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Density-functional-based Car−Parrinello and Born−Oppenheimer molecular dynamics simulations are presented for ferrocene, FeCp2, and its protonated form, FeCp2H⁺, in the gas phase, employing BP86 and B3LYP functionals. When the simulations for neutral FeCp2 are started at 300 K from the eclipsed minimum, the system stays within that well for several picoseconds. When the simulations are started from the staggered transition state for Cp rotation, in contrast, free rotation of the Cp rings is observed with a rotational period of ca. 1 ps, in accord with the low barrier for rotation (ca. 1 kcal/mol at the BP86 level). FeCp2H⁺ is indicated to be fluxional on the picosecond time scale as well. The course of the dynamics depends strongly on the underlying quantum-chemical model. At the BP86 level, rotation of a Cp ring and large-amplitude motions of the proton are observed, and depending on details such as the particular basis set, structures with an essentially iron-bonded proton can prevail, or a significant population of structures with an additional agostic interaction to one of the Cp rings can occur. At the B3LYP level, the mobility of the proton is somewhat reduced, and agostic (or ring-protonated) structures are strongly preferred. Thermally averaged chemical shifts are computed from snapshots along the trajectory. ¹H and, in particular, ⁵⁷Fe chemical shifts in FeCp2H⁺ are sensitive to the location of the proton. The experimental δ(⁵⁷Fe) value is best reproduced in simulations that stay predominantly near the metal-protonated minimum, whereas the observed ¹H chemical shifts are better reconciled with an agostic structure. The same conclusion is drawn when zero-point corrections to equilibrium chemical shifts are evaluated using a perturbational approach. Molecular dynamics and chemical shifts of protonated ferrocene thus remain a challenge for approximate density functional theory.

Catalytic conversion of MeOH and SiH4 to MeOSiH3 and H2 at a [Fe(Cp)(CO)(PR3)]⁺ center was studied computationally at a gradient-corrected level of density functional theory (BP86). Intermediates and transition states for R‘ = H and Ph were characterized along the catalytic cycle, the rate-determining step of which is indicated to be H2 dissociation from [Fe(Cp)(CO)(PR3)(H2)]⁺, in accord with a mechanistic study for higher substituted substrates (Brookhart, et al. J. Mol. Catal. A 1998, 130, 107). For the model catalyst (R = H), introduction of SiMe3, NMe2, COMe, NO2, CN, or Cl substituents at the Cp ring decreases the rate-determining barrier (up to 6 kcal/mol for NMe2). A loose correlation is found between these barriers and δ(⁵⁷Fe) values computed for the correspondingly derivatized precursor models Fe(C5H4X)(CO)(PH3)Me. In the transient, free [Fe(C5H4X)(CO)(PPh3)]⁺ complex, there can be competition between the substituent X and a phenyl group of the PPh3 ligand for a hemilabile intramolecular coordination to iron. Modification of the phosphine is thus expected to open further possibilities for tuning the catalyst properties and activities.

Structures, relative energies, vibrational spectra, and ESR parameters of the 17e^- molecule V(CO)6 in Oh, D3d, D4h, and D2h symmetry have been computed with density functional and high-level ab initio methods. At all theoretical levels applied, the same energetic order is obtained, D3d < D2h < D4h < Oh, with the D2h structure as a transition state connecting equivalent D3d species. At the RCCSD(T)/AE2 level using UBP86/AE2 geometries, the energies of the D2h, D4h and Oh species relative to that of D3d minimum are predicted to be 210, 535, and 731 cm^-1, respectively. According to molecular dynamics simulations on the UBP86/AE1 potential energy surface, the D3d minimum is preserved at very low temperatures (around 16 K), whereas at 300 K the molecule is highly fluxional with an averaged structure indistinguishable from that of [V(CO)6]^- with its Oh symmetric ground state. Nearly complete IR and Raman spectra of V(CO)6 and V(¹³CO)6 have been recorded at 300 K for the first time in the gas phase, in solution, and at cryogenic temperatures in Ne and Ar matrices. The spectra show a pronounced temperature dependence, especially for the Jahn−Teller active modes, Eg and T2g. The observed infrared matrix spectra generally agree well with the calculated spectrum (BP86/AE2 level) for the D3d structure and much less with that of the D4h isomer. The A1g modes in the Raman spectra are reasonably well reproduced computationally in the harmonic approximation, whereas this approximation breaks down for the Eg and T2g bands, as expected. Further evidence for the D3d symmetry of V(CO)6 is obtained from a reanalysis of the experimental ESR spectrum that is reported in the literature. The observed ordering of the hyperfine coupling constants A∥ and A⊥ is reproduced theoretically only when distortion to D3d symmetry is assumed. In addition, the bonding properties of V(CO)6 are compared to those of [V(CO)6]^- and Cr(CO)6.

Car−Parrinello molecular dynamics simulations have been performed for MnO4^- in the gas phase and in aqueous solution. Dynamic averaging at ca. 300 K affords only minor changes for the mean Mn−O bond length with respect to the optimized value re = 1.622 Å (BP86 value). Magnetic shieldings, evaluated at the GIAO-B3LYP level for snapshots along the trajectories, decrease upon going from the equilibrium values to the dynamic averages in water. For ⁵⁵Mn and ¹⁷O, the resulting σ^300K − σe values amount to −36 and +31 ppm, respectively. The structure of the solvation shell around permanganate ion in water is characterized by the mean number of hydrogen-bonded solvent molecules (approximately 4) and by the gMnO pair correlation function.

The low-temperature X-ray structures of bis(η²-ethene)(2,4-pentanedionato)rhodium(I) (1)and bis(η²-ethene)(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)rhodium(I) (2) were determined. Very similar Rh−ethene coordination geometries are found in the solid state, i.e., 1, Rh−C = 2.127(5) Å, and 2, Rh−C = 2.121(3) Å, in good accord with DFT calculations, i.e., 1, Rh−C = 2.132 Å, and 2, Rh−C = 2.136 Å. The calculated ¹⁰³Rh NMR chemical shifts (GIAO-B3LYP/II level) for a range of bis(η²-alkene)(2,4-pentanedionato)rhodium(I) complexes also agree well with solution NMR data. The empirical correlation between transition-metal shifts and stability constants (Öhrström, L. Comm. Inorg. Chem. 1996, 18, 305) could be confirmed for simple alkenes, since the computed relative Rh−alkene binding energies were found to correlate with δ(¹⁰³Rh). In contrast, chelating or fluorinated alkenes showed large deviations from this correlation. The steric and electronic effects on the Rh−alkene bond are discussed and analyzed in terms of Bader's atoms-in-molecules theory, which revealed qualitatively different binding modes of ethene and tetrafluoroethene to rhodium:  ethene forms typical π-complexes in the Dewar−Chatt−Duncanson model, whereas tetrafluoroethene complexes are on the borderline to metallacyclopropanes.

Experimental and theoretical methods have been employed to investigate the influence of the chelating phosphine ligand on the ¹⁰³Rh chemical shift in complexes containing the [(P2)Rh] fragment (P2 = chelating bidentate phosphine). The δ(¹⁰³Rh) values obtained by 2D(³¹P,¹⁰³Rh{¹H}) NMR spectroscopy for a series of neutral rhodium complexes [{R2P(CH2)nPR2}Rh(hfacac)] (R = Ar, Ph, Cy, Me, n = 1−4, hfacac = hexafluoroacetylacetonate) have been compared. Systematic variation of the phosphine ligand has allowed separation of electronic and geometrical effects. The purely electronic influence of para substituents in complexes [{(p-XC6H4)2P(CH2)4P(p-XC6H4)2}Rh(hfacac)] correlates directly with the Hammett σP constants of X, but leads to variations in the chemical shift of less than 80 ppm between X = CF3 and X = OMe. In contrast, geometrical changes in complexes [(P2)Rh(hfacac)] lead to variations in the chemical shift over a range of approximately 800 ppm. The individual contributions of various structural parameters on the δ(¹⁰³Rh) values have been assessed by density-functional-based calculations for suitable model compounds. The same approach has been extended to the rationalization of the trends in ¹⁰³Rh chemical shifts of cationic complexes with four P donor ligands around a Rh(+I) center and selected anionic Rh(−I) complexes [(P2)2Rh]^-. This analysis allows for the first time a direct corroboration of geometrical variations and their effect on ¹⁰³Rh chemical shifts, demonstrating that correlations of reactivity with ¹⁰³Rh chemical shifts can give valuable information on structure/reactivity relationships.

Summary The prevalence of hepatitis C virus (HCV) infection in Omani patients with renal disease was determined using a second-generation enzyme immunoassay which detects antibodies to three HCV antigens. Based on the results of this assay, 27 of 102 (26.5%) sera from patients on haemodialysis, 11 of 82 (13.4%) sera from kidney transplant patients, and 1 of 103 (1%) sera from non-dialysed, non-transplanted patients with various renal diseases had antibodies to HCV. Among healthy subjects, none of 134 medical students and 5 of 564 (0.9%) blood donors were anti-HCV positive. Thus, the prevalence of HCV infection in dialysis and renal transplant patients was significantly higher than that found in patients with renal disease who had been neither dialysed nor transplanted (p<0.05). In the latter group of patients, the frequency of anti-HCV was low, and comparable to that of healthy Omani subjects.

¹⁰³Rh chemical shifts, computed using the SOS-DFPT approach (sum-over-states density functional perturbation theory), large basis sets, and optimized geometries, correlate well with the corresponding experimental data for [Rh(C5H5)2]⁺, [Rh(CO)2(C5H4X)] (X = H, NMe2, Cl, NO2), [Rh(CO)4]^-, [RhCl2(CO)2]^-, and [Rh(acac)L2] (acac = acetylacetonato, L2 = (C2H4)2, 1,5-cyclooctadiene, cyclooctatetraene). The slope of the regression line, however, is only ca. 0.8 instead of unity. Scaled appropriately, the theoretical δ(¹⁰³Rh) values differ from experiment by less than 100 ppm (mean absolute deviation), over a range of 3600 ppm. In the [Rh(CO)2(C5H4X)] series, the computed activation barriers of PH3 addition correlate well with the experimental rate constants for CO/PPh3 displacement, thereby reproducing theoretically the relation between reactivities and ¹⁰³Rh chemical shifts that has been reported experimentally.

⁹¹Zr NMR chemical shifts and line widths (Δν1/2) are reported for a number of ring-bridged and ring-substituted zirconocene dichloride, dibromide, and dimethyl complexes. Ab initio computations at the SCF level employing basis sets of moderate size suggest that the magnitude of the electric field gradient (EFG) at the Zr atom dominates Δν1/2 when the substituents X at Zr are varied (X = Br, Cl, Me). Substituents at the cyclopentadiene (Cp) rings affect the computed EFGs much less; in these cases, the line widths Δν1/2 are governed by the molecular correlation times τc, which were obtained for several zirconocene dichlorides from T1(¹³C) measurements. Experimental trends in δ(⁹¹Zr) of zirconocenes are well reproduced computationally with the IGLO (individual gauge for localized orbitals) or GIAO (gauge including atomic orbitals) SCF methods employing large basis sets. Model calculations suggest that δ(⁹¹Zr), as well as the EFG, are quite sensitive to the inclination and twist angles of the Cp rings and, to a lesser extent, to the CpZrCp‘ angle. A substantial deshielding, δ(⁹¹Zr) ca. 700 ppm, is predicted for (C5H5)2ZrMe⁺, presumably the active olefin-polymerizing catalyst.

A⁹¹Zr chemical shift of −1500 ppm (relative to (C5H5)2ZrBr2) is predicted for Zr@C28 at the GIAO (gauge-including atomic orbitals)-B3LYP level using a medium-sized basis set and SCF optimized geometries. This value is even more shielded than the one predicted for hypothetical (η⁵-C5H5)4Zr, ca. −1100 ppm at the same level. A noticeable deshielding with repect to Zr@C28 is indicated for Zr@C28H2, a model for exohedrally substituted derivatives. Electron correlation effects on δ(⁹¹Zr), as assessed by GIAO-B3LYP vs GIAO-SCF results, are much larger for these endohedral Zr fullerenes than for any other Zr compound studied so far. According to the computed electric field gradients, quadrupolar line broadening should be large for the known (η⁵-C5H5)3(η¹-C5H5)Zr but should be small for Zr@C28H2. Since Zr@C28 has been detected mass-spectroscopically “in substantial yield” (Guo, T.; et al. Science 1992, 257, 1661), ⁹¹Zr NMR spectroscopy is suggested as the analytical method of choice for Zr@C28 and derivatives thereof.