Laboratory of Physical Chemistry

Apart from avoiding experimental problems with additional radio frequency fields, HYSCORE [7] can also map nuclear spin dependent energy differences between states that cannot be excited by single quantum transitions. Double quantum transitions, however, are very useful when analysing hyperfine (hfi) structures in disordered matrices like in our case, because the orientation dependence on quadrupolar hfi of Δ m I = ±2 transitions in the I = 1 spin system of 14 N are cancelled to first order [8]. Still, a powder line broadening caused by anisotropic dipolar hfi is to be expected, unless a specific molecular orientation is selected by exciting at a canonical orientation.

We studied low-lying states of lithium-like and beryllium-like ions with Z s 80 and 100 F Z F 120 for differential effects due to variation of the nuclear charge density distribution. The latter was represented by exponential, Gauss-type, and Fermi-type models, standardized to a common rms radius. Numerical Hartree-Fock and Dirac-Fock-Coulomb calculations were performed. Changes in total energies due to the use of different models become increasingly important for high Z in relativistic approaches. Different models may even lead to different sequences of atomic states, and thus energy differences between those states may vary by an order of magnitude. q

We briefly review our work on theoretical vibrational spectroscopy and elaborate on a recent approach for the selective calculation of high-intensity features in vibrational spectra developed in our laboratory. Conventional methods for the determination of frequencies and intensities describe all vibrations on the same footing, irrespectively of their intensity. This becomes increasingly demanding for larger molecules. In our intensity-driven approach the total intensity is iteratively distributed to individual distinct bands as well as to fingerprint regions so that only the subset of vibrations that dominates the appearance of the spectrum is determined. In turn, normal mode approximations are obtained.

While Arduengo-type carbenes are now well established for singlet-triplet energy separations and electron affinities. Hence, they may be considered as valid targets for experi-the Group IVa elements carbon, silicon and germanium, they are experimentally unknown for the isoelectronic anions with mental investigations. An analysis of the electron density distribution in the case of boron and aluminum reveals a the Group IIIa elements boron, aluminum, gallium, and indium. In the present quantum chemical investigations, the strongly polar BϪN bond for the former, and a half AlϪN bond with positive charge at the aluminum, emphasizing the bonding features of this class of compounds are explored. As a result, they are predicted to be stable species with sizeable donor-acceptor formulation for the latter. systems analogous to 1 [18] [19] . The halogen precursors of the latter dissociate spontaneously into the cyclic cation Since the pioneering studies of Wanzlick on nucleophilic and a halide anion . In contrast, Arduengo-type carbenes carbenes [1] , their successful isolation as stable species bewith Group IIIa elements are as yet unknown. If these came prominent in the reports of Arduengo et al. [2] [3] on compounds are considered as being isoelectronic with the the imidazo-2-ylidene, 1 (E ϭ C).

The mono-and bidentate chelation of the main-group related bis(diamino)-and the higher element homologue bis(diarsanyl)methanide ligand systems have been studied. elements silicon, germanium, tin, and lead through the phosphorus atoms of the diphosphanylmethanide ligand has

In this work, we investigate with density functional methods mechanistic details of catalytic dinitrogen reduction mediated by Schrock's molybdenum complex under ambient conditions. We explicitly take into account the full HIPTN 3 N ligand without approximating it by model systems. Our data show that replacement of the bulky HIPT substituent by smaller groups leads to deviations in energy of up to 100 kJ mol -1 . Alternatives to the Chatt-like mechanism are also investigated. It turns out that for the generation of the first molecule of ammonia, protonation of the ligand plays a crucial role. With an increasing number of hydrogens on the terminal nitrogen atom, the reduction becomes more difficult. The energetically most feasible step is the generation of the first molecule of ammonia, while the preceding transfer of the second electron and proton is the most difficult one. Reaction energies are not only reported for decamethyl chromocene as in previous studies but also for a series of other metallocenes. Furthermore, results are provided in a way to allow for a convenient estimation of the thermochemical boundary conditions of catalysis with an arbitrary combination of acid and reductant. We demonstrate that the [Mo](NNH 3 ) + complex easily loses ammonia even in the absence of a reductant. For some complexes, spin states with higher multiplicity are the ground state instead of those with lower spin multiplicity.

The trinuclear copper(II) complexes ([CuL1)(µ-ac)Cu(µ-ac)CuL1) (1) and ([CuL2)(µ-ac)Cu(µ-ac)CuL2) (2) of the tridentate aminosaccharide-derived Schiff-base ligands H 2 L1 [6-N-(salicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-R-D-glucopyranoside] and H 2 L2 [6-N-(3,5-di-tert-butylsalicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-R-D-glucopyranoside] were synthesized and structurally characterized. The trinuclear complex units can be described as two terminal copper−ligand moieties bridged by a central copper acetate moiety, with the Cu centers arranged in a triangular fashion. IR and UV/vis spectroscopic studies strongly indicate that the trinuclear structure is maintained in a methanolic solution. The temperature dependence of the magnetic susceptibility of both complexes shows a moderate antiferromagnetic coupling and can be well interpreted by applying a symmetric Cu a −Cu b −Cu a′ model with linear spin topology. The fit of the magnetic data affords coupling constants J of −34 and −24 cm -1 for 1 and 2, respectively [H ) −J(S a S b + S b S a′ )]. For µ-alkoxo-µ-acetato-bridged copper(II) complexes with a large dihedral angle between the adjacent coordination planes, as found in 1 and 2, such an antiferromagnetic coupling is unusual. However, density functional theory calculations of 2 using BP86, B3LYP*, and B3LYP density functionals confirmed a symmetric doublet ground state.

The mono-and bidentate formation of the amidinate ligands toward the main group elements silicon, germanium, tin, and lead is studied with quantum chemical methods. In accordance with the experimental investigations the bisamidinate complexes of the higher element homologues Pb and Sn in their divalent oxidation states adopt C 2 symmetrical structures, with a distorted trigonal bipyramidal (Ψ-tpb) arrangement. The formation of a pseudotrigonal bipyramid is less favorable with the lighter elements Si and Ge; the resulting structures easily undergo Jahn-Teller distortion to C 1 symmetrical structures in which the axial bonds become unsymmetrical. A structural rearrangement from C 2 to C 2V symmetry is even feasible and causes a facile degenerate rearrangement in which the nitrogens in the amidinate ligands become positionally equivalent. The effect of substituents on these different structures is discussed. An analysis of the electron distributions (natural bond orbital populations, Laplacian of the charge densities) establishes for these compounds a strongly positive charged central atom (M ) Si, Ge, Sn, Pb), weakly chelated by the surrounding amidinate ligands. The equilibrium between chelated and covalent bound structures is discussed on the basis of a bond stretch formalism. Structural predictions are also made for ligands isoelectronic to the amidinate anion [HNCHNH] -, (e.g., [HPCHPH] -, [HNSiHNH] -, and [OCHO] -). Accordingly, bisamidinate-like structures are promoted with increasing difference in electronegativity between the central atom (Si, Ge, etc.) and the neighboring chelating atom (P < N < O). The design of structures with a tetrahedral environment of the central atom in which the nonbonding s-orbital is stereochemically not active is discussed. Such conformations possess large open shell character as tested by the electron correlated methods and MCSCF calculations and are promoted by chelating ligands with an inherent weak π-allylic system. The matter is tested for silicon as central atom for the homologous bisamidinate complex.

We have applied density functional theory, using both pure (BP86) and hybrid (B3LYP and B3LYP*) functionals, to investigate structural parameters and reaction energies for nickel(II)-sulfur coordination compounds, as well as for small cluster models of the Ni-SI and Ni-R redox state of [NiFe] hydrogenases. Results obtained investigating experimentally wellcharacterized complexes show that BP86 is well suited to describe the structural features of this class of compounds. However, the singlet-triplet energy splitting and even the computed ground state are strongly dependent on the applied functional. Results for the cluster models of [NiFe] hydrogenases lead to the conclusion that in the reduced protein structures characterized by X-ray diffraction a hydride bridges the two metal centres. The energy splitting of the singlet and triplet states in Ni-R and Ni-SI models is calculated to be very small and may be overcome at room temperature to allow a spin crossover. Moreover, the relative stability of the Ni-SI and Ni-R structures adopted in the present investigation is fully compatible with their involvement in the reversible heterolytic cleavage of H 2 .

Methane formation in methanogenic Archaea is catalyzed by methyl-coenzyme M reductase (MCR) and takes place via the reduction of methyl-coenzyme M (CH 3 -S-CoM) with coenzyme B (HS-CoB) to methane and the heterodisulfide CoM-S-S-CoB. MCR harbors the nickel porphyrinoid coenzyme F 430 as a prosthetic group, which has to be in the Ni(I) oxidation state for the enzyme to be active. To date no intermediates in the catalytic cycle of MCR red1 (red for reduced Ni) have been identified. Here, we report a detailed characterization of MCR red1m (''m'' for methyl-coenzyme M), which is the complex of MCR red1a (''a'' for absence of substrate) with CH 3 -S-CoM. Using continuous-wave and pulse electron paramagnetic resonance spectroscopy in combination with selective isotope labeling ( 13 C and 2 H) of CH 3 -S-CoM, it is shown that CH 3 -S-CoM binds in the active site of MCR such that its thioether sulfur is weakly coordinated to the Ni(I) of F 430 . The complex is stable until the addition of the second substrate, HS-CoB. Results from EPR spectroscopy, along with quantum mechanical calculations, are used to characterize the electronic and geometric structure of this complex, which can be regarded as the first intermediate in the catalytic mechanism.

The frozen-density embedding ͑FDE͒ scheme ͓Wesolowski and Warshel, J. Phys. Chem. 97, 8050 ͑1993͔͒ relies on the use of approximations for the kinetic-energy component v T ͓ 1 , 2 ͔ of the embedding potential. While with approximations derived from generalized-gradient approximation kinetic-energy density functional weak interactions between subsystems such as hydrogen bonds can be described rather accurately, these approximations break down for bonds with a covalent character. Thus, to be able to directly apply the FDE scheme to subsystems connected by covalent bonds, improved approximations to v T are needed. As a first step toward this goal, we have implemented a method for the numerical calculation of accurate references for v T . We present accurate embedding potentials for a selected set of model systems, in which the subsystems are connected by hydrogen bonds of various strength ͑water dimer and F-H-F − ͒, a coordination bond ͑ammonia borane͒, and a prototypical covalent bond ͑ethane͒. These accurate potentials are analyzed and compared to those obtained from popular kinetic-energy density functionals.