Publications (435)2409.61 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: Improved versions of the segmented allelectron relativistically contracted (SARC) basis sets for the lanthanides are presented. The secondgeneration SARC2 basis sets maintain the efficient construction of their predecessors and their individual adaptation to the DKH2 and ZORA Hamiltonians, but feature exponents optimized with a completely new orbital shape fitting procedure and a slightly expanded f space that results in sizable improvement in CASSCF energies and in the significantly more accurate prediction of spinorbit coupling parameters. Additionally, an extended set of polarization/correlation functions is constructed that is appropriate for multireference correlated calculations, and new auxiliary basis sets for use in resolutionofidentity (densityfitting) approximations in combination with both DFT and wave function based treatments. Thus, the SARC2 basis sets extend the applicability of the firstgeneration DFToriented basis sets to routine allelectron wave function based treatments of lanthanide complexes. The new basis sets are benchmarked with respect to excitation energies, radial distribution functions, optimized geometries, orbital eigenvalues, ionization potentials, and spinorbit coupling parameters of lanthanide systems, and are shown to be suitable for the description of magnetic and spectroscopic properties using both DFT and multireference wave function based methods.  [Show abstract] [Hide abstract]
ABSTRACT: Domain based local pair natural orbital coupled clustertheory with single, double, and perturbative triple excitations (DLPNOCCSD(T)) is a highly efficient local correlation method. It is known to be accurate and robust and can be used in a black box fashion in order to obtain coupled cluster quality total energies for large molecules with several hundred atoms. While previous implementations showed near linear scaling up to a few hundred atoms, several nonlinear scaling steps limited the applicability of the method for very large systems. In this work, these limitations are overcome and a linear scaling DLPNOCCSD(T) method for closed shell systems is reported. The new implementation is based on the concept of sparse maps that was introduced in Part I of this series [P. Pinski, C. Riplinger, E. F. Valeev, and F. Neese, J. Chem. Phys. 143, 034108 (2015)]. Using the sparse map infrastructure, all essential computational steps (integral transformation and storage, initial guess, pair natural orbital construction, amplitude iterations, triples correction) are achieved in a linear scaling fashion. In addition, a number of additional algorithmic improvements are reported that lead to significant speedups of the method. The new, linearscaling DLPNOCCSD(T) implementation typically is 7 times faster than the previous implementation and consumes 4 times less disk space for large threedimensional systems. For linear systems, the performance gains and memory savings are substantially larger. Calculations with more than 20 000 basis functions and 1000 atoms are reported in this work. In all cases, the time required for the coupled cluster step is comparable to or lower than for the preceding HartreeFock calculation, even if this is carried out with the efficient resolutionoftheidentity and chainofspheres approximations. The new implementation even reduces the error in absolute correlation energies by about a factor of two, compared to the already accurate previous implementation.  [Show abstract] [Hide abstract]
ABSTRACT: The redox potential of synthetic oligonuclear transition metal complexes has been shown to correlate with the Lewis acidity of a redoxinactive cation connected to the redoxactive transition metals of the cluster via oxo or hydroxo bridges. Such heterometallic clusters are important cofactors in many metalloenzymes, where it is speculated that the redoxinactive constituent ion of the cluster serves to optimize its redox potential for electron transfer or catalysis. A principal example is the oxygenevolving complex in photosystem II of natural photosynthesis, a Mn4CaO5 cofactor that oxidizes water into dioxygen, protons and electrons. Calcium is critical for catalytic function, but its precise role is not yet established. In analogy to synthetic complexes it has been suggested that Ca2+ finetunes the redox potential of the manganese cluster. Here we evaluate this hypothesis by computing the relative redox potentials of substituted derivatives of the oxygenevolving complex with the cations Sr2+, Gd3+, Cd2+, Zn2+, Mg2+, Sc3+, Na+ and Y3+ for two sequential transitions of its catalytic cycle. The theoretical approach is validated with a series of experimentally wellcharacterized Mn3AO4 cubane complexes that are structural mimics of the enzymatic cluster. Our results reproduce perfectly the experimentally observed correlation between the redox potential and the Lewis acidities of redoxinactive cations for the synthetic complexes. However, it is conclusively demonstrated that this correlation does not hold for the oxygen evolving complex. In the enzyme the redox potential of the cluster only responds to the charge of the redoxinactive cations and remains otherwise insensitive to their precise identity, precluding redoxtuning of the metal cluster as a primary role for Ca2+ in biological water oxidation.  [Show abstract] [Hide abstract]
ABSTRACT: Identifying catalytically active structures or intermediates in homogeneous and heterogeneous catalysis is a formidable challenge. However, to obtain experimentally verified insight into the active species in heterogeneous catalysis is a tremendously challenging problem. Many highly advanced spectroscopic and microscopic methods have been developed to probe surfaces. However, developing the full information content of the wealth of experimental information that is available through these methods has been proven to be ambitious. At least three key issues must be addressed: a) sample heterogeneity, b) interpretation of complex spectroscopic patterns in terms of geometric and electronic structure and c) crosscorrelation between different experimental methods. All three challenges must be addressed simultaneously through careful experiments. Key insights can be obtained by combining rate measurements with spectroscopic measurements. Such insitu experiments require dedicated experimental setups and frequently will also require to simplify the catalytic system as much as possible in order to render a coherent interpretation of the data conceivable. This implies the necessity for a more immediate connection between theory and experiment. It is the aim of this work to emphasize that strong correlation between theory and experiment can be uniquely established by combining a range of spectroscopic methods with the results of carefully calibrated theoretical spectroscopy. In this account we employ a combination of spectroscopic methods to study two closely related systems from the heterogeneous (the silicasupported vanadium oxide VOx/SBA15) and homogeneous (the complex K[VO(O2)Hheida]) domains. Spectroscopic measurements were conducted strictly parallel for both systems and consisted of oxygen Kedge and vanadium Ledge Xray absorption measurements in conjunction to resonance Raman spectroscopy. It is shown that the full information content of the spectra can be developed through advanced quantum chemical calculations that directly address the sought after structurespectra relationships. To this end we employ the recently developed restricted open shell configuration interaction theory together with the timedependent theory of electronic spectroscopy to calculate XAS and rR spectra respectively. The results of the study demonstrate that: a) a combination of several spectroscopic techniques is of paramount importance in identifying signature structural motifs and b) quantum chemistry is an extremely powerful guide in cross connecting theory and experiment as well as the homogeneous and heterogeneous catalysis fields. It is emphasized that the calculation of spectroscopic observables provides an excellent way for the critical experimental validation of the theoretical results.  [Show abstract] [Hide abstract]
ABSTRACT: In transitionmetal complexes, the geometric structure is intimately connected with the spin state arising from magnetic coupling between the paramagnetic ions. The tetramanganesecalcium cofactor that catalyzes biological water oxidation in photosystem II cycles through five catalytic intermediates, each of which adopts a specific geometric and electronic structure and is thus characterized by a specific spin state. Here, we review spinstructure correlations in Nature's watersplitting catalyst. The catalytic cycle of the Mn4O5Ca cofactor can be described in terms of spindependent reactivity. The lower "inactive" S states of the catalyst, S0 and S1, are characterized by lowspin ground states, SGS = (1)/2 and SGS = 0. This is connected to the "open cubane" topology of the inorganic core in these states. The S2 state exhibits structural and spin heterogeneity in the form of two interconvertible isomers and is identified as the spinswitching point of the catalytic cycle. The first S2 state form is an open cubane structure with a lowspin SGS = (1)/2 ground state, whereas the other represents the first appearance of a closed cubane topology in the catalytic cycle that is associated with a higherspin ground state of SGS = (5)/2. It is only this higherspin form of the S2 state that progresses to the "activated" S3 state of the catalyst. The structure of this final metastable catalytic state was resolved in a recent report, showing that all manganese ions are sixcoordinate. The magnetic coupling is dominantly ferromagnetic, leading to a highspin ground state of SGS = 3. The ability of the Mn4O5Ca cofactor to adopt two distinct structural and spinstate forms in the S2 state is critical for water binding in the S3 state, allowing spinstate crossing from the inactive, lowspin configuration of the catalyst to the activated, highspin configuration. Here we describe how an understanding of the magnetic properties of the catalyst in all S states has allowed conclusions on the catalyst function to be reached. A summary of recent literature results is provided that constrains the sequence of molecular level events: catalyst/substrate deprotonation, manganese oxidation, and water molecule insertion.  [Show abstract] [Hide abstract]
ABSTRACT: Anionic twocoordinate complexes of firstrow transitionmetal(I) centres are rare molecules that are expected to reveal new magnetic properties and reactivity. Recently, we demonstrated that a N(SiMe3 )2 () ligand set, which is unable to prevent dimerisation or extraneous ligand coordination at the +2 oxidation state of iron, was nonetheless able to stabilise anionic twocoordinate Fe(I) complexes even in the presence of a Lewis base. We now report analogous Cr(I) and Co(I) complexes with exclusively this amido ligand and the isolation of a [Mn(I) {N(SiMe3 )2 }2 ]2 (2) dimer that features a MnMn bond. Additionally, by increasing the steric hindrance of the ligand set, the twocoordinate complex [Mn(I) {N(Dipp)(SiMe3 )}2 ]() was isolated (Dipp=2,6iPr2 C6 H3 ). Characterisation of these compounds by using Xray crystallography, NMR spectroscopy, and magnetic susceptibility measurements is provided along with ligandfield analysis based on CASSCF/NEVPT2 ab initio calculations.  [Show abstract] [Hide abstract]
ABSTRACT: The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 19, containing either 2,2':6',2″terpyridine (tpy), 4,4'ditertbutyl2,2'bipyridine ((t)bpy), or 1,10phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [Co(I)(tpy(0))2](+), [Co(I)((t)bpy(0))3](+), and [Co(I)(phen(0))3](+) are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy(0), bpy(0), and phen(0). On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly lowspin Co(III) (S = 0) species, and the dicationic species [Co(II)(tpy(0))2](2+), [Co(II)((t)bpy(0))3](2+), and [Co(II)(phen(0))3](2+) contain highspin (S = (3)/2) Co(II). Notably, the cobalt(I) complexes do not display any structural indication of significant metaltoligand (t2g → π*) πbackdonation effects. Consistent with this proposal, the temperaturedependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni(II)(tpy(0))2](2+), [Ni(II)(bpy(0))3](2+), and [Ni(II)(phen(0))3](2+), which display d → d bands with very small molar extinction coefficients (ε < 60 M(1) cm(1)), the spectra of the cobalt(I) species exhibit intense bands (ε > 10(3) M(1) cm(1)) in the visible and nearIR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalttoligand πbackbonding. Similar calculations for the sixcoordinate neutral complexes [Co(II)(tpy(•))2](0) and [Co(II)(bpy(•))2(bpy(0))](0) point to a common S = (3)/2 ground state, each possessing a central highspin Co(II) ion and two πradical anion ligands. In addition, the excitedstates and ground state magnetic properties of [Co(I)(tpy(0))2][Co(I)(CO)4] have been explored by variabletemperature variablemagneticfield magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced Cterm behavior indicative of the presence of metaltoligand chargetransfer bands [in contrast to dd transitions of the nickel(II) analogue]. Timedependent DFT calculations have allowed assignment of these transitions as Co(3d) → π*(tpy) excitations. Metaltoligand chargetransfer states intermixing with the Co(d(8)) multiplets explain the remarkably large (and negative) zerofieldsplitting parameter D obtained from SQUID and MCD measurements. Groundstate electron and spindensity distributions of [Co(I)(tpy(0))2](+) have been investigated by multireference electronic structure methods: complete activespace selfconsistent field (CASSCF) and Nelectron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spinunrestricted B3LYPbased DFT calculations show a significant delocalization of the spin density from the Co(I) dxz,yz orbitals toward the empty π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,βspin polarization type (McConnel mechanism I) and is definitely not due to magnetic metaltoradical coupling. A comparison of these results with those for [Ni(II)(tpy(0))2](2+) (S = 1) is presented.  [Show abstract] [Hide abstract]
ABSTRACT: A disulfidebridged diiron complex with [FeSSFe] core, which represents an isomer of the common biological [2Fe2S] ferredoxintype clusters, was synthesized using strongly σdonating macrocyclic tetracarbene capping ligands. Though the complex is quite labile in solution, single crystals were obtained, and the structure was elucidated by Xray diffraction. The electronrich ironsulfur core is found to show rather unusual magnetic and electronic properties. Experimental data and density functional theory studies indicate extremely strong antiferromagnetic coupling (J > 800 cm(1)) between two lowspin iron(III) ions via the S2(2) bridge, and the intense nearIR absorption characteristic for the [FeSSFe] core was assigned to a S → Fe ligandtometal charge transfer transition.  [Show abstract] [Hide abstract]
ABSTRACT: Over the past several decades, tremendous efforts have been invested in finding molecules that display slow relaxation of magnetization and hence act as singlemolecule magnets (SMMs). While initial research was strongly focused on polynuclear transition metal complexes, it has become increasingly evident that SMM behavior can also be displayed in relatively simple mononuclear transition metal complexes. One of the first examples of a mononuclear SMM that shows a slow relaxation of the magnetization in the absence of an external magnetic field is the cobalt(II) tetrathiolate [Co(SPh)4](2). Fascinatingly, substitution of the donor ligand atom by oxygen or selenium dramatically changes zerofield splitting (ZFS) and relaxation time. Clearly, these large variations call for an indepth electronic structure investigation in order to develop a qualitative understanding of the observed phenomena. In this work, we present a systematic theoretical study of a whole series of complexes (PPh4)2[Co(XPh)4] (X = O, S, Se) using multireference ab initio methods. To this end, we employ the recently proposed ab initio ligand field theory, which allows us to translate the ab initio results into the framework of ligand field theory. Magnetostructural correlations are then developed that take into account the nature of metalligand covalent bonding, ligand spinorbit coupling, and geometric distortions away from pure tetrahedral symmetry. The absolute value of zerofield splitting increases when the ligand field strength decreases across the series from O to Te. The zerofield splitting of the ground state of the hypothetical [Co(TePh)4](2) complex is computed to be about twice as large as for the wellknown (PPh4)2[Co(SPh)4] compound. It is shown that due to the πanisotropy of the ligand donor atoms (S, Se) magnetostructural correlations in [Co(OPh)4](2) complex differ from [Co(S/SePh)4](2). In the case of almost isotropic OPh ligand, only variations in the first coordination sphere affect magnetic properties, but in the case of S/SePh ligand, variations in the first and second coordination sphere become equally important for magnetic properties.  [Show abstract] [Hide abstract]
ABSTRACT: Zerofield splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H2TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm1 for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm1, E = 0.1(2) cm1 and D = 13.4(6) cm1, E = 0.3(6) cm1 for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm1 for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space selfconsistent field (CASSCF) and the Nelectron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zerofield splitting D of the 6A1 ground state. D was calculated from wave functions of the electronic multiplets spanned by the d5 configuration of Fe(III) along with spinorbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ and πantibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX63 complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π and σantibonding energies eλX (λ = σ, π) in the series from X = F to I. Analysis of this correlation using secondorder perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. D parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental D and spectroscopic transitions for highspin Fe(III) complexes are proposed. 
Article: Resolving the Manganese Oxidation States in the Oxygenevolving Catalyst of Natural Photosynthesis
[Show abstract] [Hide abstract]
ABSTRACT: A frequent challenge when dealing with multinuclear transition metal clusters in biology is to determine the absolute oxidation states of the individual metal ions and to identify how they evolve during catalytic turnover. The oxygenevolving complex of biological photosynthesis, an active site that harbors an oxobridged Mn4Ca cluster as the wateroxidizing species, offers a prime example of such a challenge that withstood satisfactory resolution for decades. A multitude of experimental studies have approached this question and have offered insights from different angles, but they were also accompanied by incomplete or inconclusive interpretations. Only very recently, through a combination of experiment and theory, has a definitive assignment of the individual Mn oxidation states been achieved for all observable catalytic states of the complex. Here we review the information obtained by structural and spectroscopic methods, describe the interpretation and synthesis achieved through quantum chemistry, and summarize our current understanding of the electronic structure of nature’s water splitting catalyst. 
Article: Direct Observation of Very Large ZeroField Splitting in a Tetrahedral NiIISe4 Coordination Complex
[Show abstract] [Hide abstract]
ABSTRACT: The highspin (S = 1) tetrahedral NiII complex [Ni{iPr2(Se)NP(Se)iPr2}2] was investigated by magnetometry, spectroscopic and quantum chemical methods. Angleresolved magnetometry studies revealed the orientation of the magnetization principal axes. The very large zerofield splitting (zfs), D = 45.40(2) cm1, E = 1.91(2) cm1, of the complex was accurately determined by farinfrared magnetic spectroscopy, directly observing transitions between the spin sublevels of the triplet ground state. These are the largest zfs values ever determined  directly for a highspin NiII complex. Ab initio calculations further probed the electronic structure of the system, elucidating the factors controlling the sign and magnitude of D. The latter is dominated by spinorbit coupling contributions of the Ni ions, whereas the corresponding effects of the Se atoms are remarkably smaller.  [Show abstract] [Hide abstract]
ABSTRACT: First principle calculations of extended xray absorption fine structure (EXAFS) data have seen widespread use in bioinorganic chemistry, perhaps most notably for modeling the Mn4Ca site in the oxygen evolving complex (OEC) of photosystem II (PSII). The logic implied by the calculations rests on the assumption that it is possible to a priori predict an accurate EXAFS spectrum provided that the underlying geometric structure is correct. The present study investigates the extent to which this is possible using state of the art EXAFS theory. The FEFF program is used to evaluate the ability of a multiple scatteringbased approach to directly calculate the EXAFS spectrum of crystallographicallydefined model complexes. The results of these parameter free predictions are compared with the more traditional approach of fitting FEFF calculated spectra to experimental data. A series of seven crystallographically characterized Mn monomers and dimers is used as a test set. The largest deviations between the FEFF calculated EXAFS spectra and the experimental EXAFS spectra arise from the amplitudes. The amplitude errors result from a combination of errors in calculated S02 and DebyeWaller values, as well as uncertainties in background subtraction. Additional errors may be attributed to structural parameters, particularly in cases where reliable highresolution crystal structures are not available. Based on these investigations, the strengths and weaknesses of using first principle EXAFS calculations as a predictive tool are discussed. We demonstrate that a range of DFT optimized structures of the OEC may all be considered consistent with experimental EXAFS data and that caution must be exercised when using EXAFS data to obtain topological arrangements of complex clusters.  [Show abstract] [Hide abstract]
ABSTRACT: Hemilabile ligands, which have one donor that can reversibly bind to a metal, are widely used in transitionmetal catalysts to create open coordination sites. This change in coordination at the metal can also cause spinstate changes. Here, we explore a cobalt(I) system that is poised on the brink of hemilability and of a spinstate change and can rapidly interconvert between different spin states with different structures ("spin isomers"). The new cobalt(I) monocarbonyl complex L(tBu)Co(CO) (2) is a singlet ((1)2) in the solid state, with an unprecedented diketiminate binding mode where one of the C═C double bonds of an aromatic ring completes a pseudosquareplanar coordination. Dissolving the compound gives a substantial population of the triplet ((3)2), which has exceptionally large uniaxial zerofield splitting due to strong spinorbit coupling with a lowlying excited state. The interconversion of the two spin isomers is rapid, even at low temperature, and temperaturedependent NMR and electronic absorption spectroscopy studies show the energy differences quantitatively. Spectroscopically validated computations corroborate the presence of a low minimumenergy crossing point (MECP) between the two potential energy surfaces and elucidate the detailed pathway through which the βdiketiminate ligand "slips" between bidentate and arenebound forms: rather than dissociation, the cobalt slides along the aromatic system in a pathway that balances strain energy and cobaltligand bonding. These results show that multiple spin states are easily accessible in this hemilabile system and map the thermodynamics and mechanism of the transition. 
Article: Hydride bridge in [NiFe]hydrogenase observed by nuclear resonance vibrational spectroscopy
[Show abstract] [Hide abstract]
ABSTRACT: The metabolism of many anaerobes relies on [NiFe]hydrogenases, whose characterization when bound to substrates has proven nontrivial. Presented here is direct evidence for a hydride bridge in the active site of the 57 Felabelled fully reduced NiR form of Desulfovibrio vulgaris Miyazaki F [NiFe]hydrogenase. A unique ' wagging' mode involving H motion perpendicular to the Ni(μ1/4H)57 Fe plane was studied using 57 Fespecific nuclear resonance vibrational spectroscopy and density functional theory (DFT) calculations. On Ni(μ1/4D) 57 Fe deuteride substitution, this wagging causes a characteristic perturbation of FeCO/CN bands. Spectra have been interpreted by comparison with Ni(μ1/4H/D) 57 Fe enzyme mimics [(dppe)Ni(μ1/4pdt)(μ1/4H/D) 57 Fe(CO)3 ]+ and DFT calculations, which collectively indicate a lowspin Ni(II)(μ1/4H)Fe(II) core for NiR, with H binding Ni more tightly than Fe. The present methodology is also relevant to characterizing FeH moieties in other important natural and synthetic catalysts.  [Show abstract] [Hide abstract]
ABSTRACT: In this Forum Article, an extensive discussion of the mechanism of sixelectron, sevenproton nitrite reduction by the cytochrome c nitrite reductase enzyme is presented. On the basis of previous studies, the entire mechanism is summarized and a unified picture of the most plausible sequence of elementary steps is presented. According to this scheme, the mechanism can be divided into five functional stages. The first phase of the reaction consists of substrate binding and NO bond cleavage. Here His277 plays a crucial role as a proton donor. In this step, the NO bond is cleaved heterolytically through double protonation of the substrate. The second phase of the mechanism consists of two protoncoupled electrontransfer events, leading to an HNO intermediate. The third phase involves the formation of hydroxylamine, where Arg114 provides the necessary proton for the reaction. The second NO bond is cleaved in the fourth phase of the mechanism, again triggered by proton transfer from His277. The Tyr218 side chain governs the fifth and last phase of the mechanism. It consists of radical transfer and ammonia formation. Thus, this mechanism implies that all conserved activesite side chains work in a concerted way in order to achieve this complex chemical transformation from nitrite to ammonia. The Forum Article also provides a detailed discussion of the density functional theory based cluster model approach to bioinorganic reactivity. A variety of questions are considered: the resting state of enzyme and substrate binding modes, interaction with the metal site and with activesite side chains, electron and protontransfer events, substrate dissociation, etc.  [Show abstract] [Hide abstract]
ABSTRACT: Multifrequency pulsed EPR data are reported for a series of oxygen bridged (µoxo/ µhydroxo) bimetallic manganese complexes where the oxygen is labeled with the magnetically active isotope (17)O (I = 5/2). Two synthetic complexes and two biological metallocofactors are examined: a planar bisµoxo bridged complex and a bent, bisµoxoµcarboxylato bridge complex; the dimanganese catalase, which catalyzes the dismutation of H2O2 to H2O and O2; and the recently identified manganese/iron cofactor of the R2lox protein, a homologue of the small subunit of the ribonuclotide reductase enzyme (class 1c). High field (Wband) hyperfine EPR spectroscopies are demonstrated to be ideal methods to characterize the (17)O magnetic interactions, allowing a magnetic fingerprint for the bridging oxygen ligand to be developed. It is shown that the μoxo bridge motif displays small positive isotropic hyperfine coupling constant of about +5 to +8 MHz and an anisotropic/dipolar coupling of 9 MHz. In addition, protonation of the bridge is correlated with an increase of the hyperfine coupling constant. Broken symmetry Density Functional Theory is evaluated as a predictive tool for estimating hyperfine coupling of bridging species. Experimental and theoretical results provide a framework for the characterization of oxygen bridge in Mn metallocofactor systems, including water oxidizing cofactor of Photosystem II, allowing substrate/solvent interface to be examined throughout its catalytic cycle.  [Show abstract] [Hide abstract]
ABSTRACT: The recently developed domainbased local pair natural orbital coupled cluster theory with single, double, and perturbative triple excitations (DLPNOCCSD(T)) delivers results that are closely approaching those of the parent canonical coupled cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2−3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2−4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the coupled cluster step scales nearly linearly with system size. Since, in many instances, the coupled cluster step in DLPNOCCSD(T) is cheaper or at least not much more expensive than the preceding Hartree−Fock calculation, it is useful to compare the method against modern density functional theory (DFT), which requires an effort comparable to that of Hartree−Fock theory (at least if Hartree−Fock exchange is part of the functional definition). Double hybrid density functionals (DHDF's) even require a MP2like step. The purpose of this article is to evaluate the cost vs accuracy ratio of DLPNOCCSD(T) against modern DFT (including the PBE, B3LYP, M062X, B2PLYP, and B2GPPLYP functionals and, where applicable, their van der Waals corrected counterparts). To eliminate any possible bias in favor of DLPNOCCSD(T), we have chosen established benchmark sets that were specifically proposed for evaluating DFT functionals. It is demonstrated that DLPNOCCSD(T) with any of the three default thresholds is more accurate than any of the DFT functionals. Furthermore, using the augccpVTZ basis set and the LoosePNO default settings, DLPNOCCSD(T) is only about 1.2 times slower than B3LYP. With NormalPNO thresholds, DLPNOCCSD(T) is about a factor of 2 slower than B3LYP and shows a mean absolute deviation of less than 1 kcal/mol to the reference values for the four different data sets used. Our conclusion is that coupled cluster energies can indeed be obtained at near DFT cost.  [Show abstract] [Hide abstract]
ABSTRACT: The development of efficient catalysts with base metals for CO2 hydrogenation has always been a major thrust of interest. A series of experimental and theoretical work has revealed that the catalytic cycle typically involves two key steps, namely, basepromoted heterolytic H2 splitting and hydride transfer to CO2, either of which can be the ratedetermining step (RDS) of the entire reaction. To explore the determining factor for the nature of RDS, we present herein a comparative mechanistic investigation on CO2 hydrogenation mediated by [M(H)(η(2)H2)(PP3(Ph))](n+) (M = Fe(II), Ru(II), and Co(III); PP3(Ph) = tris(2(diphenylphosphino)phenyl)phosphine) type complexes. In order to construct reliable free energy profiles, we used highly correlated wave function based ab initio methods of the coupled cluster type alongside the standard density functional theory. Our calculations demonstrate that the hydricity of the metalhydride intermediate generated by H2 splitting dictates the nature of the RDS for the Fe(II) and Co(III) systems, while the RDS for the Ru(II) catalyst appears to be ambiguous. CO2 hydrogenation catalyzed by the Fe(II) complex that possesses moderate hydricity traverses an H2splitting RDS, whereas the RDS for the highhydricity Co(III) species is found to be the hydride transfer. Thus, our findings suggest that hydricity can be used as a practical guide in future catalyst design. Enhancing the electronaccepting ability of lowhydricity catalysts is likely to improve their catalytic performance, while increasing the electrondonating ability of highhydricity complexes may speed up CO2 conversion. Moreover, we also established the active roles of base NEt3 in directing the heterolytic H2 splitting and assisting product release through the formation of an acidbase complex.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, a systematic infrastructure is described that formalizes concepts implicit in previous work and greatly simplifies computer implementation of reducedscaling electronic structure methods. The key concept is sparse representation of tensors using chains of sparse maps between two index sets. Sparse map representation can be viewed as a generalization of compressed sparse row, a common representation of a sparse matrix, to tensor data. By combining few elementary operations on sparse maps (inversion, chaining, intersection, etc.), complex algorithms can be developed, illustrated here by a linearscaling transformation of threecenter Coulomb integrals based on our compact code library that implements sparse maps and operations on them. The sparsity of the threecenter integrals arises from spatial locality of the basis functions and domain density fitting approximation. A novel feature of our approach is the use of differential overlap integrals computed in linearscaling fashion for screening products of basis functions. Finally, a robust linear scaling domain based local pair natural orbital secondorder MöllerPlesset (DLPNOMP2) method is described based on the sparse map infrastructure that only depends on a minimal number of cutoff parameters that can be systematically tightened to approach 100% of the canonical MP2 correlation energy. With default truncation thresholds, DLPNOMP2 recovers more than 99.9% of the canonical resolution of the identity MP2 (RIMP2) energy while still showing a very early crossover with respect to the computational effort. Based on extensive benchmark calculations, relative energies are reproduced with an error of typically <0.2 kcal/mol. The efficiency of the local MP2 (LMP2) method can be drastically improved by carrying out the LMP2 iterations in a basis of pair natural orbitals. While the present work focuses on local electron correlation, it is of much broader applicability to computation with sparse tensors in quantum chemistry and beyond.
Publication Stats
20k  Citations  
2,409.61  Total Impact Points  
Top Journals
Institutions

20122016

Max Planck Institute for Chemical Energy Conversion
MülheimonRuhr, North RhineWestphalia, Germany 
Università degli Studi di Salerno
 Department of BioMedical and Pharmaceutical Sciences FARMABIOMED
Fisciano, Campania, Italy 
Universitat Rovira i Virgili
 Department of Physical and Inorganic Chemistry
Tarraco, Catalonia, Spain


2015

Stockholm University
Tukholma, Stockholm, Sweden


2014

Jacobs University
 SES  School of Engineering & Science
Bremen, Bremen, Germany


20122013

Bulgarian Academy of Sciences
 Institute of General and Inorganic Chemistry
Ulpia Serdica, SofiaCapital, Bulgaria


20032013

Max Planck Institute for Chemistry
Mayence, RheinlandPfalz, Germany


20062012

University of Bonn
 Institute of Physical and Theoretical Chemistry
Bonn, North RhineWestphalia, Germany 
Cornell University
 Department of Chemistry and Chemical Biology
Итак, New York, United States


2009

University of Rochester
 Department of Chemistry
Rochester, New York, United States


2008

University of Wisconsin–Madison
Madison, Wisconsin, United States 
University of Münster
 Institute of Organic Chemistry
Muenster, North RhineWestphalia, Germany


2005

Max Planck Institute for Coal Research
MülheimonRuhr, North RhineWestphalia, Germany 
ChristianAlbrechtsUniversität zu Kiel
 Institute of Inorganic Chemistry
Kiel, SchleswigHolstein, Germany 
Universität Paderborn
 Department of Physics
Paderborn, North RhineWestphalia, Germany 
The University of Arizona
 Department of Chemistry and Biochemistry (College of Science)
Tucson, Arizona, United States


2002

Universitätsklinikum Erlangen
Erlangen, Bavaria, Germany


19972002

Universität Konstanz
 Faculty of Sciences
Constance, BadenWürttemberg, Germany 
Stanford University
 Department of Chemistry
Stanford, CA, United States
