Michael Bühl

Calculation of NMR and EPR parameters : theory and applications

Here, readers are given a broad overview of all the pertinent topics, such as basic theory, methodic considerations, benchmark results and applications for both spectroscopy methods in such fields as biochemistry, bioinorganic chemistry as well as with different substance classes, including fullerenes, zeolites and transition metal compounds. The chapters have been written by leading experts in a given area, but with a wider audience in mind.

Geometries of Transition-Metal Complexes from Density-Functional Theory

Several levels of density functional theory, i.e., various combinations of exchange-correlation functionals and basis sets, have been employed to compute equilibrium geometries for a diverse set of 32 metal complexes from the first transition row, for which precise gas-phase geometries are known from electron diffraction or microwave spectroscopy. Most DFT levels beyond the local density approximation can reproduce the 50 metal-ligand bond distances selected in this set with reasonable accuracy, as assessed by mean and standard deviations of optimized vs observed values. The ranking of some popular functionals, ordered according to decreasing standard deviation, is BLYP ≈ HCTH > B3LYP > BP86 > TPSS ≈ TPSSh. Together with its hybrid variant, the recently introduced meta-GGA functional TPSS performs best of all tested functionals, with mean and standard deviations of -0.5 and 1.4 pm, respectively. Even smaller errors are found for a more compact but less diverse set of transition-metal mono- and dihalides, for which experimentally derived equilibrium geometries are available.

The DFT route to NMR chemical shifts

An overview is given of the recent development and use of density functional methods in nuclear magnetic resonance (NMR) chemical‐shift calculations. The available density functional theory (DFT) methods are discussed, and examples for their validation and application are given. Relativistic effects are also considered, with an emphasis on spin–orbit coupling. The systems discussed range from transition‐metal complexes and clusters via biological systems and fullerenes to weakly bound van der Waals molecules. DFT results not published previously comprise spin–orbit effects on 31P chemical shifts in phosphorus halides, the orientation of the 31P‐shift tensor in Ru4(PPh)(CO)13, δ(95Mo) data, 13C and endohedral chemical shifts for fullerenes and for C60H36, as well as the shielding surface of the Ne2 molecule. © 1999 John Wiley & Sons, Inc. J Comput Chem 20: 91–105, 1999

Mechanism of Olefin Metathesis with Catalysis by Ruthenium Carbene Complexes: Density Functional Studies on Model Systems

Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH(3))(L)Cl(2)Ru[double bond]CH(2)] with L=PH3 (I) and L=C(3)N(2)H(4)=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (Delta G(22-25)*[double bond] kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (Delta G(298)* is approximately equal to 5-10 kcal mol(-1)). The resulting active species (L)Cl(2)Ru[double bond]CH(2) can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (Delta G(298)* is approximately equal to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the complex to yield the ruthenacyclobutane (catalyst I, Delta G(298)*=12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, Delta G(298)*=10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe(3))(2)Cl(2)Ru[double bond]CH(2) (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.

Geometries of Second-Row Transition-Metal Complexes from Density-Functional Theory

A data set of 19 second-row transition-metal complexes has been collated from sufficiently precise gas-phase electron-diffraction experiments and used for evaluating errors in DFT optimized geometries. Equilibrium geometries have been computed using 15 different combinations of exchange-correlation functionals in conjunction with up to three different effective core potentials. Most DFT levels beyond the local density approximation can reproduce the 29 metal-ligand bond distances selected in this set with reasonable accuracy and precision, as assessed by the mean and standard deviations of optimized vs experimentally observed bond lengths. The pure GGAs tested in this study all have larger standard deviations than their corresponding hybrid variants. In contrast to previous findings for first-row transition-metal complexes, the TPSSh hybrid meta-GGA is slightly inferior to the best hybrid GGAs. The ranking of some popular density functionals, for second-row transition-metal complexes, ordered according to decreasing standard deviation, is VSXC ≈ LSDA > BLYP > BP86 > B3LYP ≈ TPSSh > PBE hybrid ≈ B3PW91 ≈ B3P86. When zero-point vibrational corrections, computed at the BP86/SDD level, are added to equilibrium bond distances obtained from a number of density-functional/basis-set combinations, the overall performance in terms of mean and standard deviations from experiment is not improved. For a combined data set comprised of the first- and second-row transition-metal complexes the hybrid functionals B3P86, B3PW91, and the meta-GGA hybrid TPSSh afford the lowest standard deviations.

Photoswitchable Catalysts: Correlating Structure and Conformational Dynamics with Reactivity by a Combined Experimental and Computational Approach

Photocontrol of a piperidines Brønsted basicity was achieved by incorporation of a bulky azobenzene group and could be translated into pronounced reactivity differences between ON- and OFF-states in general base catalysis. This enabled successful photomodulation of the catalysts activity in the nitroaldol reaction (Henry reaction). A modular synthetic route to the photoswitchable catalysts was developed and allowed for preparation and characterization of three azobenzene-derived bases as well as one stilbene-derived base. Solid-state structures obtained by X-ray crystal structure analysis confirmed efficient blocking of the active site in the E isomer representing the OFF-states, whereas a freely accessible active site was revealed for a representative Z isomer in the crystal. To correlate structure with reactivity of the catalysts, conformational dynamics were thoroughly studied in solution by NMR spectroscopy, taking advantage of residual dipolar couplings (RDCs), in combination with comprehensive DFT computational investigations of conformations and proton affinities.

DENSITY FUNCTIONAL COMPUTATIONS OF TRANSITION METAL NMR CHEMICAL SHIFTS : DRAMATIC EFFECTS OF HARTREE-FOCK EXCHANGE

Abstract The theoretical description of the 103 Rh chemical shift range of a number of organorhodium complexes is significantly better when the B3LYP hybrid functional is used instead of pure density functionals: the slope of the linear regression line of computed versus experimental δ( 103 Rh) shifts is 0.80, 0.90 and 0.97 with the SOS-DFPT-IGLO, GIAO-BPW91 and GIAO-B3LYP methods, respectively, using a large basis set and optimized geometries. For 57 Fe chemical shifts of several organoiron complexes, the improvement is even more dramatic: the corresponding slopes with the same methods are 0.55, 0.65, and 0.97, respectively. Inclusion of Hartree-Fock exchange has thus a huge effect on the computed transition metal chemical shifts.

The Relation Between Endohedral Chemical Shifts and Local Aromaticities in Fullerenes

Endohedral chemical shiftsδ(endo) at the very centers of fullerenes and their derivatives (right) can to a large extent be related to ring currents localized in the individual pentagons and hexagons. These ring currents are assessed by theoretical NICS (nucleus-independent chemical shift) calculations, which have recently been introduced as a probe for local aromatic and antiaromatic character.

Medium effects on 51V NMR chemical shifts: a density functional study.

Car-Parrinello molecular dynamics simulations were performed for [H2VO4], [VO2(OH2)4]+, and [VO(O2)2(OH2)]- in periodic boxes with 30, 28, and 29 water molecules, respectively, employing the BLYP density functional. On the timescale of the simulations, up to 2 ps, well-structured first solvation spheres are discernible for [H2VO4]- and [VO(O2)2(OH2)]- containing, on average, eight and ten water molecules, respectively. One of the four water molecules directly attached to the metal in [VO2(OH2)4]+ is only loosely bound, and the average coordination number of vanadium in aqueous VO2+ is between five and six. 51V chemical shifts were evaluated at the B3LYP level for representative snapshots along the trajectories, including the water molecules of the solvent by means of point charges. The resulting averaged delta(51V) values are proposed to model the combined effects of temperature (dynamic averaging) and solvent (charge polarization). Both effects are shown to be rather small, of the order of a few dozen ppm. The observed shielding of 51V in the bis(peroxo) complex with respect to the vanadate species is not reproduced computationally.

Automated Chemical Crystallography

The first fully automated small-molecule robotic X-ray diffractometer is described. After demonstrating the utility of the instrument using multiple samples of ammonium bitartrate, we investigated the conformational chirality of diphenyl dichalcogenide (E(2)Ph(2), where E = S, Se, or Te). Structural and computational studies suggest that the two enantiomers are energetically indistinguishable. Therefore, it was unsurprising that we found (in 35 suitable data collections) the proportion 0.51:0.49 of M-S(2)Ph(2) to P-S(2)Ph(2) in the bulk sample. Interestingly, after 65 data collections of Te(2)Ph(2), (46 provided suitable data sets), we found the proportion 0.72 +/- 0.13 of M-Te(2)Ph(2), suggesting there could be a statistically significant preference for the M-enantiomer in the sample examined here. We found that Se(2)Ph(2) underwent homochiral crystallization, with all 24 crystals being M. Our experiments may represent a salutary lesson in statistical analysis.