Dispersion Correction Term Research Articles

Hexa-peri-hexabenzocoronene as a vector sensing and delivery of Carmustine anticancer drug: A density functional theory study

We conducted research on the adsorption of carmustine (BCNU) on the surface of pristine, N-doped, and BN-doped Hexa-peri-hexabenzocoronene (HBC) nanographene using density functional theory calculations. We used the B3LYP functional in conjunction with 6-31+G(d) and 6-311+G(d,p) basis sets to perform geometrical optimization calculations, vibrational frequencies, and natural bond orbitals (NBO) analysis. The dispersion correction term of Grimme-D3 was incorporated to account for the dispersion interactions. We found that the geometry of the nanographenes remains unchanged after the drug molecule adsorption. The calculated adsorption energies (Eads) were −17.1 kcal.mol−1 for HBC-BCNU, −17.0 kcal.mol−1 for NHBC-BCNU, and −17.9 kcal.mol−1 for BNHBC-BCNU when using the 6-31+G(d) basis set. The corresponding values calculated employing the 6-311+G(d,p) basis set were −17.9, −17.3, and −19.1 kcal.mol−1, respectively. Analysis of frontier molecular orbitals (HOMO and LUMO) showed that the electronic properties of NHBC were significantly affected by the presence of BCNU. We used NBO analysis to investigate the distribution of partial atomic charges in different systems. Finally, using the transition state equation and the smaller basis set, we found that the required time for BCNU to release from HBC, NHBC, and BNHBC is approximately 3.3, 1.8, and 13.0 s, respectively. The obtained values are 13.0, 4.6, and 96.0 s, respectively, for the higher level of calculations. The recovery time for all nanographenes is within a reasonable range, while NHBC exhibits the highest sensitivity for drug detection.

Decoding energy decomposition analysis: Machine-learned Insights on the impact of the density functional on the bonding analysis.

The concept of chemical bonding is a crucial aspect of chemistry that aids in understanding the complexity and reactivity of molecules and materials. However, the interpretation of chemical bonds can be hindered by the choice of the theoretical approach and the specific method utilized. This study aims to investigate the effect of choosing different density functionals on the interpretation of bonding achieved through energy decomposition analysis (EDA). To achieve this goal, a data set was created, representing four bonding groups and various combinations of functionals and dispersion correction schemes. The calculations showed significant variation among the different functionals for the EDA terms, with the dispersion correction terms exhibiting the highest variability. More information was extracted by using machine learning in combination with dimensionality reduction on the data set. Results indicate that, despite the differences in the EDA terms obtained from different functionals, the functional has the least significant impact, suggesting minimal influence on the bonding interpretation.

Simple, Efficient, and Universal Energy Decomposition Analysis Method Based on Dispersion-Corrected Density Functional Theory.

Energy decomposition analysis (EDA) is an important class of methods to explore the nature of interaction between fragments in a chemical system. It can decompose the interaction energy into different physical components to understand the factors that play key roles in the interaction. This work proposes an EDA strategy based on dispersion-corrected density functional theory (DFT), called sobEDA. This method is fairly easy to implement and very universal. It can be used to study weak interactions, chemical bond interactions, open-shell systems, and interactions between multiple fragments. The total time consumption of sobEDA is only about twice that of conventional DFT single-point calculation for the entire system. This work also proposes a variant of the sobEDA method named sobEDAw, which is designed specifically for decomposing weak interaction energies. Through a proper combination of DFT correlation energy and dispersion correction term, sobEDAw gives a ratio between dispersion energy and electrostatic energy that is highly consistent with the symmetry-adapted perturbation theory, which is quite popular and robust in studying weak interactions but expensive. We present a shell script sobEDA.sh to implement the methods proposed in this work based on the very popular Gaussian quantum chemistry program and Multiwfn wavefunction analysis code. Via the script, theoretical chemists can use the sobEDA and sobEDAw methods very conveniently in their study. Through a series of examples, the rationality of the new methods and their implementation are verified, and their great practical values in the study of various chemical systems are demonstrated.

Exploration for the Optical Properties and Fluorescent Prediction of Nitrotriazole and Nitrofurazan: First-Principles and TD-DFT Calculations.

High-energy materials containing azole and furazan have revealed numerous properties; however, the underlying optical properties need to be solved. Meanwhile, the uncertainty for the choice of fluorescent matrix materials and the flexible situational conditions prompted us to estimate the optical and fluorescent properties of 5,5′-dinitro-2H,2H′-3,3′-bi-1,2,4-triazole (DNBT), 4,4′-dinitroazolefurazan (DNAF), and 4,4′-dinitro-3,3′-4,3′-ter-1,2,5-oxadiazole (DNTO). The first-principles calculation with improved dispersion correction terms and time-dependent density functional theory were utilized to calculate the absorbance and excitation energy of DNBT, DNAF, and DNTO, as well as characterization for their crystal structure, electronic structure, molecular orbitals, and so forth, synchronously. In this work, the absorbance anisotropy of DNBT and DNTO is stronger than that of DNAF. The absorbance for each of the (0,0,1) crystal planes in the three compounds is greater than that of the other two crystal planes. Moreover, DNBT has the maximum absorbance on the (0,0,1) crystal plane. The N–N–H from DNBT and N–O–N from DNTO and DNAF are responsible for these results, while N=N in DNAF weakens the performance of N–O–N. UV–vis spectra show that the maximum absorption wavelengths λmax for DNBT, DNAF, and DNTO are 225, 228, and 201 nm, respectively. The number of five-membered rings and the coplanarity of groups in the intermolecular non-conjugation interaction potentially improve this ability due to the results from the crystal diffraction analysis. In addition, the polarization rate DNBT > DNTO > DNAF based on the molecular orbital analysis and the electrostatic potential calculation implies that the excitation energy of DNBT is less than DNTO, and the excitation energy of DNTO is less than DNAF. This work is beneficial to the expansion of energetic materials into the optical field and the accelerated application process of the related industry.

Application of borophene as catechol sensor: a computational study.

The efficacy of borophene (BP) as catechol (CC) sensor was explored using density functional theory (DFT) method. All calculations were performed at B3LYP level of theory and 6-31 + G(d) basis set employing the dispersion correction term of Grimme to consider dispersion interactions. The CC molecule is adsorbed on top of BP horizontally with the adsorption energy (Eads) of about - 13.5kcal·mol-1. The HOMO and LUMO levels of nanosheet destabilize by about 0.36 and 0.14eV, respectively, going from bare BP to BP-CC complex. Therefore, the Eg value decreases by about 10.5% upon adsorption process, which is a reasonable energy gap change for detection of CC. The negligible difference between the work function values (Φ, defined as the minimum amount of the energy needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface) of BP and its complex with CC indicates that the BP sheet is not an appropriate Φ-type sensor (in these sensors, adsorption of a chemical changes the gate voltage and produces an electrical signal that leads to the detection of chemical agent) for CC detection. The electrical conductivity of BP becomes 72 times higher after CC adsorption. The time needed for CC desorption from BP sheet is 7.6ns, based on conventional transition state theory, showing that BP benefits from a short recovery time. The effect of CC concentration was explored by adsorption of 2 and 3 CC molecules on top of BP nanosheet and the results showed that the sensor response does not change by increasing the CC concentration. Also, the effect of lateral dimensions of BP on the adsorption energy was explored and it was shown that Eads increases by enlargement of the nanosheet.

Two-Dimensional Boron–Phosphorus Monolayer for Reversible NO2 Gas Sensing

We proposed a boron–phosphorus monolayer (BP-ML) and investigated its gas sensing properties by density functional theory including the van der Waals dispersion correction term. Electronic property...

Molecular dynamics simulations of a stacked $$\uppi $$-conjugated soft material: binding energy and preferential geometry for self-assembly

Understanding intermolecular interactions among supramolecular self-assembled organization and identifying the molecular structure relevant to the self-assembly are crucial for designing materials with desired functionalities. Interactions of aromatic rings in a peptide–perylenediimide conjugate (P-1) are investigated using dispersion-corrected density functional theory. The binding energies of fully optimized dimeric P-1 are calculated to identify the most stable conformation of the dimer. We show that the dispersion correction terms have significant contributions to the total energies of the dimers. The combined results from electronic structure calculations and molecular dynamics simulations demonstrate that the stacked dimer with negative inter-planar angle with clock-wise rotation has stronger binding energy than the dimer with positive inter-planar angle. The excess stability of the dimer with clock-wise rotation is attributed to the intra- and inter-molecular $$\uppi $$ – $$\uppi $$ stacking of the side aromatic rings of the dimer facilitated by formation of less number of hydrogen bonds. The stacked P-1 dimer with negative inter-planar angle and stronger binding energy is identified as the building block of a super structure with left-handed helical arrangements. Our calculations will build the first step towards understanding the molecular origin of the stability of a specific super structure of P-1 over the other, as obtained in the experiment relevant to material science and technology.

High-Resolution THz Spectroscopy and Solid-State Density Functional Theory Calculations of Polycyclic Aromatic Hydrocarbons

High-resolution and broadband THz spectra of the crystals of nine polycyclic aromatic hydrocarbons (PAHs) are presented. Five PAHs are comprised of ortho-fused benzene rings and the other four of peri-fused benzene rings. THz mode assignment is performed by using the anthracene and pyrene crystals as examples. The performance of the PBE functional augmented by Grimme’s two dispersion correction terms, D* and D3, respectively, are rigorously evaluated against the experimental criteria of frequency and isotope shift (IS). The D* and D3 terms use empirical and semi-classical approach for correcting the London-type dispersion interactions, respectively. The nature of each THz mode simulated by PBE-D* and that by PBE-D3 is quantitatively compared in terms of the percentage contributions of the intermolecular and the intramolecular vibrations to the vibrational energy. We find that the two methods have equivalent performance in reproducing the frequencies, ISs, and nature of THz modes of both the anthracene and pyrene crystals.

A New Look on Larger Sulfur and Selenium Rings – Dispersion Forces and Shapes of Larger Cycles

In this paper we discuss the shape of sulfur and selenium rings S6 to S20 and Se6 to Se20. The sizes and conformations of the sulfur rings are known by X‐ray data. A comparison between the results of B3LYP/cc‐pVTZ, B3LYP‐D3/cc‐pVTZ, and B3LYP‐D3BJ/cc‐pVTZ calculations reveals structural relaxation dispersion energy values of 3.0 kJ/mol (S10) and 19.4 kJ/mol (S18) for the sulfur allotropes. In the case of selenium rings, we investigated rings with the same size. We found that at low temperatures the selenium rings Se13, Se14, Se18 and Se20 should be more stable than Se8 if the empirical dispersion corrections D3 and D3BJ are taken into account. All larger rings reveal structural relaxation dispersion between 6.0 kJ/mol (Se10) and 34.7 kJ/mol (Se18). Furthermore, the calculations reveal that the consideration of the dispersion energy using the correction terms D3 and D3BJ leads in some cases to a distortion of the molecular structure accompanied by a degradation of the molecular symmetry. In these cases, the transannular distances strongly depend on the method used and the difference sometimes amounts to more than 1 Å. For the structural relaxation dispersion energy, the following applies: A few short noncovalent bonds are better than many medium‐sized ones. The structural distortion is also found in the X‐ray data which stresses the importance of the usage of the dispersion correction terms for the structural investigation of these cycles.

Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer

Many transition metal di-selenides such as MoSe2 and WSe2 show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the number of electrocatalytic active sites and in turn to improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, we have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe2 for HER and OER catalyst. The state-of-art density functional theory (DFT) based computations are performed for estimating the catalytic activity of the pristine and doped SnSe2 by means of evaluating the adsorption and Gibbs free energies subjected to hydrogen and oxygen adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, we have included the dispersion correction term to exchange-correlation functional. Results show that the pristine SnSe2 is not a good HER catalyst when hydrogen is adsorbed on its basal plane. However, edge-sites show the good hydrogen adsorption and indicates that the edges of SnSe2 are the most preferential site for hydrogen adsorption. As far as the catalytic activity of SnSe2 with dopants is concerned, the Na-doped SnSe2 among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe2 show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe2 can be improved by selective doping of alkali metals.

Molecular interactions in the complexes of toluene with butyronitrile: A DFT approach

Density Functional Theory (DFT) has been employed to investigate the self association of butyronitrile and the heterointeractions in the 1:2 (toluene: butyronitrile) and 1:1 complexes of toluene with butyronitrile. For this investigation the B3LYP functional with Grimme's dispersion correction (D3) term and ωB97XD functionals were used. The theoretical frequency analysis shows the unsuitability of B3LYP with D3 for the present investigation. Therefore, Natural Bonding Orbital analysis was done at the functional ωB97XD. It is found through this work that only the methylene hydrogens of butyronitrile are responsible for the self association among the butyronitrile molecules. In 1:1 complex, the red shift in the butyronitrile methyl asymmetric stretching mode is not due to the active participation of this group in heterointeractions and it is solely due to the other interactions happening in its vicinity. Only the interaction (TOL)C−H⋯N(BN) is present in the complex. In 1:2 complex the butyronitrile methyl/methylene hydrogens interact with the delocalized electron cloud of toluene and the toluene hydrogens interact with the butyronitrile nitrogen. Comparison of interaction energies shows that the stability of 1:2 complex is more than that of butyronitrile dimer and 1:1 complex.

Heats of Formation of Medium-Sized Organic Compounds from Contemporary Electronic Structure Methods

Computational electronic structure calculations are routinely undertaken to predict thermodynamic properties of various species. However, the application of highly accurate wave function theory methods, such as the "gold standard" coupled cluster approach including single, double, and partly triple excitations in perturbative fashion, CCSD(T), to large molecules is limited due to high computational cost. In this work, the promising domain based local pair natural orbital coupled cluster approach, DLPNO-CCSD(T), has been tested to reproduce 113 accurate formation enthalpies of medium-sized molecules (few dozens heavy atoms) important for bio- and combustion chemistry via the reaction based Feller-Peterson-Dixon approach. For comparison, eight density functional theory (B3LYP, B3LYP-D3, PBE0, PBE0-D3, M06, M06-2X, ωB97X-D3, and ωB97M-V) and MP2-based (B2PLYP-D3, PWPB95-D3, B2T-PLYP, B2T-PLYP-D, B2GP-PLYP, DSD-PBEP86-D3, SCS-MP2, and OO-SCS-MP2) methods have been tested. The worst performance has been obtained for the standard hybrid DFT functionals, PBE0 (mean unsigned error (MUE)/mean signed error (MSE) = 9.1/6.0 kcal/mol) and B3LYP (MUE/MSE = 13.5/-13.3 kcal/mol). The influence of an empirical dispersion correction term on these functionals' performance is not homogeneous: B3LYP performance is improved (B3LYP-D3 (MUE/MSE = 6.0/0.8 kcal/mol)); meanwhile PBE0 performance is worse (PBE0-D3 (MUE/MSE = 14.1/13.6 kcal/mol)). The Minnesota functionals, M06 (MUE/MSE = 3.8/-2.0 kcal/mol) and M06-2X (MUE/MSE = 3.5/3.0 kcal/mol), and recently developed ωB97X-D3 (MUE/MSE = 3.2/0.2 kcal/mol) and ωB97M-V (MUE/MSE = 2.2/1.3 kcal/mol) methods provided significantly better formation enthalpies. Enthalpies of similar quality can also be obtained from some double hybrid methods (B2PLYP-D3 (MUE/MSE = 4.7/2.0 kcal/mol), PWPB95-D3 (MUE/MSE = 4.3/3.2 kcal/mol), B2T-PLYP (MUE/MSE = 4.1/-3.0 kcal/mol), and B2T-PLYP-D (MUE/MSE = 3.3/1.7 kcal/mol)). The two spin component scaled (SCS) MP2 methods resulted in even smaller errors (SCS-MP2 (MUE/MSE = 1.9/1.2 kcal/mol) and OO-SCS-MP2 (MUE/MSE = 1.6/0.1 kcal/mol)). The best performance was found for the frozen core (FC) DLPNO-CCSD(T) method with a MUE/MSE of 1.6/-1.2 kcal/mol. The performance of the DLPNO-CCSD(T) method can be further improved by running the post-SCF calculations on the B3LYP orbitals: the MUE/MSE for the DLPNO-CCSD(T,B3LYP) approximation is 1.2/-0.4 kcal/mol. We recommend the DLPNO-CCSD(T,B3LYP) method for black box applications in thermodynamics of medium-sized organic molecules when the canonical CCSD(T) calculations with basis sets of reasonable quality are prohibitively expensive.

Rungs 1 to 4 of DFT Jacob's ladder: Extensive test on the lattice constant, bulk modulus, and cohesive energy of solids.

A large panel of old and recently proposed exchange-correlation functionals belonging to rungs 1 to 4 of Jacob's ladder of density functional theory are tested (with and without a dispersion correction term) for the calculation of the lattice constant, bulk modulus, and cohesive energy of solids. Particular attention will be paid to the functionals MGGA_MS2 [J. Sun et al., J. Chem. Phys. 138, 044113 (2013)], mBEEF [J. Wellendorff et al., J. Chem. Phys. 140, 144107 (2014)], and SCAN [J. Sun et al., Phys. Rev. Lett. 115, 036402 (2015)] which are meta-generalized gradient approximations (meta-GGA) and are developed with the goal to be universally good. Another goal is also to determine for which semilocal functionals and groups of solids it is beneficial (or not necessary) to use the Hartree-Fock exchange or a dispersion correction term. It is concluded that for strongly bound solids, functionals of the GGA, i.e., rung 2 of Jacob's ladder, are as accurate as the more sophisticated functionals of the higher rungs, while it is necessary to use dispersion corrected functionals in order to expect at least meaningful results for weakly bound solids. If results for finite systems are also considered, then the meta-GGA functionals are overall clearly superior to the GGA functionals.

Unraveling the polymorphism of [(p-cymene)Ru(κN-INA)Cl₂] through dispersion-corrected DFT and NMR GIPAW calculations.

The structural and (13)C/(1)H NMR parameters of the four crystal forms (1α, 1·H2O, 1β, and 1γ) of the solid wheel-and-axle (WAA) metal-organic compound [(p-cymene)Ru(κN-INA)Cl2] have been studied by means of periodic DFT calculations. The quality of the results obtained strongly depends on a correct description of long-range interactions; thus, in the geometry refinement protocol used, the pure DFT functionals need to be coupled with a dispersion-correction term (B3LYP-D2, B3LYP-D*). The solid-state (13)C/(1)H NMR δ(iso) parameters and (13)C MAS NMR spectra, calculated by means of the PBE-GIPAW method, agree well with the experimental data for the four crystal forms (mean absolute deviations of the (13)C and (1)H δ(iso) data values lie in the ranges 1.3-2.9 and 0.3-1.0 ppm, respectively). In this context, some revisions in the experimental assignment of the (13)C/(1)H NMR δ(iso) parameters of the 1·H2O, 1β, and 1γ crystal forms can be suggested. The mismatch in the assignment seems to be due to the rotation of the -COOH moiety, which occurs at the 1α-1·H2O transition and was not considered in the experiments. Finally, the results obtained suggest the presence of two COOH···Cl hydrogen bonds of comparable strength established by the two molecules in the asymmetric unit of the 1γ polymorph, in partial disagreement with previous findings.

Computational Study of the Effect of Dispersion Interactions on the Thermochemistry of Aggregation of Fused Polycyclic Aromatic Hydrocarbons as Model Asphaltene Compounds in Solution

Density functional theory (DFT), Møller-Plesset second-order perturbation theory (MP2), and semiempirical methods are employed for the geometry optimization and thermochemistry analysis of π-π stacked di-, tri-, tetra-, and pentamer aggregates of the fused polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, tetracene, pyrene, and coronene as well as benzene. These aggregates (stabilized by dispersion interactions) are highly relevant to the intermolecular aggregation of asphaltenes, major components of heavy petroleum. The strength of π-π stacking interaction is evaluated with respect to the π-stacking distance and thermochemistry results, such as aggregation enthalpies, entropies, and Gibbs free energies (ΔG(298)). For both π-stacking interplanar distances and thermochemistry, the ωB97X-D functional with an augmented damped R(-6) dispersion correction term and MP2 are in the closest agreement with the highly accurate spin-component scaled MP2 (SCS-MP2) method that we selected as a reference. The ΔG(298) values indicate that the aggregation of coronene is spontaneous at 298 K and the formation of pyrene dimers occurs spontaneously at temperature lower than 250 K. Aggregates of smaller PAHs would be stable at even lower temperature. These findings are supported by X-ray crystallographic determination results showing that among the PAHs studied only coronene forms continuous stacked aggregates in single crystals, pyrene forms dimers, and smaller PAHs do not form π-π stacked aggregates. Thermochemistry analysis results show that PAHs containing more than four fused benzene rings would spontaneously form aggregates at 298 K. Also, round-shaped PAHs, such as phenanthrene and pyrene, form more stable aggregates than linear PAHs, such as anthracene and tetracene, due to decreased entropic penalty. These results are intended to help guide the synthesis of model asphaltene compounds for spectroscopic studies so as to help understand the aggregation behavior of heavy petroleum.

NbF5 and TaF5: Assignment of 19F NMR resonances and chemical bond analysis from GIPAW calculations

The 19F isotropic chemical shifts (δiso) of two isomorphic compounds, NbF5 and TaF5, which involve six nonequivalent fluorine sites, have been experimentally determined from the reconstruction of 1D 19F MAS NMR spectra. In parallel, the corresponding 19F chemical shielding tensors have been calculated using the GIPAW method for both experimental and DFT-optimized structures. Furthermore, the [M4F20] units of NbF5 and TaF5 being held together by van der Waals interactions, the relevance of Grimme corrections to the DFT optimization processes has been evaluated. However, the semi-empirical dispersion correction term introduced by such a method does not show any significant improvement. Nonetheless, a complete and convincing assignment of the 19F NMR lines of NbF5 and TaF5 is obtained, ensured by the linearity between experimental 19F δiso values and calculated 19F isotropic chemical shielding σiso values. The effects of the geometry optimizations have been carefully analyzed, confirming among other matters, the inaccuracy of the experimental structure of NbF5. The relationships between the fluorine chemical shifts, the nature of the fluorine atoms (bridging or terminal), the position of the terminal ones (opposite or perpendicular to the bridging ones), the fluorine charges, the ionicity and the length of the M-F bonds have been established. Additionally, for three of the 19F NMR lines of NbF5, distorted multiplets, arising from 1J-coupling and residual dipolar coupling between the 19F and 93Nb nuclei, were simulated yielding to values of 93Nb-19F 1J-coupling for the corresponding fluorine sites.

Determination of the distribution of elements with similar electron counts: a practical guide for resonant X-ray scattering

This article attempts to present straightforward and easy-to-understand guidelines for the determination of element distribution in compounds lacking X-ray scattering contrast because they have similar electron counts. Different sources of anomalous dispersion correction terms (especially Δf′ values) are compared with respect to their suitability, reliability and quality. Values from databases are compared with Δfvalues calculated from fluorescence spectra and those refined from single-crystal diffraction data, using both reference crystals without scattering contrast problems and crystals containing elements with similar electron counts. The number of data sets required to determine reliably the element distribution and the optimum wavelengths to be used are discussed. Joint multiple data set refinements are suitable for the refinement of multiply mixed occupancies of elements lacking scattering contrast. The most straightforward method of obtaining Δf′ values depends on the complexity of the problem to be solved and the precision required.

Differences in the Sublimation Energy of Benzene and Hexahalogenbenzenes Are Caused by Dispersion Energy

The crystals of benzene and hexahalogenbenzenes have been studied by means of the density functional theory augmented by an empirical dispersion correction term as well as by the symmetry-adapted perturbation theory. In order to elucidate the nature of noncovalent binding, pairwise interactions have been investigated. It has been demonstrated that the structures of dimers with the highest stabilization energy differ notably along the crystals. It has been shown that the differences in the experimental sublimation energies might be attributed to the dispersion interaction. To our surprise, the dihalogen bonding observed in the hexachloro- and hexabromobenzenes plays a rather minor role in structure stabilization because it is energetically comparable with the other binding motifs. However, the dihalogen bond is by far the most frequent binding motif in hexachloro- and hexabromobenzenes.

Temperature dependent resonant X-ray diffraction of single-crystalline Ge2Sb2Te5

The element distribution in the crystal structure of the stable phase of the well-known phase-change material Ge2Sb2Te5 was determined at temperatures up to 471 °C using single crystals synthesized by chemical transport reactions. Because of the similar electron count of Sb and Te, the scattering contrast was enhanced by resonant diffraction using synchrotron radiation (beamline ID11, ESRF). A simultaneous refinement on data measured at the K-absorption edges of Sb and Te as well as at additional wavelengths off the absorption edges yielded reliable occupancy factors of each element on each position (a = 4.2257(2) A, c = 17.2809(18) A, Pm1, R1 (overall) = 0.037). The dispersion correction terms Δf′ were refined and match experimental ones obtained from fluorescence spectra by the Kramers–Kronig transform. The structure contains distorted rocksalt-type blocks of nine alternating cation and anion layers, respectively, which are separated by van der Waals gaps between Te atom layers. Ge atoms prefer the cation positions near the center of the rocksalt-type block (occupancy factors Ge0.60(4)Sb0.36(2)), Sb atoms the one near the van der Waals gap (Ge0.33(7)Sb0.66(4)). Anti-site disorder is not significant. During heating up to 471 °C and subsequent cooling, a reversible structural distortion was observed. The refinements show that with increasing temperature the first pair of anion and cation layers next to the van der Waals gap becomes slightly detached from the block and increasingly resembles a GeTe-type layer. Thus, the difference between interatomic distances in the 3 + 3 cation coordination sphere of the mixed Ge–Sb position next to the gap becomes more pronounced. The element distribution, in contrast, neither changes during the heating experiment nor upon long-time annealing. Thus, the behavior of 9P-Ge2Sb2Te5 single crystals is predominantly under thermodynamic control.

Spectroscopic signatures of the carbon buckyonions C60@C180 and C60@C240: a dispersion-corrected DFT study

We have investigated, using dispersion corrected DFT methods, the structure and the spectroscopic properties of carbon buckyonions C60@C180 and C60@C240. C60, C180 and C240 showed a noticeable variation of their geometries in C60@C180 and C60@C240, upon encapsulation. Inclusion of the dispersion correction term in the calculations has a significant effect on the geometry. C60@C180 has a large positive interaction energy, while for C60@C240 a negative value is found indicating that only C240 can easily accommodate C60. In both cases dispersion interactions strongly contribute to the stabilization of the complexes. Vibrational frequencies, electronic transitions and NMR properties have been computed. The results show that encapsulation leads to appreciable variation in the characteristic resonances thus offering a useful tool for a spectroscopic identification of these species.