Hartree-Fock approach ( Becke 3LYP). The quintet Fe(C 2 H 5 ) + ( 5 A′) corresponds to the global minimum of the [Fe,C 2 ,H 5 ] + potential energy hypersurface. Fe(C 2 H 5 ) + can interconvert via β-hydrogen transfer into HFe(C 2 H 4 ) + ( 5 A′), which is ca. 13 kcal mol −1 less stable. The transition structure (TS) associated with their mutual interconversion on the quintet surface requires 36 kcal mol −1 relative to Fc(C 2 H 5 ) + . However, this barrier may be circumvented by a reaction path on the energetically low-lying triplet surface in which the corresponding transition structure for β-H transfer is 8 kcal mol −1 lower in energy than the quintet TS. Thus, the path of minimal energy requirement connects the quintet species Fe(C 2 H 5 ) + and HFe(C 2 H 4 ) + via the triplet surface such that spin inversion is part of the reaction coordinate. Agostic interaction, which is only possible in the low-spin system, constitutes an essential factor for this unprecedented reaction mechanism. Further support to this interpretation is providedby mass spectrontetric experiments which demonstrate that the interconversion Fe(C 2 H 5 ) + ⇄ HFe(C 2 H 4 ) + is facile and occurs well below the respective dissociation asymptotes.

Portrait of diatomic FeN. A theoretical study

Abstract The ground and low-lying excited states of the diatomic molecule FeN were systematically studied. For this purpose a density functional/Hartree-Fock hybrid method and the internally contracted averaged quadratic coupled cluster approach with large basis sets were used. In agreement with previous reports the calculations revealed the 2 Δ state to be the lowest in energy and only 3 kcal/mol higher lies the 4 ∏ state. In addition, we found another very low-lying 4 ф state and an even less energy demanding state of 6 ∑ + symmetry. The computations showed excitation energies of 5 and 0.5 kcal/mol, respectively. If the uncertainties of the methods are considered, all four states are good candidates for the real ground state.

A neutralization—reionization (NR) case study for cationic iron complexes with simple ligands: NR mass spectra of Fe(C2H4)+ and Fe(CO)+

Abstract The cationic iron(I) complexes Fe(C 2 H 4 ) + and Fe(CO) + were examined by neutralization—reionization mass spectrometry (NRMS). Whereas the NR mass spectrum of the carbonyl complex exhibits a signal corresponding to the reionized neutral molecule Fe(CO), for the neutral Fe(C 2 H 4 ) complex no recovery signal is observed; rather, in the course of the experiment the complex dissociates to Fe and C 2 H 4 . An explanation for this seemingly contradictory behaviour of structurally related metal complexes in a NR process is provided by ab initio MO calculations, in which the geometries of Fe(C 2 H 4 ) + ( 4 B 2 ), Fe(C 2 H 4 ) ( 5 B 2 ), Fe(CO) + ( 4 Σ − , Fe(CO) ( 3 Σ − , and Fe(CO) ( 5 Σ − ) have been fully optimized at the QCISD(T) level of theory. From the theoretical results, a neutralization-reionization scheme for organometallic ions MX + emerges which considers the effects caused by curve-crossing from a bound state to a repulsive ground-state asymptote of the neutral building blocks M and X. Thus, even for bound organometallic complexes MX, recovery signals in the NR mass spectrum can only be detected if the internal energy deposited in MX in the vertical electron-transfer reaction MX + → MX is too small to permit this curve-crossing. If dissociation occurs on the time scale of the NR experiments, the spectrum exhibits features of both the metal M and the ligand X, thus revealing the structural properties of the (organic) ligand X bound to the metal ion.

Systematische ab initio ECP-Untersuchungen zum Fe(H2O)+-Komplex und zu Ionisierungs- und Anregungsenergien von Fe und Fe+

Several basis sets are tested by employing ab initio effective core potential calculations, in order to evaluate the excitation energies (AE) of the transitions 5D -» 5 F and 5D -> iF for the iron atom and bD -» F for the Fe+ cation, respectively. In addition, the ionization energies (IE) for the process Fe(5Z)) -> Fe+ (bD) are calculated. Furthermore, the geometry and the bond dissociation energy of the Fe(H20)+ complex are computed and compared with experimental as well as other theoretical data. It is demonstrated that for a given theoretical level the basis set used is of minor importance for the determination of AE. However, the inclusion of correlation effects give satisfactory if not excellent agreement with the experimental data. Similarly, the bond dissociation energy of the process Fe(H20)+ ->· Fe + (60) + H20 is very well reproduced at the MP2 and CISD( + D) level of theory for all the basis sets used. In distinct contrast, even at the highest level of theory the ionization energies are always underestimated up to 11 %.