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  • Hermann, Gunter and Marsoner Steinkasserer, Lukas Eugen and Paulus, Beate and Tremblay, Jean Christophe: Dipole-Induced Transition Orbitals: A Novel Tool for Investigating Optical Transitions in Extended Systems, The Journal of Physical Chemistry Letters 9, 6624-6630 (2018) DOI: 10.1021/acs.jpclett.8b02253

Optical absorption spectra for nanostructures and solids can be obtained from the macroscopic dielectric function within the random phase approximation. While experimental spectra can be reproduced with good accuracy, important properties, such as the charge-transfer character associated with a particular transition, are not retrievable. This contribution presents a computationally inexpensive method for the analysis of optical and excitonic properties for extended systems based on solely their electronic ground-state structure. We formulate a perturbative orbital transformation theory based on dipole-induced transition moments between orbitals, which yields correlated pairs of particle and hole functions. To demonstrate the potency of this new transformation formalism, we investigate the nature of excitations in inorganic molecular complexes and in extended systems. With our method, it is possible to extract mechanistic insights from the transitions observed in the optical spectrum, without requiring explicit calculation of the many-electron excited states.

  • Frenio A. Redeker and Alexey Kropman and Carsten Müller and Sarah E. Zewge and Helmut Beckers and Beate Paulus and Sebastian Riedel: Theoretical investigation of the structures, stabilities and vibrational properties of triatomic interhalide ions and their alkali ion pairs, Journal of Fluorine Chemistry 216, 81 - 88 (2018) DOI: https://doi.org/10.1016/j.jfluchem.2018.10.007

While homonuclear free trihalide anions and their alkali ion pairs have been thoroughly described experimentally and theoretically, the coverage of their heteronuclear relatives in the literature remains fragmentary. This might be merely a consequence of the large variety of possible compositions of trihalide anions, however the experimental difficulties associated with the preparation of heteronuclear trihalide anions especially those containing the lightest halogen atom F could also play a role. This paper is a systematic approach to a complete theoretical description of the free inter trihalide anions XXX−, XXY−, YXY−, and XYZ− (X, Y, Z = F–I) and their alkali ion pairs MXXX, MYXX, MXXY, MYXY, and MXYZ (M = Li–Cs; X, Y, Z = F–Br). The minimum structures obtained from calculations at CCSD(T)/triple-ζ level of theory were analyzed and ordered with respect to trends in their structural appearance, dissociation energies, and vibrational properties. Predictions about the existence of mixed alkali trihalide anions and new insights about existing experimental data could be derived from calculated data.

  • Riddhish Pandharkar and Christian Becker and Johannes Budau and Zeinab Kaawar and Beate Paulus: A Computational Study of AlF3 and ACF Surfaces, Inorganics 6, 124 (2018) DOI: 10.3390/inorganics6040124

By applying first principles density functional theory (DFT) methods, different metal fluorides and their surfaces have been characterized. One of the most investigated metal fluorides is AlF3 in different polymorphs. Its chloride-doped analogon AlClxF3−x (ACF) has recently attracted much attention due to its application in catalysis. After presenting a summary of different first-principle studies on the bulk and surface properties of different main group fluorides, we will revisit the problem of the stability of different α -AlF3 surfaces and extend the investigation to chloride-doped counterparts to simulate the surface properties of amorphous ACF. For each material, we have considered ten different surface cuts with their respective terminations. We found that terminations of ( 011¯0 ) and ( 112¯0 ) yield the most stable surfaces for α -AlF3 and for the chlorine substituted surfaces. A potential equilibrium shape of the crystal for both α -AlF3 and ACF is visualized by a Wulff construction.

  • Schröder, Hendrik V. and Mekic, Amel and Hupatz, Henrik and Sobottka, Sebastian and Witte, Felix and Urner, Leonhard H. and Gaedke, Marius and Pagel, Kevin and Sarkar, Biprajit and Paulus, Beate and Schalley, Christoph A.: Switchable synchronisation of pirouetting motions in a redox-active [3]rotaxane, Nanoscale 10, 21425-21433 (2018) DOI: 10.1039/C8NR05534C

In this study, the crown/ammonium [3]rotaxane R2 is reported which allows a switchable synchronisation of wheel pirouetting motions. The rotaxane is composed of a dumbbell-shaped axle molecule with two mechanically interlocked macrocycles which are decorated with a redox-active tetrathiafulvalene (TTF) unit. Electrochemical, spectroscopic, and electron paramagnetic resonance experiments reveal that rotaxane R2 can be reversibly switched between four stable oxidation states (R2, R2˙+, R22(˙+), and R24+). The oxidations enable non-covalent, cofacial interactions between the TTF units in each state—including a stabilised mixed-valence (TTF2)˙+ and a radical-cation (TTF˙+)2 dimer interaction—which dictate a syn (R2, R2˙+, and R22(˙+)) or anti (R24+) ground state co-conformation of the wheels in the rotaxane. Furthermore, the strength of these wheel–wheel interactions varies with the oxidation state, and thus electrochemical switching allows a controllable synchronisation of the wheels’ pirouetting motions. DFT calculations explore the potential energy surface of the counter-rotation of the two interacting wheels in all oxidation states. The controlled coupling of pirouetting motions in rotaxanes can lead to novel molecular gearing systems which transmit rotational motion by switchable non-covalent in rotaxanes can lead to novel molecular gearing systems which transmit rotational motion by switchable non-covalent interactions.

  • ChunMei Liu, Jörn Manz and Jean Christophe Tremblay: From Molecular Symmetry Breaking to Symmetry Restoration by Attosecond Quantum Control, in Progress in Ultrafast Intense Laser Science XIV (2018) DOI: 10.1007/978-3-030-03786-4 Chapter 7
  • Grohmann,Thomas and Haase,Dietrich and Jia,Dongming and Manz,Jörn and Yang,Yonggang : Nuclear spin blockade of laser ignition of intramolecular rotation in the model boron rotor 11B13+, The Journal of Chemical Physics 149, 184302 (2018) DOI: 10.1063/1.5048358

The boron rotor 11B13+ consists of a tri-atomic inner “wheel” that may rotate in its pseudo-rotating ten-atomic outer “bearing”—this concerted motion is called “contorsion.” 11B13+ in its ground state has zero contorsional angular momentum. Starting from this initial state, it is a challenge to ignite contorsion by a laser pulse. We discover, however, that this is impossible, i.e., one cannot design any laser pulse that induces a transition from the ground to excited states with non-zero contorsional angular momentum. The reason is that the ground state is characterized by a specific combination of irreducible representations (IRREPs) of its contorsional and nuclear spin wavefunctions. Laser pulses conserve these IRREPs because hypothetical changes of the IRREPs would require nuclear spin flips that cannot be realized during the interaction with the laser pulse. We show that all excited target states of 11B13+ with non-zero contorsional angular momentum have different IRREPs that are inaccessible by laser pulses. Conservation of nuclear spins thus prohibits laser-induced transitions from the non-rotating ground to rotating target states. We discover various additional constraints imposed by conservation of nuclear spins, e.g., laser pulses can change clockwise to counter-clockwise contorsions or vice versa, but they cannot stop them. The results are derived in the frame of a simple model.

  • Dennis J. Diestler and Jörn Manz and Jhon F. Pérez-Torres: Comparison of approximate methods for computation of the concerted adiabatic electronic and nuclear fluxes in aligned H2+(2Σg+), Chemical Physics 514, 67 - 77 (2018) DOI: https://doi.org/10.1016/j.chemphys.2018.05.026

Approximate methods of computing the concerted electronic and nuclear fluxes associated with electronically adiabatic processes are developed and applied to the prototypal system, namely aligned H2+ vibrating in its electronic ground state (2Σg+), the only realistic system for which highly accurate (exact) electronic (EPD) and nuclear (NPD) probability densities, electronic (EFD) and nuclear (NFD) flux densities, as well as corresponding fluxes, are available. Alternative formulas for the electronic flux, Fe,EPD and Fe,EFD, based on either the EPD or the EFD, are derived from the continuity equation. The results of Born-Oppenheimer approximation (BOA) and of an ordered sequence of Born-Huang expansions (BHE) are presented. The BOA and first-order BHE are in excellent agreement with the exact for both the NPD and NFD, as well as for the EPD and Fe,EPD up to about 1ps. Higher-order BHE are necessary to achieve similar accuracy at longer times. In contrast, the BOA and first-order BHE yield zero EFD and therefore also zero Fe,EFD. Although the higher-order BHE give non-zero values for these properties, they disagree flagrantly with their exact correlates. The error is traceable to numerical ill-conditioning of the working expression for the EFD. In summary, the BOA is adequate to compute accurate NPD, NFD, EPD and Fe,EPD for times corresponding to several dozens of vibrational periods; the higher-order BHE is required for longer times. But neither the BOA nor the BHE can provide reliable estimates of the EFD and Fe,EFD.

  • Liu, ChunMei and Manz, Jörn and Ohmori, Kenji and Sommer, Christian and Takei, Nobuyuki and Tremblay, Jean Christophe and Zhang, Yichi: Attosecond Control of Restoration of Electronic Structure Symmetry, Phys. Rev. Lett. 121, 173201 (2018) DOI: 10.1103/PhysRevLett.121.173201

Laser pulses can break the electronic structure symmetry of atoms and molecules by preparing a superposition of states with different irreducible representations. Here, we discover the reverse process, symmetry restoration, by means of two circularly polarized laser pulses. The laser pulse for symmetry restoration is designed as a copy of the pulse for symmetry breaking. Symmetry restoration is achieved if the time delay is chosen such that the superposed states have the same phases at the temporal center. This condition must be satisfied with a precision of a few attoseconds. Numerical simulations are presented for the C6H6 molecule and 87Rb atom. The experimental feasibility of symmetry restoration is demonstrated by means of high-contrast time-dependent Ramsey interferometry of the 87Rb atom.

  • Efthimiopoulos, Ilias and Küllmey, Tim and Speziale, Sergio and Pakhomova, Anna S. and Quennet, Marcel and Paulus, Beate and Ritscher, Anna and Lerch, Martin and Koch-Müller, Monika : Pressure-induced structural and electronic transitions in kesterite-type Cu2ZnSnS4, Journal of Applied Physics 124, 085905 (2018) DOI: 10.1063/1.5047842

We have performed structural investigations of ordered kesterite-type Cu2ZnSnS4 up to 30 GPa. Our current X-ray diffraction results clearly excluded the presence of a kesterite - disordered kesterite transition reported previously between 7 and 9 GPa. Nevertheless, specific anomalies connected with the Cu-S bond length of the starting kesterite-type phase are evidenced close to 6 GPa, indicating subtle structural effects at play in this system. Moreover, we have indexed the high-pressure modification of Cu2ZnSnS4 adopted above 16 GPa to a disordered GeSb-type structure, a tetragonally distorted rocksalt-type modification. Full decompression leads to the adoption of a disordered sphalerite/zincblende-type structure. Our complementary density functional theory calculations reproduce accurately the experimental observations and indicate the possibility of a metallic high-pressure GeSb-type phase, unlike the starting semiconducting kesterite-type Cu2ZnSnS4 structure.

  • Li, Lin and Stüker, Tony and Kieninger, Stefanie and Andrae, Dirk and Schlöder, Tobias and Gong, Yu and Andrews, Lester and Beckers, Helmut and Riedel, Sebastian: Oxygen radical character in group 11 oxygen fluorides, Nature Communications (2018) DOI: 10.1038/s41467-018-03630-0

Transition metal complexes bearing terminal oxido ligands are quite common, yet group 11 terminal oxo complexes remain elusive. Here we show that excited coinage metal atoms M (M = Au, Ag, Cu) react with OF2 to form hypofluorites FOMF and group 11 oxygen metal fluorides OMF2, OAuF and OAgF. These compounds have been characterized by IR matrix-isolation spectroscopy in conjunction with state-of-the-art quantum-chemical calculations. The oxygen fluorides are formed by photolysis of the initially prepared hypofluorites. The linear molecules OAgF and OAuF have a 3S?- ground state with a biradical character. Two unpaired electrons are located mainly at the oxygen ligand in antibonding O-M p* orbitals. For the 2B2 ground state of the OMIIIF2 compounds only an O-M single bond arises and a significant spin-density contribution was found at the oxygen atom as well.

  • Cao, Kun and Füchsel, Gernot and Kleyn, Aart W and Juurlink, Ludo BF: Hydrogen adsorption and desorption from Cu(111) and Cu(211), Phys. Chem. Chem. Phys. 20, 22477-22488 (2018), part of the themed collection: '2018 PCCP HOT Articles' DOI: 10.1039/C8CP03386B

We present a combined experimental-theoretical study on structural and coverages dependences of the adsorption and desorption of molecular hydrogen on atomically flat Cu(111) and highly stepped Cu(211) surfaces. For molecules with identical incident energy from supersonic molecular beams, we find a reduced dissociative sticking probability for the stepped surface compared to Cu(111). DFT calculations of activation barriers to dissociation for the clean and partially precovered surfaces, as well as quantitative analysis of TPD spectra support that the A-type step of the (211) surface causes an upward shift in activation barriers to dissociation and a lowering of the desorption barrier. The new data allow us to determine low sticking probabilities at conditions where King and Wells measurements fail to determine the reactivity. They are also fully consistent with the unexpected observation that monoatomic steps on a surface lower the reactivity toward the dissociation of a diatomic molecule.

  • Göth, Melanie and Witte, Felix and Quennet, Marcel and Jungk, Phillip and Podolan, Gabriel and Lentz, Dieter and Hoffmann, Waldemar and Pagel, Kevin and Reissig, Hans-Ulrich and Paulus, Beate and Schalley, Christoph A.: To Anion-pi or not to Anion-pi: The Case of Anion-Binding to Divalent Fluorinated Pyridines in the Gas Phase, Chemistry - A European Journal 24, 12879-12889 (2018) DOI: 10.1002/chem.201800893

A series of fluorinated pyridines is investigated by ESI (tandem) mass spectrometry and quantum chemical calculations with respect to their capability to bind anions in the gas phase. The pyridines differ in valency, the degree of fluorination, the positions of the fluorine atoms, the rigidity of the spacers and the relative configuration. While the monovalent compounds did not form anion complexes, the divalent analogues exhibit anion binding even to weakly coordinating anions such as tetrafluoroborate. Three different tandem MS experiments were applied to rank the gas-phase binding energies: (i) CID experiments in an FTICR mass spectrometer, (ii) determination of the collision energy required to fragment 50 % of the mass-selected complexes, and (iii) CID of heterodimers formed from two different, competing pyridine receptors and indigo carmine, a dianion with two identical binding sites. All three experiments result in consistent binding energy ranking. This ranking reveals surprising features, which are not in agreement with binding through anion-pi interactions. Density functional theory (DFT) calculations comparing different potential binding modes provide evidence that the ranking can instead nicely be explained, when C-H-anion interactions with the spacers are invoked. These results are supported by gas-phase IR spectroscopy and ion mobility-mass spectrometry (IM-MS) on a selected set of chloride pyridine complexes.

  • E. Fertitta and D. Koch and B. Paulus and G. Barcza and Ö. Legeza: Towards a multiconfigurational method of increments, Molecular Physics 116, 1471-1482 (2018) DOI: 10.1080/00268976.2018.1444208

The method of increments (MoI) allows one to successfully calculate cohesive energies of bulk materials with high accuracy, but it encounters difficulties when calculating dissociation curves. The reason is that its standard formalism is based on a single Hartree-Fock (HF) configuration whose orbitals are localised and used for the many-body expansion. In situations where HF does not allow a size-consistent description of the dissociation, the MoI cannot be guaranteed to yield proper results either. Herein, we address the problem by employing a size-consistent multiconfigurational reference for the MoI formalism. This leads to a matrix equation where a coupling derived by the reference itself is employed. In principle, such an approach allows one to evaluate approximate values for the ground as well as excited states energies. While the latter are accurate close to the avoided crossing only, the ground state results are very promising for the whole dissociation curve, as shown by the comparison with density matrix renormalisation group benchmarks. We tested this two-state constant-coupling MoI on beryllium rings of different sizes and studied the error introduced by the constant coupling.

  • Schacht Julia and Budau Johannes Horst and Gaston Nicola and Paulus Beate: Aluminum oxo-fluoride clusters: A first principle investigation of stability, synthetic considerations, and the interaction with water, Journal of Computational Chemistry 39, 1208-1214 (2018) DOI: 10.1002/jcc.25183

The introduction of the so called fluorolytic sol-gel synthesis in 2003 gave access to previously inaccessible aluminum oxo-fluorides, thus to nanoscopic materials and, more importantly, novel catalysts. The intermediate cluster structures synthesized and stabilized by Kemnitz and coworkers have mainly been protected by iso-propoxide groups. However, since catalytic reactions take place in a large variety of media, hydrophilic analogs of those clusters would be of interest. In this manuscript, we present a computational analysis for the fluorination reaction, which represents the second part of fluorolytic sol-gel synthesis, and a theoretical study of the synthesized Al4F4(mu-O)(mu-OiPr)5[H(OiPr)2] nanostructure's conversion to its hydroxylated analog Al4F4(mu-O)(mu-OH)5[H(OH)2] utilizing the nudged elastic band method. Furthermore, the role of the fluorine atoms of the cluster in an aqueous medium is evaluated by studying the incremental addition of water molecules to the cluster with and without fluorine atoms. In addition, NMR shifts of clusters exhibiting different substituents are compared. It has been found that the inclusion of an explicit solvent is necessary to capture the magnetic response of the individual cluster atoms in an aqueous solvent correctly.

  • Schroder, Hendrik V. and Witte, Felix and Gaedke, Marius and Sobottka, Sebastian and Suntrup, Lisa and Hupatz, Henrik and Valkonen, Arto and Paulus, Beate and Rissanen, Kari and Sarkar, Biprajit and Schalley, Christoph A.: An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities, Org. Biomol. Chem. 16, 2741-2747 (2018) DOI: 10.1039/C8OB00415C

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)[radical dot]+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF[radical dot]+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole [small pi]-surface in comparison to pristine tetrathiafulvalene.

  • Tesch, Julia and Paschke, Fabian and Fonin, Mikhail and Wietstruk, Marko and Bottcher, Stefan and Koch, Roland J. and Bostwick, Aaron and Jozwiak, Chris and Rotenberg, Eli and Makarova, Anna and Paulus, Beate and Voloshina, Elena and Dedkov, Yuriy: The graphene/n-Ge(110) interface: structure, doping, and electronic properties, Nanoscale 10, 6088-6098 (2018) DOI: 10.1039/C8NR00053K

The implementation of graphene in semiconducting technology requires precise knowledge about the graphene-semiconductor interface. In our work the structure and electronic properties of the graphene/n-Ge(110) interface are investigated on the local (nm) and macro (from [small mu ]m to mm) scales via a combination of different microscopic and spectroscopic surface science techniques accompanied by density functional theory calculations. The electronic structure of freestanding graphene remains almost completely intact in this system, with only a moderate n-doping indicating weak interaction between graphene and the Ge substrate. With regard to the optimisation of graphene growth it is found that the substrate temperature is a crucial factor, which determines the graphene layer alignment on the Ge(110) substrate during its growth from the atomic carbon source. Moreover, our results demonstrate that the preparation route for graphene on the doped semiconducting material (n-Ge) leads to the effective segregation of dopants at the interface between graphene and Ge(110). Furthermore, it is shown that these dopant atoms might form regular structures at the graphene/Ge interface and induce the doping of graphene. Our findings help to understand the interface properties of the graphene-semiconductor interfaces and the effect of dopants on the electronic structure of graphene in such systems.

  • D. J. Diestler, D. Jia, J. Manz, and Y. Yang: Na2 Vibrating in the Double-Well Potential of State 2 1Σu+ (JM = 00): A Pulsating “Quantum Bubble” with Antagonistic Electronic Flux, J. Phys. Chem. A 122, 2150-2159 (2018) DOI: 10.1021/acs.jpca.7b11732

The theory of concerted electronic and nuclear flux densities associated with the vibration and dissociation of a multielectron nonrotating homonuclear diatomic molecule (or ion) in an electronic state 2S+1Σg,u+ (JM = 00) is presented. The electronic population density, nuclear probability density, and nuclear flux density are isotropic. A theorem of Barth, presented in this issue, shows that the electronic flux density (EFD) is also isotropic. Hence, the evolving system appears as a pulsating, or exploding, “quantum bubble”. Application of the theory to Na2 vibrating in the double-minimum potential of the 2  1Σu+ (JM = 00) excited state reveals that the EFD consists of two antagonistic components. One arises from electrons that flow essentially coherently with the nuclei. The other, which is oppositely directed (i.e., antagonistic) and more intense, is due to the transition in electronic structure from “Rydberg” to “ionic” type as the nuclei traverse the potential barrier between inner and outer potential wells. This "transition" component of the EFD rises and falls sharply as the nuclei cross the barrier.

  • Marsoner Steinkasserer,Lukas Eugen and Pohl,Vincent and Paulus,Beate : Cyanographone and isocyanographone — Two asymmetrically functionalized graphene pseudohalides and their potential use in chemical sensing, The Journal of Chemical Physics 148, 084703 (2018) DOI: 10.1063/1.5009405

Graphene pseudohalides are natural candidates for use in molecular sensing due to their greater chemical activity as compared to both graphene halides and pristine graphene. Though their study is still in its infancy, being hindered until recently by the unavailability of both selective and efficient procedures for their synthesis, they promise to considerably widen the application potential of chemically modified graphenes. Herein, we employ van der Waals density functional theory to study the structural and electronic properties of two selected graphene pseudohalides, namely, cyanographone and isocyanographone and investigate the potential use of the latter as a chemical sensor via electron transport calculations.

  • C. Stemmle, B. Paulus, and Ö. Legeza: Analysis of electron-correlation effects in strongly correlated systems (N2 and N2+) by applying the density-matrix renormalization-group method and quantum information theory, Phys. Rev. A 97, 022505 (2018) Editors' Suggestion DOI: 10.1103/PhysRevA.97.022505

The dissociation of N2 and N2+ has been studied by using the ab initio density-matrix renormalization-group (DMRG) method. Accurate potential energy surfaces (PESs) have been obtained for the electronic ground states of N2 (X1Σ+g) and N2+ (X2Σ+g) as well as for the N2+ excited state B2Σ+u. Inherent to the DMRG approach, the eigenvalues of the reduced density matrix (ρ) and their correlation functions are at hand. Thus we can apply quantum information theory directly and investigate how the wave function changes along the PES and depict differences between the different states. Moreover, by characterizing quantum entanglement between different pairs of orbitals and analyzing the reduced density matrix, we achieved a better understanding of the multireference character featured by these systems.

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