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  • Liese, Susanne and Gensler, Manuel and Krysiak, Stefanie and Schwarzl, Richard and Achazi, Andreas and Paulus, Beate and Hugel, Thorsten and Rabe, Jürgen P. and Netz, Roland R.: Hydration Effects Turn a Highly Stretched Polymer from an Entropic into an Energetic Spring, ACS Nano 11, 702-712 (2017) DOI: 10.1021/acsnano.6b07071

Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such applications, PEG’s elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom.

  • Muna Tayyem and Bothina Hamad and Beate Paulus: Adsorption of O, O2 and CO on iridium clusters and the investigations of their stability, Chemical Physics 495, 10 - 15 (2017) DOI: 10.1016/j.chemphys.2017.08.008

Density functional theory calculations were performed to study the stability of Irn clusters as well as the adsorption of O, O2 and CO adsorbates on selected structures. The clusters form three dimensional structures for n>4. Larger clusters of n>13 exhibit simple cubic structures up to n around 32, beyond which fcc structures become more favorable. The binding energy is found to increase as a function of cluster size to approach bulk cohesive energy asymptotically. The total magnetic moment is found to decrease as a function of the cluster size approaching the bulk nonmagnetic ground state. The top adsorption site is the most site of O, O2 and CO on small clusters, unlike Ir64 that exhibits hollow, bridge and top sites, respectively. The vibrational frequencies of CO (O2) on Ir2 and Ir4 are found to be less than those of free molecules of 2102.82 (1562.08)cm−1.

  • Battaglia, Stefano and Faginas-Lago, Noelia and Andrae, Dirk and Evangelisti, Stefano and Leininger, Thierry: Increasing Radical Character of Large [n]cyclacenes Unveiled by Wave Function Theory, The Journal of Physical Chemistry A (2017) DOI: 10.1021/acs.jpca.7b00123

We have investigated the radicality and the vertical singlet-triplet energy gap of [n]cyclacenes (cyclic polyacenes) as a function of the system size for n even, from 6 to 22. The calculations are performed using the complete active space self-consistent field method and second-order n-electron valence perturbation theory. We present a systematic way for the selection of the active space in order to have a balanced description of the wave function as the size of the system increases. Moreover, we provide didactic insight into the failure of an approach based on a minimal active space. We find that the ground state is an open-shell singlet and its multireference character increases progressively with n. The singlet-triplet gap decreases as a function of the system size and approaches a finite positive value for the limit n → ∞. Finally, an analysis based on the one-particle reduced density matrix suggests a polyradical character for the largest cyclacenes.

  • Achazi, Andreas J. and Andrae, Dirk and Reissig, Hans-Ulrich and Paulus, Beate: A computational study of samarium diiodide-induced cyclizations of N-oxoalkyl-substituted methyl indole-3-carboxylates - A rationale of the diastereoselectivity, Journal of Computational Chemistry (2017) DOI: 10.1002/jcc.25055

A detailed model for the reaction mechanism of the samarium diiodide (SmI2) mediated reductive coupling of N-oxoalkyl-substituted methyl indole-3-carboxylates is developed in this study by determining the Gibbs energies for the intermediates of possible reaction pathways. The Gibbs energies at ambient temperature are calculated with dispersion corrected density functional theory in combination with implicit (D-COSMO-RS) and explicit solvent description. Temperature dependent ro-vibrational contributions are considered with the help of statistical thermodynamics. In contrast to previous proposals for the reaction mechanism, the high diastereoselectivity in the cyclization is found to be due to the formation of an energetically highly favorable chelate complex in which the final relative configuration is already preformed. After cyclization and a second electron transfer, alkylation of the resulting anion takes place under kinetic control from the more "open" face whereas protonation is under thermodynamic control. The calculations are in good agreement with these experimental findings.

  • D. J. Diestler, G. Hermann, and J. Manz: Charge Migration in Eyring, Walter and Kimball’s 1944 Model of the Electronically Excited Hydrogen-Molecule Ion,J. Phys. Chem. A 121, 5332-5340 (2017) DOI: 10.1021/acs.jpca.7b04714

In an elementary variational treatment of the electronic structure of H2+, Eyring, Walter, and Kimball (EWK) serendipitously discovered charge migration (CM) in 1944. Using an analytic expression for the electronic probability density (EPD), they found that if the electron is initially localized on one of the protons (by taking the initial state to be a superposition of the ground and first excited electronic energy eigenstates), then it oscillates adiabatically between fixed protons with a period T inversely proportional to the energy gap between the eigenstates. At the equilibrium internuclear separation, T = 550.9 as. As shown here, the EWK model also yields an analytic formula for the electronic flux density (EFD). While the EPD indicates where the electron is at any instant, the EFD reveals the pathways the electron follows during its migration. Thus, the EFD complements the EPD, providing valuable new insight into the mechanism of CM. The formula for the EFD is a simple product of a time factor and a spatial factor. This factoring exposes a plethora of spatial-temporal symmetry relations which imply novel and surprising properties. An especially significant finding is that, in contrast to multielectron systems, where electron correlation may play a role in CM, in the EWK model of H2+, CM is due strictly to quantum interference between the ground and first excited electronic states.

  • J. H. Budau, B. Paulus, and K. G. Steenbergen: Theoretical investigation of the crystal structure of AlOF, Chem. Phys. 491, 112-117 (2017) DOI: 10.1016/j.chemphys.2017.05.004

The experimental structure of AlOF is only partially known, given the limitations of XRD measurements. We have completed an extensive first principle theoretical study of the structure of AlOF. As a complement to experimental studies we investigate the structure of AlOF with density functional theory to identify the most stable distribution of oxide and fluoride ions on the lattice. Next to the experimentally found space group Pnma, we include its subgroups in our study for a deeper understanding of the structure-stability-relation. The theoretically identified most stable structure confirms the experimentally determined space group with a homogeneous distribution of the fluoride anions. Additionally, we determine Bader charges and elastic constants for two selected distributions to gain a deeper insight into the binding of this material.

  • D. M. Jia, J. Manz, B. Paulus, V. Pohl, J. C. Tremblay, and Y. G. Yang: Quantum control of electronic fluxes during adiabatic attosecond charge migration in degenerate superposition states of benzene, Chem. Phys. 482, 146-159 (2017) DOI: 10.1016/j.chemphys.2016.09.021

We design four linearly x- and y -polarized as well as circularly right (+) and left (−) polarized, resonant π/2-laser pulses that prepare the model benzene molecule in four different degenerate superposition states. These consist of equal (0.5) populations of the electronic ground state S0(1A1g) plus one of four degenerate excited states, all of them accessible by dipole-allowed transitions. Specifically, for the molecule aligned in the xy -plane, these excited states include different complex-valued linear combinations of the 1E1u,x and 1E1u,y degenerate states. As a consequence, the laser pulses induce four different types of periodic adiabatic attosecond (as) charge migrations (AACM) in benzene, all with the same period, 504 as, but with four different types of angular fluxes. One of the characteristic differences of these fluxes are the two angles for zero fluxes, which appear as the instantaneous angular positions of the “source” and “sink” of two equivalent, or nearly equivalent branches of the fluxes which flow in pincer-type patterns from one molecular site (the “source”) to the opposite one (the “sink”). These angles of zero fluxes are either fixed at the positions of two opposite carbon nuclei in the yz-symmetry plane, or at the centers of two opposite carbon-carbon bonds in the xz-symmetry plane, or the angles of zero fluxes rotate in angular forward (+) or backward (−) directions, respectively. As a resume, our quantum model simulations demonstrate quantum control of the electronic fluxes during AACM in degenerate superposition states, in the attosecond time domain, with the laser polarization as the key knob for control.

  • A. Setaro, M. Adeli, M. Glaeske, D. Przyrembel, T. Bisswanger, G. Gordeev, F. Maschietto, A. Faghani, B. Paulus, M. Weinelt, R. Arenal, R. Haag, and S. Reich: Preserving pi-conjugation in covalently functionalized carbon nanotubes for optoelectronic applications, Nat. Commun. 8, 14281 (2017) DOI: 10.1038/ncomms14281

Covalent functionalization tailors carbon nanotubes for a wide range of applications in varying environments. Its strength and stability of attachment come at the price of degrading the carbon nanotubes sp2 network and destroying the tubes electronic and optoelectronic features. Here we present a non-destructive, covalent, gram-scale functionalization of single-walled carbon nanotubes by a new [2+1] cycloaddition. The reaction rebuilds the extended π-network, thereby retaining the outstanding quantum optoelectronic properties of carbon nanotubes, including bright light emission at high degree of functionalization (1 group per 25 carbon atoms). The conjugation method described here opens the way for advanced tailoring nanotubes as demonstrated for light-triggered reversible doping through photochromic molecular switches and nanoplasmonic gold-nanotube hybrids with enhanced infrared light emission.

  • Z. Kaawar, C. Müller, and B. Paulus: Theoretical investigations of the CO adsorption on ZnF2 surfaces, Surf. Sci. 656, 48-53 (2017) DOI: 10.1016/j.susc.2016.06.021

Periodic density functional theory calculations were performed to investigate the Lewis acidity of unsaturated surface cations of ZnF2, using CO as probe molecule at different coverages. We have calculated adsorption energies for CO on all low index ZnF2 surfaces using DFT with the B3LYP functional and subsequent dispersion correction. Additionally local second order Møller-Plesset perturbation theory (LMP2) calculations were performed. In most of the cases, the adsorption of CO on different surfaces is described well using B3LYP. Dispersion correction to B3LYP is found to overestimate the adsorption energy. The interaction among adsorbed CO molecules appears to have a significant effect on the adsorption energies at full coverage.

  • H. V. Schröder, H. Hupatz, A. J. Achazi, S. Sobottka, B. Sarkar, B. Paulus and C. A. Schalley: A Divalent Pentastable Redox-Switchable Donor-Acceptor Rotaxane, Chem. Eur. J. 23, 2960-2967 (2017) DOI: 10.1002/chem.201605710

Donor–acceptor materials with small HOMO–LUMO gaps are important in molecular electronics, but are often difficult to synthesise. A simple and efficient way to position tetrathiafulvalene (TTF) as the donor and naphthalene diamide (NDI) as the acceptor in close proximity to each other in a divalent crown/ammonium pseudo[2]rotaxane is presented. The divalent design provides high chelate cooperativity and much stronger binding compared with a monovalent analogue. The pseudo[2]rotaxane was then doubly interlocked by stoppering it in a catalyst-free 1,3-dipolar cycloaddition. UV/Vis and cyclic voltammetry experiments with the resulting [2]rotaxane revealed the optoelectronic properties of an intramolecular charge transfer with a small HOMO–LUMO energy gap. Redox-switching experiments showed the rotaxane to be pentastable. DFT calculations provided insights into the electronic structures of the five redox states.

  • L. K. S. von Krbek, A. J. Achazi, S. Schroder, M. Gaedke, T. Biberger, B. Paulus and C. A. Schalley: The Delicate Balance of Preorganisation and Adaptability in Multiply Bonded Host-Guest Complexes, Chem. Eur. J. 23, 2877-2883 (2017) DOI: 10.1002/chem.201605092

Rigidity and preorganisation are believed to be required for high affinity in multiply bonded supramolecular complexes as they help reduce the entropic penalty of the binding event. This comes at the price that such rigid complexes are sensitive to small geometric mismatches. In marked contrast, nature uses more flexible building blocks. Thus, one might consider putting the rigidity/high-affinity notion to the test. Multivalent crown/ammonium complexes are ideal for this purpose as the monovalent interaction is well understood. A series of divalent complexes with different spacer lengths and rigidities has thus been analysed to correlate chelate cooperativities and spacer properties. Too long spacers reduce chelate cooperativity compared to exactly matching ones. However, in contrast to expectation, flexible guests bind with chelate cooperativities clearly exceeding those of rigid structures. Flexible spacers adapt to small geometric host–guest mismatches. Spacer–spacer interactions help overcome the entropic penalty of conformational fixation during binding and a delicate balance of preorganisation and adaptability is at play in multivalent complexes.

  • M. Quennet, A. Ritscher, M. Lerch and B. Paulus: The order-disorder transition in Cu2ZnSnS4: A theoretical and experimental study, J. Solid State Chem. 250, 140-144 (2017) DOI: 10.1016/j.jssc.2017.03.018

In this work the Cu/Zn order-disorder transition in Cu2ZnSnS4 kesterites on Wyckoff positions 2c and 2d was investigated by a structural and electronic analysis in theory and experiment. For experimental investigations stoichiometric samples with different Cu/Zn order, annealed in the temperature range of 473–623 K and afterwards quenched, were used. The optical gaps were determined using the Derivation of Absorption Spectrum Fitting (DASF) method. Furthermore, the order-disorder transition was examined by DFT calculations for a closer analysis of the origins of the reduced band gap, showing a good agreement with experimental data with respect to structural and electronic properties. Our studies show a slight increase of lattice parameter c in the kesterite lattice with increasing disorder. Additionally, a reduced band gap was observed with increasing disorder, which is an effect of newly occurring binding motifs in the disordered kesterite structure.

  • Z. Kaawar, S. Mahn, E. Kemnitz and B. Paulus: On the Morphology of Group II Metal Fluoride Nanocrystals at Finite Temperature and Partial Pressure of HF, Molecules 22, 663 (2017) DOI: 10.3390/molecules22040663

We have investigated the bulk and surface properties of the group II metal fluorides CaF 2 , SrF 2 and BaF 2 using periodic density functional theory (DFT) calculations and surface thermodynamics. Our bulk results show that the best agreement with experiment is achieved with the B3LYP and PBE functionals. We determined the relative importance of the low index surfaces in vacuum and found that an fluoride microcrystal exposes only the (111) surface in which the undercoordinated cations are sevenfold coordinated. With methods of ab initio surface thermodynamics, we analyzed the stability of different surfaces under hydrogen fluoride (HF) pressure and determined the presumable shape of the crystals with respect to different HF concentrations and temperatures. In the case of CaF 2 and SrF 2 , the calculated shapes of the crystals agree well with TEM images of fluorolytic sol-gel synthesized nanocrystals at room temperature and high HF concentration.

  • X. Chen, S. Lei, C. Lotze, C. Czekelius, B. Paulus, and K. J. Franke: Conformational adaptation and manipulation of manganese tetra(4-pyridyl)porphyrin molecules on Cu(111), J. Chem. Phys. 146, (2017) DOI: 10.1063/1.4974313

Porphyrins are highly flexible molecules and well known to adapt to their local environment via conformational changes. We studied the self-assembly of manganese meso-tetra(4-pyridyl)porphyrin (Mn-TPyP) molecules on a Cu(111) surface by low temperature scanning tunneling microscopy (STM) and atomic force microscopy (ATM). We observe molecular chains along the ⟨11¯0⟩⟨11¯0⟩ direction of the substrate. Within these chains, we identify two molecular conformations, which differ by the orientation of the upward bending of the macrocycle. Using density functional theory, we show that this saddle shape is a consequence of the rotation and inclination of the pyridyl groups towards Cu adatoms, which stabilize the metal-organic chains. The molecular conformations obey a strict alternation, reflecting the mutual enforcement of conformational adaptation in densely packed structures. Tunneling electrons from the STM tip can induce changes in the orientation of the pyridyl endgroups. The switching behaviour varies with the different adsorption configurations.

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