Honors & Awards
CIC Advancement Award for Computational Chemistry 2019, German Chemical Society (GDCh) (2019-11)
Leopoldina Research Fellowship, German National Academy of Sciences Leopoldina (2018-11)
Travel Scholarship for an Invited Seminar Talk, German Academic Exchange Service (DAAD) (2017-12)
Poster Award, The 8th Molecular Quantum Mechanics Conference in Uppsala, Sweden (2016-06)
Poster Award, The 51st Symposium on Theoretical Chemistry in Potsdam, Germany (2015-09)
Friedrich Wilhelm Prize, RWTH Aachen University (2013-11)
PROMOS Scholarship, RWTH Aachen University and the German Academic Exchange Service (DAAD) (2012-04)
Doctor of Philosophy, University of Bonn (2018)
Master of Science, Rheinisch-Westfalische Technische Hochschule (2012)
Bachelor of Science, Rheinisch-Westfalische Technische Hochschule (2009)
A Simplified Spin-Flip Time-Dependent Density Functional Theory Approach for the Electronic Excitation Spectra of Very Large Diradicals.
The journal of physical chemistry. A
Experimentalists working with diradicals are often facing the question of what kind of species among singlet or triplet diradicals or closed-shell molecules are observed. To treat large diradicals with a high density of electronic states, we propose a simplified version of the spin-flip time-dependent density functional theory (SF-TD-DFT) method for a fast computation of their state energies and absorption spectra with an accuracy similar to the nonsimplified scheme. An ultrafast tight-binding variant called SF-sTD-DFT-xTB is also developed to treat even larger systems. For a benchmark set of nine diradicals, good agreement between simplified and conventional SF excitation energies for standard functionals is found. This shows that the proposed parameterization is robust for a wide range of Fock exchange mixing values. With the asymptotically correct response integrals used in SF-sTD-DFT and a correction factor of 2 for the transition moments, the SF-sTD-DFT/B5050LYP/cc-pVDZ method even outperforms the nonsimplified scheme at drastically reduced computational effort when comparing to the experimental absorption spectra for this set of diradicals. To showcase the actual performance of the method, absorption spectra of two mu-hydroxo-bridged dimers of corrole tape Ga(III) complex derivatives were computed and compared to the experiment, providing good qualitative agreement. Finally, a comparison with the high-spin triplet spectrum of a perylene bisimide biradical and the one determined at the SF-sTD-DFT level showed that at room temperature, mostly triplet diradicals are present in solution.
View details for DOI 10.1021/acs.jpca.9b03176
View details for PubMedID 31199632
- Extension of the element parameter set for ultra-fast excitation spectra calculation (sTDA-xTB) MOLECULAR PHYSICS 2019; 117 (9-12): 1104–16
A generally applicable atomic-charge dependent London dispersion correction.
The Journal of chemical physics
2019; 150 (15): 154122
The so-called D4 model is presented for the accurate computation of London dispersion interactions in density functional theory approximations (DFT-D4) and generally for atomistic modeling methods. In this successor to the DFT-D3 model, the atomic coordination-dependent dipole polarizabilities are scaled based on atomic partial charges which can be taken from various sources. For this purpose, a new charge-dependent parameter-economic scaling function is designed. Classical charges are obtained from an atomic electronegativity equilibration procedure for which efficient analytical derivatives with respect to nuclear positions are developed. A numerical Casimir-Polder integration of the atom-in-molecule dynamic polarizabilities then yields charge- and geometry-dependent dipole-dipole dispersion coefficients. Similar to the D3 model, the dynamic polarizabilities are precomputed by time-dependent DFT and all elements up to radon (Z = 86) are covered. The two-body dispersion energy expression has the usual sum-over-atom-pairs form and includes dipole-dipole as well as dipole-quadrupole interactions. For a benchmark set of 1225 molecular dipole-dipole dispersion coefficients, the D4 model achieves an unprecedented accuracy with a mean relative deviation of 3.8% compared to 4.7% for D3. In addition to the two-body part, three-body effects are described by an Axilrod-Teller-Muto term. A common many-body dispersion expansion was extensively tested, and an energy correction based on D4 polarizabilities is found to be advantageous for larger systems. Becke-Johnson-type damping parameters for DFT-D4 are determined for more than 60 common density functionals. For various standard energy benchmark sets, DFT-D4 slightly but consistently outperforms DFT-D3. Especially for metal containing systems, the introduced charge dependence of the dispersion coefficients improves thermochemical properties. We suggest (DFT-)D4 as a physically improved and more sophisticated dispersion model in place of DFT-D3 for DFT calculations as well as other low-cost approaches like semi-empirical models.
View details for DOI 10.1063/1.5090222
View details for PubMedID 31005066
GFN2-xTB-An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions.
Journal of chemical theory and computation
An extended semiempirical tight-binding model is presented, which is primarily designed for the fast calculation of structures and noncovalent interaction energies for molecular systems with roughly 1000 atoms. The essential novelty in this so-called GFN2-xTB method is the inclusion of anisotropic second order density fluctuation effects via short-range damped interactions of cumulative atomic multipole moments. Without noticeable increase in the computational demands, this results in a less empirical and overall more physically sound method, which does not require any classical halogen or hydrogen bonding corrections and which relies solely on global and element-specific parameters (available up to radon, Z = 86). Moreover, the atomic partial charge dependent D4 London dispersion model is incorporated self-consistently, which can be naturally obtained in a tight-binding picture from second order density fluctuations. Fully analytical and numerically precise gradients (nuclear forces) are implemented. The accuracy of the method is benchmarked for a wide variety of systems and compared with other semiempirical methods. Along with excellent performance for the "target" properties, we also find lower errors for "off-target" properties such as barrier heights and molecular dipole moments. High computational efficiency along with the improved physics compared to its precursor GFN-xTB makes this method well-suited to explore the conformational space of molecular systems. Significant improvements are furthermore observed for various benchmark sets, which are prototypical for biomolecular systems in aqueous solution.
View details for PubMedID 30741547
Catalytic deracemization of chiral allenes by sensitized excitation with visible light
2018; 564 (7735): 240-+
Chiral compounds exist as enantiomers that are non-superimposable mirror images of each other. Owing to the importance of enantiomerically pure chiral compounds1-for example, as active pharmaceutical ingredients-separation of racemates (1:1 mixtures of enantiomers) is extensively performed2. Frequently, however, only a single enantiomeric form of a chiral compound is required, which raises the question of how a racemate can be selectively converted into a single enantiomer. Such a deracemization3 process is entropically disfavoured and cannot be performed by a conventional catalyst in solution. Here we show that it is possible to photochemically deracemize chiral compounds with high enantioselectivity using irradiation with visible light (wavelength of 420 nanometres) in the presence of catalytic quantities (2.5 mole per cent) of a chiral sensitizer. We converted an array of 17 chiral racemic allenes into the respective single enantiomers with 89 to 97 per cent enantiomeric excess. The sensitizer is postulated to operate by triplet energy transfer to the allene, with different energy-transfer efficiencies for the two enantiomers. It thus serves as a unidirectional catalyst that converts one enantiomer but not the other, and the decrease in entropy is compensated by light energy. Photochemical deracemization enables the direct formation of enantiopure materials from a racemic mixture of the same compound, providing a novel approach to the challenge of creating asymmetry.
View details for DOI 10.1038/s41586-018-0755-1
View details for Web of Science ID 000452972600047
View details for PubMedID 30542163
B97-3c: A revised low-cost variant of the B97-D density functional method
JOURNAL OF CHEMICAL PHYSICS
2018; 148 (6): 064104
A revised version of the well-established B97-D density functional approximation with general applicability for chemical properties of large systems is proposed. Like B97-D, it is based on Becke's power-series ansatz from 1997 and is explicitly parametrized by including the standard D3 semi-classical dispersion correction. The orbitals are expanded in a modified valence triple-zeta Gaussian basis set, which is available for all elements up to Rn. Remaining basis set errors are mostly absorbed in the modified B97 parametrization, while an established atom-pairwise short-range potential is applied to correct for the systematically too long bonds of main group elements which are typical for most semi-local density functionals. The new composite scheme (termed B97-3c) completes the hierarchy of "low-cost" electronic structure methods, which are all mainly free of basis set superposition error and account for most interactions in a physically sound and asymptotically correct manner. B97-3c yields excellent molecular and condensed phase geometries, similar to most hybrid functionals evaluated in a larger basis set expansion. Results on the comprehensive GMTKN55 energy database demonstrate its good performance for main group thermochemistry, kinetics, and non-covalent interactions, when compared to functionals of the same class. This also transfers to metal-organic reactions, which is a major area of applicability for semi-local functionals. B97-3c can be routinely applied to hundreds of atoms on a single processor and we suggest it as a robust computational tool, in particular, for more strongly correlated systems where our previously published "3c" schemes might be problematic.
View details for DOI 10.1063/1.5012601
View details for Web of Science ID 000425299800004
View details for PubMedID 29448802
Fully Automated Quantum-Chemistry-Based Computation of Spin-Spin-Coupled Nuclear Magnetic Resonance Spectra
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2017; 56 (46): 14763–69
We present a composite procedure for the quantum-chemical computation of spin-spin-coupled 1 H NMR spectra for general, flexible molecules in solution that is based on four main steps, namely conformer/rotamer ensemble (CRE) generation by the fast tight-binding method GFN-xTB and a newly developed search algorithm, computation of the relative free energies and NMR parameters, and solving the spin Hamiltonian. In this way the NMR-specific nuclear permutation problem is solved, and the correct spin symmetries are obtained. Energies, shielding constants, and spin-spin couplings are computed at state-of-the-art DFT levels with continuum solvation. A few (in)organic and transition-metal complexes are presented, and very good, unprecedented agreement between the theoretical and experimental spectra was achieved. The approach is routinely applicable to systems with up to 100-150 atoms and may open new avenues for the detailed (conformational) structure elucidation of, for example, natural products or drug molecules.
View details for DOI 10.1002/anie.201708266
View details for Web of Science ID 000414764600079
View details for PubMedID 28906074
View details for PubMedCentralID PMC5698732
A general intermolecular force field based on tight-binding quantum chemical calculations
JOURNAL OF CHEMICAL PHYSICS
2017; 147 (16): 161708
A black-box type procedure is presented for the generation of a molecule-specific, intermolecular potential energy function. The method uses quantum chemical (QC) information from our recently published extended tight-binding semi-empirical scheme (GFN-xTB) and can treat non-covalently bound complexes and aggregates with almost arbitrary chemical structure. The necessary QC information consists of the equilibrium structure, Mulliken atomic charges, charge centers of localized molecular orbitals, and also of frontier orbitals and orbital energies. The molecular pair potential includes model density dependent Pauli repulsion, penetration, as well as point charge electrostatics, the newly developed D4 dispersion energy model, Drude oscillators for polarization, and a charge-transfer term. Only one element-specific and about 20 global empirical parameters are needed to cover systems with nuclear charges up to radon (Z = 86). The method is tested for standard small molecule interaction energy benchmark sets where it provides accurate intermolecular energies and equilibrium distances. Examples for structures with a few hundred atoms including charged systems demonstrate the versatility of the approach. The method is implemented in a stand-alone computer code which enables rigid-body, global minimum energy searches for molecular aggregation or alignment.
View details for DOI 10.1063/1.4991798
View details for Web of Science ID 000414177600009
View details for PubMedID 29096497
Diastereoselective Self-Assembly of a Neutral Dinuclear Double-Stranded Zinc(II) Helicate via Narcissistic Self-Sorting
CHEMISTRY-A EUROPEAN JOURNAL
2017; 23 (50): 12380–86
A new bis(salicylimine) ligand based on the Tröger's base scaffold was synthesized in racemic and enantiomerically pure form. Upon coordination to zinc(II) ions this ligand undergoes highly diastereoselective self-assembly into neutral dinuclear double-stranded helicates as proven by XRD analysis and via comparison of experimental ECD spectra with those simulated with quantum-chemical methods. When the racemic ligand was used, self-assembly occurs under narcissistic self-sorting resulting in the formation of a racemic pair of helicates as revealed by NMR spectroscopy and XRD analysis.
View details for DOI 10.1002/chem.201702125
View details for Web of Science ID 000409406100046
View details for PubMedID 28650081
Biomolecular Structure Information from High-Speed Quantum Mechanical Electronic Spectra Calculation
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2017; 139 (34): 11682–85
A fully quantum mechanical (QM) treatment to calculate electronic absorption (UV-vis) and circular dichroism (CD) spectra of typical biomolecules with thousands of atoms is presented. With our highly efficient sTDA-xTB method, spectra averaged along structures from molecular dynamics (MD) simulations can be computed in a reasonable time frame on standard desktop computers. This way, nonequilibrium structure and conformational, as well as purely quantum mechanical effects like charge-transfer or exciton-coupling, are included. Different from other contemporary approaches, the entire system is treated quantum mechanically and neither fragmentation nor system-specific adjustment is necessary. Among the systems considered are a large DNA fragment, oligopeptides, and even entire proteins in an implicit solvent. We propose the method in tandem with experimental spectroscopy or X-ray studies for the elucidation of complex (bio)molecular structures including metallo-proteins like myoglobin.
View details for PubMedID 28799760
- Pyridyl Containing 1,5-Diaza-3,7-diphosphacyclooctanes as Bridging Ligands for Dinuclear Copper(I) Complexes ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE 2017; 643 (14): 895–902
Extension of the D3 dispersion coefficient model
JOURNAL OF CHEMICAL PHYSICS
2017; 147 (3): 034112
A new model, termed D4, for the efficient computation of molecular dipole-dipole dispersion coefficients is presented. As in the related, well established D3 scheme, these are obtained as a sum of atom-in-molecule dispersion coefficients over atom pairs. Both models make use of dynamic polarizabilities obtained from first-principles time-dependent density functional theory calculations for atoms in different chemical environments employing fractional atomic coordination numbers for interpolation. Different from the D3 model, the coefficients are obtained on-the-fly by numerical Casimir-Polder integration of the dynamic, atomic polarizabilities α(iω). Most importantly, electronic density information is now incorporated via atomic partial charges computed at a semi-empirical quantum mechanical tight-binding level, which is used to scale the polarizabilities. Extended statistical measures show that errors for dispersion coefficients with the proposed D4 method are significantly lower than with D3 and other, computationally more involved schemes. Alongside, accurate isotropic charge and hybridization dependent, atom-in-molecule static polarizabilities are obtained with an unprecedented efficiency. Damping function parameters are provided for three standard density functionals, i.e., TPSS, PBE0, and B3LYP, allowing evaluation of the new DFT-D4 model for common non-covalent interaction energy benchmark sets.
View details for DOI 10.1063/1.4993215
View details for Web of Science ID 000406129100015
View details for PubMedID 28734285
A Robust and Accurate Tight-Binding Quantum Chemical Method for Structures, Vibrational Frequencies, and Noncovalent Interactions of Large Molecular Systems Parametrized for All spd-Block Elements (Z=1-86)
JOURNAL OF CHEMICAL THEORY AND COMPUTATION
2017; 13 (5): 1989–2009
We propose a novel, special purpose semiempirical tight binding (TB) method for the calculation of structures, vibrational frequencies, and noncovalent interactions of large molecular systems with 1000 or more atoms. The functional form of the method is related to the self-consistent density functional TB scheme and mostly avoids element-pair-specific parameters. The parametrization covers all spd-block elements and the lanthanides up to Z = 86 using reference data at the hybrid density functional theory level. Key features of the Hamiltonian are the use of partially polarized Gaussian-type orbitals, a double-ζ orbital basis for hydrogen, atomic-shell charges, diagonal third-order charge fluctuations, coordination number-dependent energy levels, a noncovalent halogen-bond potential, and the well-established D3 dispersion correction. The accuracy of the method, called Geometry, Frequency, Noncovalent, eXtended TB (GFN-xTB), is extensively benchmarked for various systems in comparison with existing semiempirical approaches, and the method is applied to a few representative structural problems in chemistry.
View details for DOI 10.1021/acs.jctc.7b00118
View details for Web of Science ID 000401221300013
View details for PubMedID 28418654
An Octanuclear Metallosupramolecular Cage Designed To Exhibit Spin-Crossover Behavior
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2017; 56 (18): 4930–35
By employing the subcomponent self-assembly approach utilizing 5,10,15,20-tetrakis(4-aminophenyl)porphyrin or its zinc(II) complex, 1H-4-imidazolecarbaldehyde, and either zinc(II) or iron(II) salts, we were able to prepare O-symmetric cages having a confined volume of ca. 1300 Å3 . The use of iron(II) salts yielded coordination cages in the high-spin state at room temperature, manifesting spin-crossover in solution at low temperatures, whereas corresponding zinc(II) salts led to the corresponding diamagnetic analogues. The new cages were characterized by synchrotron X-ray crystallography, high-resolution mass spectrometry, and NMR, Mössbauer, IR, and UV/Vis spectroscopy. The cage structures and UV/Vis spectra were independently confirmed by state-of-the-art DFT calculations. A remarkably high-spin-stabilizing effect through encapsulation of C70 was observed. The spin-transition temperature T1/2 is lowered by 20 K in the host-guest complex.
View details for DOI 10.1002/anie.201700832
View details for Web of Science ID 000399384700003
View details for PubMedID 28370757
Benzimidazolylquinoxalines: novel fluorophores with tuneable sensitivity to solvent effects
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2017; 19 (8): 6095–6104
We report on the photophysical properties, conjugation, conformational behavior, intra- and intermolecular hydrogen bonds (HBs) of a series of novel fluorophores, consisting of 3-arylquinoxaline and benzimidazole moieties linked by a single CC bond. Computations employing density functional theory (DFT) reveal that conjugation between these moieties stabilizes syn-conformers with two HB centers located on the same side of the molecule. Anti-conformers form stronger intermolecular HBs with DMSO and DMF than syn-conformers, and this influences the energy gap between syn- and anti-forms, especially upon excitation of the molecules to the S1 state. Substituents introduced in various positions of the molecules modify their conformational behavior, and mutual disposition of excited singlet states relative to the ground states. Various substitution patterns produce very different effects on relative quantum yield of luminescence: from a moderate increase in polar DMSO and DMF relative to 1,2-dichloroethane solutions to complete quenching of emission which is observable in polar media. The observed behavior is understood with the aid of DFT and time-dependent DFT calculations. The tuneability of the spectroscopic range of the luminescence and especially of its sensitivity to environmental effects allows rational design of the novel fluorophores of this family for various applications.
View details for DOI 10.1039/c6cp06658e
View details for Web of Science ID 000395869500045
View details for PubMedID 28191569
Effect of Conjugation Pathway in Metal-Free Room-Temperature Dual Singlet Triplet Emitters for Organic Light-Emitting Diodes
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2016; 7 (22): 4802–8
Metal-free dual singlet-triplet organic light-emitting diode (OLED) emitters can provide direct insight into spin statistics, spin correlations and spin relaxation phenomena, through a comparison of fluorescence to phosphorescence intensity. Remarkably, such materials can also function at room temperature, exhibiting phosphorescence lifetimes of several milliseconds. Using electroluminescence, quantum chemistry, and electron paramagnetic resonance spectroscopy, we investigate the effect of the conjugation pathway on radiative and nonradiative relaxation of the triplet state in phenazine-based compounds and demonstrate that the contribution of the phenazine nπ* excited state is crucial to enabling phosphorescence.
View details for PubMedID 27788014
Ultra-fast computation of electronic spectra for large systems by tight-binding based simplified Tamm-Dancoff approximation (sTDA-xTB)
JOURNAL OF CHEMICAL PHYSICS
2016; 145 (5): 054103
The computational bottleneck of the extremely fast simplified Tamm-Dancoff approximated (sTDA) time-dependent density functional theory procedure [S. Grimme, J. Chem. Phys. 138, 244104 (2013)] for the computation of electronic spectra for large systems is the determination of the ground state Kohn-Sham orbitals and eigenvalues. This limits such treatments to single structures with a few hundred atoms and hence, e.g., sampling along molecular dynamics trajectories for flexible systems or the calculation of chromophore aggregates is often not possible. The aim of this work is to solve this problem by a specifically designed semi-empirical tight binding (TB) procedure similar to the well established self-consistent-charge density functional TB scheme. The new special purpose method provides orbitals and orbital energies of hybrid density functional character for a subsequent and basically unmodified sTDA procedure. Compared to many previous semi-empirical excited state methods, an advantage of the ansatz is that a general eigenvalue problem in a non-orthogonal, extended atomic orbital basis is solved and therefore correct occupied/virtual orbital energy splittings as well as Rydberg levels are obtained. A key idea for the success of the new model is that the determination of atomic charges (describing an effective electron-electron interaction) and the one-particle spectrum is decoupled and treated by two differently parametrized Hamiltonians/basis sets. The three-diagonalization-step composite procedure can routinely compute broad range electronic spectra (0-8 eV) within minutes of computation time for systems composed of 500-1000 atoms with an accuracy typical of standard time-dependent density functional theory (0.3-0.5 eV average error). An easily extendable parametrization based on coupled-cluster and density functional computed reference data for the elements H-Zn including transition metals is described. The accuracy of the method termed sTDA-xTB is first benchmarked for vertical excitation energies of open- and closed-shell systems in comparison to other semi-empirical methods and applied to exemplary problems in electronic spectroscopy. As side products of the development, a robust and efficient valence electron TB method for the accurate determination of atomic charges as well as a more accurate calculation scheme of dipole rotatory strengths within the Tamm-Dancoff approximation is proposed.
View details for DOI 10.1063/1.4959605
View details for Web of Science ID 000381680000004
View details for PubMedID 27497535
Synthesis and Comprehensive Structural and Chiroptical Characterization of Enones Derived from (-)-alpha-Santonin by Experiment and Theory
JOURNAL OF ORGANIC CHEMISTRY
2016; 81 (11): 4588–4600
The aim of the present work is to explain the causes of the observed deviations from sector and helicity rules to determine the absolute configuration of optically active α,β-unsaturated ketones by means of electronic circular dichroism (ECD). To this end, a series of model compounds with a common decahydronaphthalene skeleton representing both cisoid and transoid enones were synthesized. In the framework of this work, detailed dichroic studies supported by single crystal X-ray analysis were performed where possible. To assist the achievement of the desired objectives the conformational flexibility of the selected cis-enones through the dependence of solvent and temperature on the ECD spectra were examined. All experimental studies were supplemented by detailed DFT calculations. A notable result of the study is assessing the applicability of the enone sector and helicity rules in dichroic studies and potential restrictions. To this end, a number of factors that could determine the signs of the individual Cotton effects has been considered. Among these nonminimum structure effects, i.e., twisting of the enone chromophore and nonplanarity of the enone double bond can be mentioned.
View details for PubMedID 27115057
- Recent research directions in Fribourg: nuclear dynamics in resonances revealed by 2-dimensional EEL spectra, electron collisions with ionic liquids and electronic excitation of pyrimidine EUROPEAN PHYSICAL JOURNAL D 2016; 70 (5)
Dispersion-Corrected Mean-Field Electronic Structure Methods
2016; 116 (9): 5105–54
Mean-field electronic structure methods like Hartree-Fock, semilocal density functional approximations, or semiempirical molecular orbital (MO) theories do not account for long-range electron correlation (London dispersion interaction). Inclusion of these effects is mandatory for realistic calculations on large or condensed chemical systems and for various intramolecular phenomena (thermochemistry). This Review describes the recent developments (including some historical aspects) of dispersion corrections with an emphasis on methods that can be employed routinely with reasonable accuracy in large-scale applications. The most prominent correction schemes are classified into three groups: (i) nonlocal, density-based functionals, (ii) semiclassical C6-based, and (iii) one-electron effective potentials. The properties as well as pros and cons of these methods are critically discussed, and typical examples and benchmarks on molecular complexes and crystals are provided. Although there are some areas for further improvement (robustness, many-body and short-range effects), the situation regarding the overall accuracy is clear. Various approaches yield long-range dispersion energies with a typical relative error of 5%. For many chemical problems, this accuracy is higher compared to that of the underlying mean-field method (i.e., a typical semilocal (hybrid) functional like B3LYP).
View details for DOI 10.1021/acs.chemrev.5b00533
View details for Web of Science ID 000375888300008
View details for PubMedID 27077966
Electronic Circular Dichroism of Helicene With Simplified TD-DFT: Beyond the Single Structure Approach
2016; 28 (5): 365–69
The electronic circular dichroism (ECD) spectrum of the recently synthesized helicene and a derivative comprising two triisopropylsilyloxy protection groups was computed by means of the very efficient simplified time-dependent density functional theory (sTD-DFT) approach. Different from many previous ECD studies of helicenes, nonequilibrium structure effects were accounted for by computing ECD spectra on "snapshots" obtained from a molecular dynamics (MD) simulation including solvent molecules. The trajectories are based on a molecule specific classical potential as obtained from the recently developed quantum chemically derived force field (QMDFF) scheme. The reduced computational cost in the MD simulation due to the use of the QMDFF (compared to ab-initio MD) as well as the sTD-DFT approach make realistic spectral simulations feasible for these compounds that comprise more than 100 atoms. While the ECD spectra of helicene and its derivative computed vertically on the respective gas phase, equilibrium geometries show noticeable differences, these are "washed" out when nonequilibrium structures are taken into account. The computed spectra with two recommended density functionals (ωB97X and BHLYP) and extended basis sets compare very well with the experimental one. In addition we provide an estimate for the missing absolute intensities of the latter. The approach presented here could also be used in future studies to capture nonequilibrium effects, but also to systematically average ECD spectra over different conformations in more flexible molecules. Chirality 28:365-369, 2016. © 2016 Wiley Periodicals, Inc.
View details for DOI 10.1002/chir.22594
View details for Web of Science ID 000375148300002
View details for PubMedID 27071653
Indirect synthesis of a pair of formal methane activation products at a phosphane/borane frustrated Lewis pair
2016; 45 (48): 19230–33
The regioisomeric formal Mes2PCH2CH2B(C6F5)2 FLP methane splitting products Mes2P(H)CH2CH2B(CH3)(C6F5)2 and Mes2P(CH3)CH2CH2B(H)(C6F5)2 were produced by indirect stepwise reactions. They were stable at 60 °C and were both characterized by X-ray diffraction. A DFT analysis revealed that their formation from the FLP and CH4 would be endergonic by +6.9 and +3.6 kcal mol-1, respectively, and it indicated that methane elimination from the zwitterionic products is kinetically hindered by barriers of 54.0 and 72.3 kcal mol-1, respectively.
View details for DOI 10.1039/c6dt04206f
View details for Web of Science ID 000390470400007
View details for PubMedID 27872934
Synthesis of novel pyridyl containing phospholanes and their polynuclear luminescent copper(I) complexes
2016; 45 (5): 2250–60
A novel type of cyclic P,N-ligands, pyridyl containing phospholanes, has been synthesized in a moderate yield by the reaction of primary phosphines with 1,4-dichlorobutane in a superbasic medium. A series of homo tetranuclear octahedral Cu4I4L2, dinuclear tetrahedral Cu2I2L3, and dinuclear "head-to-tail" Cu2I2L2 luminescent complexes with these ligands were obtained. All the compounds were characterized using a range of spectroscopic and computational techniques, and in the case of some Cu4I4L2 and Cu2I2L3 complexes, by single crystal X-ray diffraction. The structural diversity of the obtained complexes was reflected in their photophysical properties: phosphorescence spectra of the compounds display emission in broad spectral range of 471-615 nm. TD-DFT computations allow the assignment of a single emission band around 550 nm for Cu2I2L3 complexes and 471 nm for Cu2I2L2 complex to a vertical triplet-singlet transition from a metal-to-ligand and halide-to-ligand charge-transfer (3)(M + X)LCT excited state, whereas a second band at around 600 nm in the spectra of octahedral Cu4I4L2 complexes was assigned predominantly to Cu4I4 cluster-centered ((3)CC) excited state.
View details for DOI 10.1039/c5dt03346b
View details for Web of Science ID 000369413700051
View details for PubMedID 26621131
- Synthesis, Chiral Resolution, and Absolute Configuration of Functionalized Troger's Base Derivatives: Part III SYNTHESIS-STUTTGART 2015; 47 (20): 3118–32
Consistent structures and interactions by density functional theory with small atomic orbital basis sets
JOURNAL OF CHEMICAL PHYSICS
2015; 143 (5): 054107
A density functional theory (DFT) based composite electronic structure approach is proposed to efficiently compute structures and interaction energies in large chemical systems. It is based on the well-known and numerically robust Perdew-Burke-Ernzerhoff (PBE) generalized-gradient-approximation in a modified global hybrid functional with a relatively large amount of non-local Fock-exchange. The orbitals are expanded in Ahlrichs-type valence-double zeta atomic orbital (AO) Gaussian basis sets, which are available for many elements. In order to correct for the basis set superposition error (BSSE) and to account for the important long-range London dispersion effects, our well-established atom-pairwise potentials are used. In the design of the new method, particular attention has been paid to an accurate description of structural parameters in various covalent and non-covalent bonding situations as well as in periodic systems. Together with the recently proposed three-fold corrected (3c) Hartree-Fock method, the new composite scheme (termed PBEh-3c) represents the next member in a hierarchy of "low-cost" electronic structure approaches. They are mainly free of BSSE and account for most interactions in a physically sound and asymptotically correct manner. PBEh-3c yields good results for thermochemical properties in the huge GMTKN30 energy database. Furthermore, the method shows excellent performance for non-covalent interaction energies in small and large complexes. For evaluating its performance on equilibrium structures, a new compilation of standard test sets is suggested. These consist of small (light) molecules, partially flexible, medium-sized organic molecules, molecules comprising heavy main group elements, larger systems with long bonds, 3d-transition metal systems, non-covalently bound complexes (S22 and S66×8 sets), and peptide conformations. For these sets, overall deviations from accurate reference data are smaller than for various other tested DFT methods and reach that of triple-zeta AO basis set second-order perturbation theory (MP2/TZ) level at a tiny fraction of computational effort. Periodic calculations conducted for molecular crystals to test structures (including cell volumes) and sublimation enthalpies indicate very good accuracy competitive to computationally more involved plane-wave based calculations. PBEh-3c can be applied routinely to several hundreds of atoms on a single processor and it is suggested as a robust "high-speed" computational tool in theoretical chemistry and physics.
View details for DOI 10.1063/1.4927476
View details for Web of Science ID 000359377200011
View details for PubMedID 26254642
Electronic Circular Dichroism of Highly Conjugated pi-Systems: Breakdown of the Tamm-Dancoff/Configuration Interaction Singles Approximation
JOURNAL OF PHYSICAL CHEMISTRY A
2015; 119 (15): 3653–62
We show that the electronic circular dichroism (ECD) of delocalized π-systems represents a worst-case scenario for Tamm-Dancoff approximated (TDA) linear response methods. We mainly consider density functional theory (TDA-DFT) variants together with range-separated hybrids, but the conclusions also apply for other functionals as well as the configuration interaction singles (CIS) approaches. We study the effect of the TDA for the computation of ECD spectra in some prototypical extended π-systems. The C76 fullerene, a chiral carbon nanotube fragment, and helicene serve as model systems for inherently chiral, π-chromophores. Solving the full linear response problem is inevitable in order to obtain accurate ECD spectra for these systems. For the C76 fullerene and the nanotube fragment, TDA and CIS approximated methods yield spectra in the origin-independent velocity gauge formalism of incorrect sign which would lead to the assignment of the opposite (wrong) absolute configuration. As a counterexample, we study the ECD of an α-helix polypeptide chain. Here, the lowest-energy transitions are dominated by localized excitations within the individual peptide units, and TDA methods perform satisfactorily. The results may have far-reaching implications for simple semiempirical methods which often employ TDA and CIS for huge molecules. Our recently presented simplified time-dependent DFT approach proves to be an excellent low-cost linear response method which together with range-separated density functionals like ωB97X-D3 produces ECD spectra in very good agreement with experiment.
View details for DOI 10.1021/acs.jpca.5b01680
View details for Web of Science ID 000353249300014
View details for PubMedID 25798823
- The Association of Two "Frustrated" Lewis Pairs by State-of-the-Art Quantum Chemical Methods ISRAEL JOURNAL OF CHEMISTRY 2015; 55 (2): 235–42
Enantiotopos-Selective C-H Oxygenation Catalyzed by a Supramolecular Ruthenium Complex
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2015; 54 (2): 691–95
Spirocyclic oxindoles undergo an enantioselective oxygenation reaction (nine examples; e.r. up to 97:3) upon catalysis by a chiral ruthenium porphyrin complex (1 mol %). The catalyst exhibits a lactam ring, which is responsible for substrate association through hydrogen bonds, and an active ruthenium center, which is in a defined spatial relationship to the oxygenation substrate. DFT calculations illustrate the perfect alignment of the active site with the reactive C-H bond and suggest--in line with the kinetic isotope effect--an oxygen rebound mechanism for the reaction.
View details for DOI 10.1002/anie.201409224
View details for Web of Science ID 000347238800058
View details for PubMedID 25413591
Direct synthesis of a geminal zwitterionic phosphonium/hydridoborate system - developing an alternative tool for generating frustrated Lewis pair hydrogen activation systems
ORGANIC & BIOMOLECULAR CHEMISTRY
2015; 13 (20): 5783–92
A convenient way to a new class of geminal Mes2PH(+)/B(C6F5)2H(-) pairs is presented. It utilizes triflic acid addition to trans-Mes2PCH=CHB(C6F5)2 followed by triflate/hydride exchange. Thermally induced ring-closure gave a phosphonium/boratacyclopropane zwitterion 8 which formed the Mes2PH(CHMe)B(C6F5)2H P/B FLP-H2 product 10 by subsequent treatment with triflic acid and a silane, or alternatively with dihydrogen at 90 °C. The product 10 is an active catalyst for the hydrogenation of a variety of unsaturated organic substrates, including a quinoline derivative. Treatment of compound 8 with HB(C6F5)2 gave a bifunctional borane 14 which selectively reduced carbon monoxide to the formyl stage.
View details for DOI 10.1039/c5ob00634a
View details for Web of Science ID 000354438400026
View details for PubMedID 25906412
Enamine/butadienylborane cycloaddition in the frustrated Lewis pair regime
ORGANIC & BIOMOLECULAR CHEMISTRY
2015; 13 (42): 10477–86
The dienylborane 2a was prepared by regioselective alkyne hydroboration of the conjugated enyne 1a with Piers' borane [HB(C6F5)2]. Its reaction with a series of acetophenone derived enamines 3 resulted in the formation of the strong enamine β-carbon adduct with the borane Lewis acid (4). In contrast B-C adduct formation between the dienylborane 2a and a series of much more bulky cyclohexanone derived enamines (6) is rapidly reversible above ca.-30 °C and then leads to the formation of the [4 + 2]cycloaddition products 8. A DFT study revealed that this reaction is probably taking a stepwise route, proceeding by means of enamine addition to the dienylborane terminus to generate a zwitterionic borata-alkene/iminium ion intermediate that undergoes rapid subsequent ring closure. Heating of the products 8 led to amidoborane elimination from the vicinal amino/borane pair at the product framework to give the respective hexahydronaphthalene product 10. Subsequent treatment with TEMPO (2 equiv.) resulted in selective oxidation of the unsaturated ring to give the respective tetrahydronaphthalene derivative 12.
View details for DOI 10.1039/c5ob01602a
View details for Web of Science ID 000363214100009
View details for PubMedID 26330093
Free electrons and ionic liquids: study of excited states by means of electron-energy loss spectroscopy and the density functional theory multireference configuration interaction method
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2015; 17 (24): 15771–80
The technique of low energy (0-30 eV) electron impact spectroscopy, originally developed for gas phase molecules, is applied to room temperature ionic liquids (IL). Electron energy loss (EEL) spectra recorded near threshold, by collecting 0-2 eV electrons, are largely continuous, assigned to excitation of a quasi-continuum of high overtones and combination vibrations of low-frequency modes. EEL spectra recorded by collecting 10 eV electrons show predominantly discrete vibrational and electronic bands. The vibrational energy-loss spectra correspond well to IR spectra except for a broadening (∼0.04 eV) caused by the liquid surroundings, and enhanced overtone activity indicating a contribution from resonant excitation mechanism. The spectra of four representative ILs were recorded in the energy range of electronic excitations and compared to density functional theory multireference configuration interaction (DFT/MRCI) calculations, with good agreement. The spectra up to about 8 eV are dominated by π-π* transitions of the aromatic cations. The lowest bands were identified as triplet states. The spectral region 2-8 eV was empty in the case of a cation without π orbitals. The EEL spectrum of a saturated solution of methylene green in an IL band showed the methylene green EEL band at 2 eV, indicating that ILs may be used as a host to study nonvolatile compounds by this technique in the future.
View details for DOI 10.1039/c5cp01417d
View details for Web of Science ID 000356056000033
View details for PubMedID 26018044
The Thermochemistry of London Dispersion-Driven Transition Metal Reactions: Getting the 'Right Answer for the Right Reason'
2014; 3 (5): 177–89
Reliable thermochemical measurements and theoretical predictions for reactions involving large transition metal complexes in which long-range intramolecular London dispersion interactions contribute significantly to their stabilization are still a challenge, particularly for reactions in solution. As an illustrative and chemically important example, two reactions are investigated where a large dipalladium complex is quenched by bulky phosphane ligands (triphenylphosphane and tricyclohexylphosphane). Reaction enthalpies and Gibbs free energies were measured by isotherm titration calorimetry (ITC) and theoretically 'back-corrected' to yield 0 K gas-phase reaction energies (ΔE). It is shown that the Gibbs free solvation energy calculated with continuum models represents the largest source of error in theoretical thermochemistry protocols. The ('back-corrected') experimental reaction energies were used to benchmark (dispersion-corrected) density functional and wave function theory methods. Particularly, we investigated whether the atom-pairwise D3 dispersion correction is also accurate for transition metal chemistry, and how accurately recently developed local coupled-cluster methods describe the important long-range electron correlation contributions. Both, modern dispersion-corrected density functions (e.g., PW6B95-D3(BJ) or B3LYP-NL), as well as the now possible DLPNO-CCSD(T) calculations, are within the 'experimental' gas phase reference value. The remaining uncertainties of 2-3 kcal mol(-1) can be essentially attributed to the solvation models. Hence, the future for accurate theoretical thermochemistry of large transition metal reactions in solution is very promising.
View details for PubMedID 25478313
- A simplified time-dependent density functional theory approach for electronic ultraviolet and circular dichroism spectra of very large molecules COMPUTATIONAL AND THEORETICAL CHEMISTRY 2014; 1040: 45–53
Enantiomerically Pure [M6L12] or [M12L24] Polyhedra from Flexible Bis(Pyridine) Ligands
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2014; 53 (6): 1693–98
Coordination-driven self-assembly is one of the most powerful strategies to prepare nanometer-sized discrete (supra)molecular assemblies. Herein, we report on the use of two constitutionally isomeric BINOL-based bis(pyridine) ligands for this purpose. Upon coordination to Pd(II) ions these self-assemble into enantiomerically pure endo- and exo-functionalized hexa- and dodecanuclear metallosupramolecular spheres with a chiral skeleton depending on the substitution pattern of the BINOL core. These aggregates were characterized by NMR, MS, DLS, TEM, and EELS as well as ECD. Furthermore, experimental ECD data could be compared to those obtained from theoretical simulations using a simplified Tamm-Dancoff approximation to time-dependent DFT to rationalize the extraordinary high molar circular dichroisms. Despite the rotational freedom around the central aryl-aryl bond of these ligands, the self-assembly process happens completely selective in a "narcissistic" self-recognition manner.
View details for DOI 10.1002/anie.201308651
View details for Web of Science ID 000330558400044
View details for PubMedID 24453210
Combinations of Ethers and B(C6F5)(3) Function as Hydrogenation Catalysts
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2013; 52 (29): 7492–95
It works ether way: Labile adducts of dialkyl ethers with the electrophilic borane B(C6F5)3 are shown to scramble HD to H2 and D2 and catalyze the hydrogenation of 1,1-diphenylethylene.
View details for DOI 10.1002/anie.201303166
View details for Web of Science ID 000328708300024
View details for PubMedID 23775686
- From attraction to repulsion: anion-pi interactions between bromide and fluorinated phenyl groups CHEMICAL COMMUNICATIONS 2011; 47 (30): 8542–44