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Polyethylene glycol (PEG) is a structurally simple and nontoxic watersoluble 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 textbook knowledge, a polymer reacts to the stretching of its endtoend 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 singlemolecule 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 stretchinginduced 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.
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.
We have investigated the radicality and the vertical singlettriplet 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 selfconsistent field method and secondorder nelectron 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 openshell singlet and its multireference character increases progressively with n. The singlettriplet 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 oneparticle reduced density matrix suggests a polyradical character for the largest cyclacenes.
A detailed model for the reaction mechanism of the samarium diiodide (SmI2) mediated reductive coupling of Noxoalkylsubstituted methyl indole3carboxylates 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 (DCOSMORS) and explicit solvent description. Temperature dependent rovibrational 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.
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 spatialtemporal 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.
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 structurestabilityrelation. 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.
We design four linearly x and y polarized as well as circularly right (+) and left (−) polarized, resonant π/2laser 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 dipoleallowed transitions. Specifically, for the molecule aligned in the xy plane, these excited states include different complexvalued 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 pincertype 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 yzsymmetry plane, or at the centers of two opposite carboncarbon bonds in the xzsymmetry 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.
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 nondestructive, covalent, gramscale functionalization of singlewalled carbon nanotubes by a new [2+1] cycloaddition. The reaction rebuilds the extended πnetwork, 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 lighttriggered reversible doping through photochromic molecular switches and nanoplasmonic goldnanotube hybrids with enhanced infrared light emission.
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 MøllerPlesset 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.
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 catalystfree 1,3dipolar 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. Redoxswitching experiments showed the rotaxane to be pentastable. DFT calculations provided insights into the electronic structures of the five redox states.
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/highaffinity 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.
In this work the Cu/Zn orderdisorder 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 orderdisorder 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.
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 solgel synthesized nanocrystals at room temperature and high HF concentration.
Porphyrins are highly flexible molecules and well known to adapt to their local environment via conformational changes. We studied the selfassembly of manganese mesotetra(4pyridyl)porphyrin (MnTPyP) 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 metalorganic 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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This page was last modified on March 26, 2019, at 10:44 AM 