Average Internuclear Distance Research Articles

Surface Structure of Lecithin-Capped Cesium Lead Halide Perovskite Nanocrystals Using Solid-State and Dynamic Nuclear Polarization NMR Spectroscopy.

Inorganic colloidal cesium lead halide perovskite nanocrystals (NCs) encapsulated by surface capping ligands exhibit tremendous potential in optoelectronic applications, with their surface structure playing a pivotal role in enhancing their photophysical properties. Soy lecithin, a tightly binding zwitterionic surface-capping ligand, has recently facilitated the high-yield synthesis of stable ultraconcentrated and ultradilute colloids of CsPbX3 NCs, unlocking a myriad of potential device applications. However, the atomic-level understanding of the ligand-terminated surface structure remains uncertain. Herein, we use a versatile solid-state nuclear magnetic resonance (NMR) spectroscopic approach, in combination with dynamic nuclear polarization (DNP) and atomistic molecular dynamics (MD) simulations, to explore the effect of lecithin on the core-to-surface structures of CsPbX3 (X = Cl or Br) perovskites, sized from micron to nanoscale. Surface-selective (cross-polarization, CP) solid-state and DNP NMR (133Cs and 207Pb) methods were used to differentiate the unique surface and core chemical environments, while the head-groups {trimethylammonium [-N(CH3)3+] and phosphate (-PO4-)} of lecithin were assigned via 1H, 13C, and 31P NMR spectroscopy. A direct approach to determining the surface structure by capitalizing on the unique heteronuclear dipolar couplings between the lecithin ligand (1H and 31P) and the surface of the CsPbCl3 NCs (133Cs and 207Pb) is demonstrated. The 1H-133Cs heteronuclear correlation (HETCOR) DNP NMR indicates an abundance of Cs on the NC surface and an intimate proximity of the -N(CH3)3+ groups to the surface and subsurface 133Cs atoms, supported by 1H{133Cs} rotational-echo double-resonance (REDOR) NMR spectroscopy. Moreover, the 1H-31P{207Pb} CP REDOR dephasing curve provides average internuclear distance information that allows assessment of -PO4- groups binding to the subsurface Pb atoms. Atomistic MD simulations of ligand-capped CsPbCl3 surfaces aid in the interpretation of this information and suggest that ligand -N(CH3)3+ and -PO4- head-groups substitute Cs+ and Cl- ions, respectively, at the CsCl-terminated surface of the NCs. These detailed atomistic insights into surface structures can further guide the engineering of various relevant surface-capping zwitterionic ligands for diverse metal halide perovskite NCs.

Аналітичні та експериментальні дослідження властивостей шлаків та шлакоутворюючих сумішей сталеплавильного виробництва

The aim of the study is to analyze and generalize the properties of slags within the CaO-SiO2-Al2O3-MgO-CaF2 system in different ratios of components and their concentrations, which is the basis of refining slags in modern steelmaking. To ensure the technological functions, the slag melt should have a set of optimal values of physical and chemical properties, in particular: viscosity, surface tension, electrical conductivity, melting temperature (beginning and end), enthalpy, density and others, which significantly affect the kinetics of heat and mass transfer processes in the "metal-slag" system. The information resource of researches is provided by own experimental data and data of databases "Slag" and "SFM" containing information about technological properties of melts of slag and oxide systems. The analysis of the data on the dependence of the properties of oxide systems on their chemical composition and temperature was performed. Oxide systems within the chemical composition CaO-SiO2-Al2O3-MgO-CaF2 are considered. The connection of the chemical composition of slag systems with the properties of their melts from the point of view of the concept of directed chemical bonding using integral parameters is established: ρ is the stoichiometry index; ∆e is the average number of electrons localized in the direction of the cation-anion (K-A) bond; d is the average inter-nuclear distance K-A; tgα is the change in the electron density in the direction of the K-A bond; ZA(A-A), ZK(K-A), ZK(K-K) are the weighted average values of cation and anion charges in the A-A, K-A, K-K bonds. Based on the results of experimental and analytical studies, predictive models of the properties of the considered oxide systems were obtained, namely for the calculation of viscosity, electrical conductivity, crystallization temperature and density of slag melts in the range of their temperatures from 1200°C to 1800°C. The presented method of predicting the properties of refining slags can be used to evaluate the use of slags of different composition in the ladle, for example, in ladle-furnace installations.

Evaluation of optical, and radiation shielding features of New phosphate-based glass system

The present investigation aims to fabricate and report the optical, and radiation shielding features of a novel phosphate-based glasses. The composition of the glasses described by the equation xBi2O3+20CaO+10K2O+(30-x)Na2O+40P2O5, x=0, 2.5, 5, 7.5, and 10 mol%. The density of the fabricated samples was found in the range of 2.333 to 3.518 g/cm3. The theoretical calculations were used to calculate some physical properties like ionic concentration, the volume of phosphorus atoms, average phosphorous-phosphorus separation, polaron radius, and inter-nuclear distance. Moreover, the UV-Vis spectra were recorded using the spectrophotometer in a wavelength range between 200 and 1100 nm. The refractive index increased from 2.405 to 2.455, the dielectric constant increased from 5.784 to 6.029, while the direct band-gap decreased from 3.633 to 3.517 eV. The gamma-ray shielding characteristics were evaluated using the monte Carlo simulation in the energy interval between 0.276 and 1.458 MeV. The highest LAC achieved at 0.276 MeV and is enhanced from 0.2565 to 0.80625 cm-1, raising the Bi2O3 concentrations between 0 and 10 mol%, respectively.

Interatomic Interaction and Structural Ordering in He II Phase

The Schrodinger equation for a system of two mobile spinless nuclei with $$\mathcal{Z}=2$$ and four electrons ( $$^4$$ He $$_2$$ -dimer) has been solved with exact diagonalization approach. It is found that the ground state of the system ( $$\sim -~4.2$$ a.u.) is higher than the energy of two separate $$^4$$ He atoms ( $$\sim -~5.7$$ a.u.), and the average internuclear distance $$\langle R_0\rangle \sim 1.7~\AA $$ is a half smaller than the typical lattice constant in solid helium phases. This result corresponds to a metastable state which can be stabilized only at extremely high external pressure estimated as 27,000 GPa. It means that formation of condensed helium phases at saturated vapor pressure (SVP) is possible only due to many-particle interaction. Below 4.2 K at SVP, $$^4$$ He behaves as simple liquid without long-range ordering (He I phase), but below 2.17 K it undergoes the $$\lambda $$ -transformation into He II phase due to long-range antiferromagnetic ordering in the $$^4$$ He spin subsystem with exchange of order 10 K. As a result, the He II phase can be interpreted as ‘spin ice’ built as polytypic structure of close-packed 2D planes on triangular lattice collected in a stack with alternated ordering in the direction perpendicular to the basal planes, but without breaking the closest packing principle between neighboring planes.

Mixed transition and rare earth ion doped borate glass: structural, optical and thermoluminescence study

The present work report on physical, structural, optical and thermoluminescence properties of Dy3+ doped manganese potassium borate glasses. The conventional melt quenching technique is used to prepare the glass samples. The amorphous nature of the prepared glass sample is confirmed by X-ray diffraction study. The different B–O vibrational bands and change in the coordination number of boron (BO3 → BO4) with the inclusion of Dy3+ ions in the prepared glasses were analyzed through FTIR. The physical parameters such as density, molar volume, average molecular weight, ion concentration, polaron radius, internuclear distance and optical parameters have been determined. The optical analysis reveals that the optical band gap energy decreases with the increase of dysprosium oxide concentration. The TL glow curve of gamma irradiated samples showed single prominent peak in the temperature range 425–460 K with varying concentration of Dy2O3. Analysis of kinetic parameters reveals that the glasses demonstrate second order kinetics.

The role of taking into account the interatomic interaction in predicting the complex of structurally-sensitive properties of steels and alloys for special purpose

The aim of the work is to identify the influence of the chemical composition of steels and special-purpose alloys on the formation of their physicochemical and structural-sensitive properties. This problem is solved by mathematical modeling of the inseparable chain «composition - structure – property» taking into account the parameters of interatomic interaction in the melt based on the concept of a directed chemical bond. A steel melt is considered as a chemically homogeneous system, and the state of the melts is expressed through a set of integral parameters, the main of which are: Zy - system charge state parameter (e); r - statistically average internuclear distance (10-1nm); tgα is a constant for each element, which characterizes the change in the radius of the ion as its charge changes. On the basis of experimental information on properties and using the parameters of interatomic interaction, computational models are proposed for predicting the properties of steels and alloys. The forecast models took into account the parameters of micro-inhomogeneity of steel, which ensured a high accuracy of the operational forecast. A comparative analysis of the results of steel melting with the corresponding calculations based on the JMatPro software package confirmed the effectiveness of using the interatomic interaction parameters as models. The proposed models for determining the melting of chromium-nickel steels are recommended for use with the content of basic elements Cr, Ni from 0 to 30%. The research results are recommended for use in industrial environments through the integration of the developed models in the process control system of steelmaking, which will contribute to the directed formation of the composition and properties of smelting products, as well as reducing energy costs.

Laser-pulse-shape control of photofragmentation in the weak-field limit

We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible even in the (one-photon) weak-field limit starting from a single vibrational eigenstate. Thus, phase dependence in the interaction with a fixed energy phase-modulated pulse can persist for some time after the pulse is over. This is illustrated for the nonadiabatic process: $\mathrm{I}+{\mathrm{Br}}^{*}\ensuremath{\leftarrow}\mathrm{IBr}\ensuremath{\rightarrow}\mathrm{I}+\mathrm{Br}$, where the relative yield of excited ${\mathrm{Br}}^{*}$ can be changed by pure phase modulation. It is shown that the phase is able to influence wave-packet spreading in the continuum as well as the average internuclear distance in each channel.

Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent on the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization upon shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.

Insight into the Molecular Properties of Chitlac, a Chitosan Derivative for Tissue Engineering

Chitlac is a biocompatible modified polysaccharide composed of a chitosan backbone to which lactitol moieties have been chemically inserted via a reductive N-alkylation reaction with lactose. The physical-chemical and biological properties of Chitlac that have been already reported in the literature suggest a high accessibility of terminal galactose in the lactitol side chain. This finding may account for its biocompatibility which makes it extremely interesting for the production of biomaterials. The average structure and the dynamics of the side chains of Chitlac have been studied by means of NMR (nuclear Overhauser effect and nuclear relaxation) and molecular dynamics to ascertain this hypothesis. A complete assignment of the (1)H and (13)C NMR signals of the modified polysaccharide has been accomplished together with the determination of the apparent pKa values of the primary and secondary amines (6.69 and 5.87, respectively). NMR and MD indicated a high mobility of Chitlac side chains with comparable average internuclear distances between the two techniques. It was found that the highly flexible lactitol side chain in Chitlac can adopt two distinct conformations differing in the orientation with respect to the polysaccharide chain: a folded conformation, with the galactose ring parallel to the main chain, and an extended conformation, where the lactitol points away from the chitosan backbone. In both cases, the side chain resulted to be highly hydrated and fully immersed in the solvent.

Nuclear spin-spin coupling in a van der Waals-bonded system: Xenon dimer

Nuclear spin-spin coupling over van der Waals bond has recently been observed via the frequency shift of solute protons in a solution containing optically hyperpolarized (129)Xe nuclei. We carry out a first-principles computational study of the prototypic van der Waals-bonded xenon dimer, where the spin-spin coupling between two magnetically non-equivalent isotopes, J((129)Xe - (131)Xe), is observable. We use relativistic theory at the four-component Dirac-Hartree-Fock and Dirac-density-functional theory levels using novel completeness-optimized Gaussian basis sets and choosing the functional based on a comparison with correlated ab initio methods at the nonrelativistic level. J-coupling curves are provided at different levels of theory as functions of the internuclear distance in the xenon dimer, demonstrating cross-coupling effects between relativity and electron correlation for this property. Calculations on small Xe clusters are used to estimate the importance of many-atom effects on J((129)Xe - (131)Xe). Possibilities of observing J((129)Xe - (131)Xe) in liquid xenon are critically examined, based on molecular dynamics simulation. A simplistic spherical model is set up for the xenon dimer confined in a cavity, such as in microporous materials. It is shown that the on the average shorter internuclear distance enforced by the confinement increases the magnitude of the coupling as compared to the bulk liquid case, rendering J((129)Xe - (131)Xe) in a cavity a feasible target for experimental investigation.

Theoretical study of NMR chemical shift induced by H/D isotope effect

AbstractThe isotope effect induced by deuterium substituted species is observed in molecular properties, such as geometry, kinetics, and electronic state, of the molecules through nuclear‐electron interaction. Theoretical considerations and experimental alignments have been studied by ab initio molecular orbital, density functional theory, and other empirical strategies. The Born‐Oppenheimer approximation with nuclear vibrational wavefunction can treat isotope effect because nuclear mass effect account for the average distance of vibrational motion. In this study, we introduce Morse anharmonic oscillator model to calculate average internuclear distance of diatomic molecules having X‐H bonding and X‐D bonding. Morse parameters are determined by fitting to potential energy surface of molecular orbital and/or density functional calculations, and then the average distance are obtained as the expectation value of the analytical Morse vibrational wavefunction. Nuclear magnetic resonance shielding constants of the H/D isotopomer are calculated again on the average distance by using GIAO with B3LYP and CCSD calculation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Simultaneous analytical optimization of variational parameters in Gaussian-type functions with full configuration interaction of multicomponent molecular orbital method by elimination of translational and rotational motions: Application to isotopomers of the hydrogen molecule

We have extended the multicomponent molecular orbital (MCMO) method to the full-configuration interaction (full-CI) fully variational molecular orbital method by elimination of translational and rotational motion components from total Hamiltonian. In the MCMO scheme, the quantum effects of protons and deuterons as well as electrons can be directly taken into account. All variational parameters in the full-CI scheme, i.e., exponents and centers (alpha and R) in the Gaussian-type function (GTF) basis set as well as the CI coefficients, are simultaneously optimized by using their analytical gradients. The total energy of the H(2) molecule calculated using the electronic [6s3p2d1f] and nuclear [1s1p1d1f] GTFs is -1.161 726 hartree, which can be compared to the energy of -1.164 025 hartree reported using a 512 term-explicitly correlated GTF calculation. Although the d- and f-type nuclear GTFs contribute to the improvement of energy convergence, the convergence of electron-nucleus correlation energy is slower than that of electron-electron one. The nuclear wave functions are delocalized due to the electron-nucleus correlation effect compared to the result of Hartree-Fock level of MCMO method. In addition, the average internuclear distances of all diatomic molecules are within 0.001 A of the previously reported experimental results. The dipole moment of the HD molecule estimated by our method is 8.4 x 10(-4) D, which is in excellent agreement with the experimental result of (8-10) x 10(-4) D.

The Weakest Link: Bonding between Helium Atoms

Van der Waals attraction between helium atoms can lead to the formation of weakly-bound He2 diatomic molecules stable at temperatures in the millikelvin range. Helium dimers were first detected experimentally in 1993 and 1994. Sophisticated quantum-mechanical computations confirm that the He–He bond is the weakest known, with a binding energy corresponding to 1.176 mK, about 10-4 eV. The average internuclear distance is computed to be in excess of 50 Å, which makes He2 the largest known diatomic molecule as well. The authors have introduced an idealized deltafunction model for the helium dimer that succinctly represents its remarkable properties. Despite the weakness of each individual He–He bond, an intriguing mechanism exists for helium to form trimers and larger clusters with much greater binding energies.

Diode laser spectroscopy of the fundamental bands of [formula omitted], [formula omitted], [formula omitted], [formula omitted] free radicals in the ground 2Σ + electronic state

Rotationally resolved spectra of the fundamental band of the CN free radical in four isotopic forms have been measured using tunable diode laser absorption spectroscopy. The source of the radical was a microwave discharge in a mixture of isotopically selected methane and nitrogen diluted with argon. The lines were measured to an accuracy of 5×10 −4 cm −1 and fitted to the formula for the vibration rotation spectrum of a diatomic molecule, including quartic distortion constants. The band origins of each of the isotopomers from the five parameter fits were found to be 12 C 14 N : 2042.42115(38) cm −1, 13 C 14 N : 2000.08479(23) cm −1, 12 C 15 N : 2011.25594(25) cm −1, 13 C 15 N : 1968.22093(33) cm −1 with one standard deviation from the fit given in parenthesis. Some of the lines showed a resolved splitting due to the spin rotation interaction. This was averaged for fitting purposes. The average equilibrium internuclear distance derived from the v=0 and 1 rotational constants of the four isotopomers is 1.171800(6) Å which is in good agreement with the value determined from microwave spectroscopy.

Multi-component molecular orbital study of isotope effects on lithium hydride molecules with the configuration interaction scheme

The multi-component molecular orbital method, which can take account of the quantum effect of the electrons and nuclei, is applied to the calculation of lithium hydride isotope species with the configuration interaction (CI) scheme. The optimum basis set functions for quantum nuclei are proposed by the fully variational procedure under single electronic–single nuclear excitation CI level. The average internuclear distances and dipole moments for isotopic lithium hydride molecules calculated with small basis functions are reasonable agreement with the corresponding experimental values.

Macrocycle conformation and self-inclusion phenomena in octakis(3- O-butanoyl-2,6-di- O-pentyl)-γ-cyclodextrin (Lipodex E) by NMR spectroscopy and molecular dynamics

Octakis(3- O-butanoyl-2,6-di- O-pentyl)-γ-cyclodextrin (Lipodex E) is a lipophilic chiral selector successfully used for the enantioselective gas chromatographic separation of a multitude of racemic analytes. NMR data ( 13C chemical shifts, 3 J HH, rotating frame NOEs (ROEs)) and molecular dynamics (MD) simulations point out that the macrocycle is distorted with respect to the canonical truncated-cone shape of native cyclodextrins, although C 8 symmetry is retained on the NMR timescale. ROE data and MD trajectories provide evidence for self-inclusion of one 6- O-pentyl pendant chain within the cavity of Lipodex E. The interpretation of long-range and low-intensity ROEs is supported by the calculation of average internuclear distances by using the radial distribution function (RDF) calculated from MD trajectories. MD simulations are eventually used to compare the flexibility of the macrocycle of Lipodex E with that of native γCD.

Multi-component molecular orbital theory for electrons and nuclei including many-body effect with full configuration interaction treatment: isotope effects on hydrogen molecules

A multi-component molecular orbital (MC_MO) theory is developed for a combined quantum system of electrons and nuclei with the full configuration interaction (CI) scheme of Cartesian Gaussian-type functions. The technique of graphical unitary group approach (GUGA) is modified to obtain the CI matrix elements for many kinds of quantum particles efficiently. The optimum basis sets for quantum nuclei are proposed with the fully variational procedure. The average internuclear distances, dipole polarizabilities, and nuclear vibrational excitation energies for isotopic hydrogen molecules calculated with optimized basis sets are found to adequately reproduce the corresponding experimental values.

Computational analysis of two similar neuropeptides yields distinct conformational ensembles.

Conformational states and thermodynamic properties for two similar neuropeptides, GDPFLRF-NH(2) and GYPFLRF-NH(2), have been computed by Monte Carlo simulated annealing (MCSA) conformational searches and Metropolis Monte Carlo (MMC) calculations. These peptides were recently shown to have dramatically different conformations in solution by NMR [Edison et al., J Neuroscience 1999;19:6318-6326]. Final conformations of multiple independent MCSA runs were the starting points for MMC calculations, and conformations saved at intervals during MMC runs were characterized in terms of total energy, configuration entropy, side-chain fraction population, and ensemble average inter-nuclear distances. Without the use of any NMR data-generated pseudo-potentials, the present calculations were in excellent qualitative agreement with all previous NMR experimental data and provided a foundation by which to more quantitatively interpret the experimental NMR results. Proteins 2000;40:367-377.

Determination of Relative N−HN, N−C‘, Cα−C‘, and Cα−Hα Effective Bond Lengths in a Protein by NMR in a Dilute Liquid Crystalline Phase

Weak alignment of solute macromolecules with the magnetic field can be achieved in a dilute, aqueous liquid crystalline phase of planar phospholipid micelles, consisting of mixtures of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC). Alignment of proteins in such a medium is sufficiently weak to retain the simplicity of the isotropic solution NMR spectrum but strong enough to permit accurate measurement of residual one-bond dipolar couplings. Highly accurate one-bond N−HN, Cα−Hα, Cα−C‘, and C‘−N and two-bond C‘−HN dipolar couplings were measured in 13C/15N-enriched ubiquitin. Together with knowledge of the protein's three-dimensional structure, the dipolar couplings permit calculation of the relative, vibrationally corrected average bond lengths for these interactions. Assuming a C‘−N bond length of 1.329 Å (Engh, R. A.; Huber, R. Acta Crystallogr. 1992, A47, 392−400), the relative Cα−C‘ distance of 1.526 Å is found to be in excellent agreement with results from Engh and Huber (1.525 Å). Using a C‘−N bond length of 1.329 Å as a reference, N−HN (1.041 ± 0.006 Å) and Cα−Hα (1.117 ± 0.007 Å) are considerably longer than equilibrium or average internuclear distances derived from ab initio calculations, electron diffraction, neutron diffraction, or microwave spectroscopy. The increase in effective N−HN and Cα−Hα bond lengths is attributed to a decrease in the corresponding dipolar couplings resulting from fast librations, which must be of considerably larger amplitude than the Cα−C‘ and C‘−N angular fluctuations. Accurate knowledge of the relative effective N−HN, Cα−Hα, Cα−C‘, C‘−N, and two-bond C‘−HN effective internuclear distances is essential for determining the magnitude of the molecular alignment tensor, for using the dipolar couplings in macromolecular structure determination, and for extracting angular information from recently described cross correlation experiments.

Heteronuclear NMR studies of cobalt corrinoids. 20. 31P chemical shift anisotropy of aquacobalamin and its complex with a haptocorrin from chicken serum

Static light scattering measurements have been used to determine the molecular mass (65.3 kDa) and second virial coefficient (3.66 × 10 −4 mol mL g −2) for the complex between chicken serum haptocorrin (HC) and aquacobalamin (H 2OCbl). Dynamic light scattering measurements have also been used to determine the translational diffusion coefficient of the H 2OCbl–HC complex as a function of concentration. From the diffusion coefficient at infinite dilution (4.71 × 10 −7 cm 2 s −1), the hydrodynamic radius (45.5 Å) and rotational correlation time (85.4 ns) have been calculated. Using the latter, and measured values of the 31P NMR linewidths of the H 2OCbl–HC complex at several field strengths, a detailed analysis of the 31P nuclear relaxation is possible. The chemical shift anisotropy term from the transverse relaxation equation is determined to be 95.7 ppm, and the average phosphorus-proton internuclear distance is 2.05 Å. For comparison to protein-free H 2OCbl, the chemical shift anisotropy of the phosphorus atom was studied by solid state NMR spectroscopy and the 31P relaxation by solution T 1 measurements. These studies give values of 110.3 ppm for the chemical shift anisotropy term and 2.45 Å for the average phosphorus-proton internuclear distance. The results are consistent with a significant change in the conformation of the H 2OCbl phosphodiester upon binding to haptocorrin which could be due to a shortening of the axial Co–N bond.