Calculated Band Profiles Research Articles

Comprehensive DFT exploration of structural, electronic, magnetic and elastic properties of HfM2 (M = Cr, Mn, and Fe) intermetallic compounds

This study comprehensively explores the structural, magnetic, electronic, hyperfine field interactions (HFI), electric field gradient (EFG) and elastic properties of the intermetallics HfM2 (M = Cr, Mn, and Fe) through density functional theory. The inclusion of Hubbard U along with generalized gradient approximation is used to treat the correlation and with the introduction of the spin orbit coupling effect; the relativistic influences of electrons on the physical characteristics of these compounds are also investigated. The stable ground state optimized energies in different magnetic configurations show that HfCr2 is non spin polarized, while HfMn2 and HfFe2 are ferromagnetic in nature. The calculated band profiles and electrical resistivities demonstrate the metallic nature of these intermetallics. The HFI and EFG analysis are also made to examine the charge distribution around the nucleus. The elastic properties demonstrate the mechanical stability, elastic anisotropy and high resistance to compression.

Room temperature PL efficiency of InGaN/GaN quantum well structures with prelayers as a function of number of quantum wells

AbstractWe report on the effects of varying the number of quantum wells (QWs) in an InGaN/GaN multiple QW (MQW) structure containing a 23 nm thick In0.05Ga0.95N prelayer doped with Si. The calculated conduction and valence bands for the structures show an increasing total electric field across the QWs with increasing number of QWs. This is due to the reduced strength of the surface polarisation field, which opposes the built‐in field across the QWs, as its range is increased over thicker samples. Low temperature photoluminescence (PL) measurements show a red shifted QW emission peak energy, which is attributed to the enhanced quantum confined Stark effect with increasing total field strength across the QWs. Low temperature PL time decay measurements and room temperature internal quantum efficiency (IQE) measurements show decreasing radiative recombination rates and decreasing IQE, respectively, with increasing number of QWs. These are attributed to the increased spatial separation of the electron and hole wavefunctions, consistent with the calculated band profiles. It is also shown that, for samples with fewer QWs, the reduction of the total field across the QWs makes the radiative recombination rate sufficiently fast that it is competitive with the efficiency losses associated with the thermal escape of carriers. (© 2016 The Authors. Phys. Status Solidi C published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Widening of the hydrogen bonded OH-streching bands due to the wagging and OO-stretching modes in H 2O·H 2O

We have calculated band profiles of the hydrogen bonded OH-stretching bands in H 2O · H 2O, using a local mode Hamiltonian which includes the low-energy acceptor wagging and the high energy OH-stretching motion. We use the method previously developed for coupling of OH- and OO-stretching modes. Our current results confirms that low frequency modes will lead to wider OH-stretching bands. The spread of intensity and shape of the profile from coupling to the acceptor wagging mode is different to that found from coupling to the OO-stretching mode. Combination of both results yields an OH-stretching band profile, which is around 250 cm −1 wide in the third OH-stretching overtone.

Characteristics of the adsorption mechanism of acido-basic compounds with two p [formula omitted] in reversed-phase liquid chromatography

Overloaded band profiles of phthalic acid, nicotinic acid, and sodium nicotinate eluted with a methanol-aqueous buffer solution were recorded at the exit of a 150 mm × 4.6 mm column packed with 3.5 μ m X-Bridge-C 18 porous particles. The pH W S s of the mobile phase was adjusted at different values, with addition of hydrochloric acid ( pH W S ∼ 1.37), phosphate ( pH W S ∼ 2.01, 2.85, and 7.32), formate ( pH W S ∼ 3.61), acetate ( pH W S ∼ 4.41 and 5.15), or succinate buffers ( pH W S ∼ 6.11). The ionic strengths of all the buffers were fixed at 0.1 M by adding the necessary amount of potassium chloride when necessary. The compositions of the solutions at equilibrium were solved by using the extended Debye–Hückel theory, which estimates the activity coefficients of the ions in the bulk phase. The adsorption of the samples onto X-Bridge-C 18 assumed a simple bi-Langmuir isotherm for the neutral species (or zwitterion) and a Langmuir isotherm for the ionizable species. Competition for adsorption between the neutral and the ionic species was neglected. The band profiles were calculated based on the equilibrium-dispersive model of chromatography. An excellent agreement was observed between the experimental and the calculated band profiles for all buffer solutions. This work validates the adsorption mechanism assumed in earlier work for neutral and ionizable species. It is consistent with the neutral species adsorbing weakly (due to dispersive interactions) onto and/or into the C 18-bonded layer (large number of sites) while the singly charged species adsorb strongly onto the few residual accessible silanol groups (due to charge–dipole interactions). Doubly charged species are not retained and are probably excluded from the pores of the hydrophobic stationary phase.

Calculated Band Profiles of the OH-Stretching Transitions in Water Dimer

We have calculated the band profiles of the OH-stretching fundamental and overtone transitions in the proton donor unit of the water dimer complex. We have used a local mode Hamiltonian that includes both OH-stretching and OO-stretching motion but separates these adiabatically. The variation of OH-stretching frequency and anharmonicity with OO displacement from equilibrium contributes to the effective OO-stretching potentials for each OH-stretching state. The resulting OO-stretching energy levels and wave functions are used to simulate the vibrational profile of each OH-stretching transition. The coupled cluster with singles, doubles, and perturbative triples ab initio method with an augmented triple-zeta correlation consistent basis set has been used to obtain the necessary parameters, potentials, and dipole moment functions. We find that the OO-stretching transitions associated with a given hydrogen bonded OH-stretching transition are spread significantly and this spread increases with overtone. The spread is minor for the free OH-stretching transition. The inclusion of the OO-stretching mode has a limited effect on the overall OH-stretching band intensity.

Thermodynamic characterization of separations on alkaline-stable silica-based C18 columns: Why basic solutes may have better capacity and peak performance at higher pH

A systematic study was made to explain the large improvements in separation performance and capacity of basic compounds at alkaline conditions. The adsorption of three probe components was investigated on four alkaline-stable silica-based C18 columns at three different pH-levels: 3, 7 and 11. The probes were 3-phenyl-1-propanol (neutral), 2-phenylbutyric acid (acidic) and metoprolol (basic). Adsorption isotherms were acquired over a broad concentration range, in order to detect both high and low energy sites. Before the choice of the proper adsorption isotherm model, the adsorption energy distribution (AED) was calculated yielding the number of different kinds of interaction sites between the solute and the stationary phase. The neutral probe was entirely unaffected by pH and its AED was unimodal (one site) indicating homogenous adsorption. For the acidic probe the interactions were unimodal at pH 3 where the probe is uncharged and at least bimodal (two sites) at pH 7 and 11 where the probe is charged. For the basic probe, the interactions were heterogeneous at both pH 3 and 11. The equilibrium constants of the high and low energy sites were different by a factor of 55–100 at pH 3 and only 6–7 at pH 11. The difference in saturation capacities between the two sites was much smaller at pH 11 where 20% of the total capacity is from the high energy site, as compared to pH 3 where the high energy site was only 2–5% of the total capacity. This explains why peaks of amines (basic solutes) tail at low pH while their peaks are symmetrical at alkaline pH. The Langmuir model fit the unimodal data and the bi-Langmuir model fit the bimodal AED data. The calculated band profiles based on these parameters agreed excellently with the experimental data. The electrostatic-modified Langmuir, on the other hand, did not describe this adsorption process well.

Time-Domain Calculations of the Infrared and Polarized Raman Spectra of Tetraalanine in Aqueous Solution

The IR and polarized (isotropic and anisotropic) Raman spectra are calculated for the amide I band of tetraalanine ((Ala)4) in aqueous solution by using a time-domain computational method, which includes the effects of the diagonal frequency modulations (of individual peptide groups), the off-diagonal (interpeptide) vibrational couplings, and structural dynamics. It is shown that the calculated band profiles, especially the existence of a large negative noncoincidence effect (i.e., large frequency separations between the IR, isotropic Raman, and anisotropic Raman bands, with the isotropic Raman being higher in frequency), are in reasonable agreement with the experimental results. This negative noncoincidence effect derives from two conditions: the positive coupling between the amide I vibrations of peptide groups and the angle larger than 90 degrees between the transition dipoles of the coupled vibrations. This result means that the dynamically changing structures mainly in the polyproline II and beta-type conformations containing some repeated interconversions obtained from the molecular dynamics calculation are consistent with the existence of a large negative noncoincidence effect, as far as the structures satisfy the above two conditions. It is also shown that the electric fields from solvent water molecules induce larger frequency shifts than those of intrachain interactions, with rapid underdamped oscillatory modulations ( approximately 43 fs) due to the librational motions of water molecules that give rise to motional narrowing effect on the spectra. The reason for the difference from the behavior seen for the O-H stretching mode of liquid water is discussed. The time-domain analysis of the mode identity shows that the system proceeds halfway to complete mode mixing with a similar time scale ( approximately 60 fs), suggesting the importance of the nonadiabatic effect, which is included in a natural way in the present computational method.

Effects of Intermolecular Vibrational Coupling and Liquid Dynamics on the Polarized Raman and Two-Dimensional Infrared Spectral Profiles of Liquid N,N-Dimethylformamide Analyzed with a Time-Domain Computational Method

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented and applied to the case of the amide I band of liquid N,N-dimethylformamide. It is shown that the effect of the resonant off-diagonal vibrational coupling and the resulting delocalization of vibrational modes is clearly seen as the noncoincidence effect in the polarized Raman spectrum and some spectral features (especially as asymmetric intensity patterns) in the 2D-IR spectra. The type of 2D-IR spectra (concerning the polarization condition) most appropriate for observing this effect is discussed. On the basis of the agreement between the observed and calculated band profiles of the polarized Raman spectrum, the time dependence of the transient IR absorption anisotropy is also calculated. The method of evaluating the extent of delocalization of vibrational modes that is relevant to the features of these optical signals in the time and frequency domains is discussed. The nature of the molecular motions (concerning the liquid dynamics) that are effective on the diagonal frequency modulations is also examined.

Infrared extinction by homogeneous particle aggregates of SiC, FeO and [formula omitted]: Comparison of different theoretical approaches

Particle shape and aggregation have a strong influence on the spectral profiles of infrared phonon bands of solid dust grains. Calculating these effects is difficult due to the often extreme refractive index values in these bands. In this paper, we use the discrete dipole approximation (DDA) and the T-matrix method to compute the absorption band profiles for simple clusters of touching spherical grains. We invest reasonable amounts of computation time in order to reach high dipole grid resolutions and take high multipolar orders into account, respectively. The infrared phonon bands of three different refractory materials of astrophysical relevance are considered—silicon carbide, wustite and silicon dioxide. We demonstrate that even though these materials display a range of material properties and therefore different strengths of the surface resonances, a complete convergence is obtained with none of the approaches. For the DDA, we find a strong dependence of the calculated band profiles on the exact dipole distribution within the aggregates, especially in the vicinity of the contact points between their spherical constituents. By applying a recently developed method to separate the material optical constants from the geometrical parameters in the DDA approach, we are able to demonstrate that the most critical material properties are those where the real part of the refractive index is much smaller than unity.

Thermodynamic analysis of the heterogenous binding sites of molecularly imprinted polymers

The thermodynamic interactions of two polymers, one Fmoc- L-Trp-imprinted (MIP), the other one an unimprinted reference (NIP), with the two Fmoc-tryptophan enantiomers were studied by frontal analysis, which allows accurate measurements of the adsorption isotherms. These isotherms were acquired at temperatures of 40, 50, 60, and 70 ° C, for sample concentrations ranging between 0.005 and 40 mM. The mobile phase used was acetonitrile with one percent acetic acid as an organic modifier. Within the measured concentration ranges, the tri-Langmuir isotherm model accounts best for the isotherm data of both enantiomers on the MIP, the bi-Langmuir model for the isotherm data of Fmoc- L-Trp on the NIP. These isotherm models were selected using three independent processes: statistical tests on the results from regression of the isotherm data to different isotherm models; calculation of the affinity energy distribution from the raw isotherm data; comparison of the experimental and the calculated band profiles. The isotherm parameters obtained from these best selected isotherm models showed that the enantiomeric selectivity does not change significantly with temperature, while the affinity of the substrates for both the MIP and the NIP decrease considerably with increasing temperatures. These temperature effects on the binding performance of the MIP were clarified by considering the thermodynamic functions (i.e., the standard molar Gibbs free energy, the standard molar entropy of adsorption, and the standard molar enthalpy of adsorption) for each identified type of adsorption sites, derived from the Van’t Hoff equation. This showed that the entropy of transfer of Fmoc- L-Trp from the mobile to the MIP stationary phase is the dominant driving force for the selective adsorption of Fmoc- L-Trp onto the enantioselective binding sites. This entropy does not change significantly with increasing temperatures from 40 to 70 ° C.

Study of the adsorption equilibria of the enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate using a microbore column

Using competitive frontal analysis, the binary adsorption isotherms of the enantiomers of 1-phenyl-1-propanol were measured on a microbore column packed with a chiral stationary phase based on cellulose tribenzoate. These measurements were carried out using only the racemic mixture. The experimental data were fitted to four different isotherm models: Langmuir, BiLangmuir, Langmuir–Freundlich and Tóth. The BiLangmuir and the Langmuir–Freundlich models accounted best for the competitive adsorption data. An excellent agreement between the experimental and the calculated overloaded band profiles for various samples of racemic mixture was obtained when the equilibrium dispersive model of chromatography was used together with the BiLangmuir competitive isotherm. The isotherm parameters measured under competitive conditions were used to calculate the overloaded band profiles of large samples of the pure S- and R-enantiomers, too. A satisfactory agreement between the experimental and calculated band profiles was observed when using in the computation the corresponding single component BiLangmuir isotherm derived from the binary isotherm previously determined. Thus only data derived from the racemic mixture are required for computer optimization of the preparative chromatography separation of the enantiomers.

Effects of the composition of the mobile phase on the production rate in reversed-phase overloaded chromatography

Band profiles in overloaded reversed-phase chromatography on octadecyl silica columns were calculated using the experimental parameters of the distribution isotherms of binary mixtures of (1) resorcinol and phenol and (2) phenol and o-cresol determined by binary frontal analysis with automatic, on-line analysis of the composition of the breakthrough curve plateaus. The dependencies of the isotherm coefficients on the concentration of methanol in aqueous-organic mobile phases were described by three-parameter equations and used in the numerical calculations. Calculated band profiles of the sample components allowed determination of the fraction range, recovery yield and production rate necessary to obtain the sample components in required purity with various loading factors and concentration ratios of sample components in dependence on the concentration of methanol in the mobile phase. It was found that there is an optimum composition of the mobile phase, at which the optimum production rate can be achieved, which depends on the isotherms and on the concentration ratio of components in the sample feed.

Spectroscopic Investigation of Nitromethane Flames

AbstractNitromethane (CH3NO2) Pool Fire Flames have been investigated analysing both, the radiation emitted from intermediate combustion radicals (diatomic molecules) in the ultraviolet (UV) and visible (VIS) spectral range, and the near infrared spectra dominated by broad water bands. Comparing the UV/VIS spectra to calculated band profiles, rotational and vibrational temperatures (Trot, Tvib) of OH, NH and CN have been determined. At local thermal equilibrium Trot and Tvib must be equal. The determined temperatures show that the OH and NH emissions originate from pure thermal excitation (Trot = Tvib = 2300 K–2380 K) but the CN emissions show additionally a chemical excitation indicated by different vibrational and rotational temperatures in the Boltzman factor (Trot = 2100 K, Tvib = 4300 K). For a more detailed study of the flame, the emission of OH radicals has been investigated more extensively by monitoring a two‐dimensional rotational temperature and emissivity profile. In cooler flame regions, the observed spectra are dominated by the near infrared radiation emitted from stable combustion products like water and carbon dioxide. Therefore, NIR spectra (1000 nm to 2500 nm) have been compared to broad band profiles calculated with a self‐developed code basing on the data of the ‘Handbook of Infrared Radiation of Combustion Gases’. The flame temperatures obtained by this method range from 1800 K to 1900 K. The results are correlated to flames of methane and nitrogen oxide which emit similar spectra indicating similar reaction mechanisms in the gaseous phase.

Effects of the gradient profile on the production rate in reversed-phase gradient elution overloaded chromatography

Band profiles in overloaded gradient elution reversed-phase chromatography were calculated using the experimental parameters of the distribution isotherms of binary mixtures of phenol and o-cresol determined by binary frontal analysis with automatic, on-line analysis of the composition of the breakthrough curve plateaus. The dependencies of the isotherm coefficients on the composition of the mobile phase were described by three-parameter equations and used in the numerical calculations. Calculated band profiles of the sample components determined the fraction range, recovery yield and production rate necessary to obtain the required purity of the sample components with various loading factors and concentration ratios of sample components depending on the gradient profile. The production rate in preparative gradient elution HPLC depends more on the initial concentration of the stronger eluent than on the steepness of the gradient. The highest production rates can be achieved with steep gradients starting in pure water, where the sample-focusing effect is the strongest, provided feed solubility is adequate.

Use of the equilibrium-dispersive model of nonlinear gas chromatography for the modelling of the elution band profiles in the preparative-scale gas chromatographic separation of enantiomers

The equilibrium-dispersive model of chromatography is applied to the study of enantioselective separations of methyl 2-chloropropionate in gas chromatography. The single-component and competitive bi-Langmuir isotherm equations that describe the sorption behavior of single enantiomers and racemates, respectively, are utilized by the equilibrium-dispersive model to predict the elution band profiles of single-component and binary mixture samples. The measured and the calculated band profiles are compared both for the analytical open tubular column used to acquire the isotherm data and for the 1 m×22.5 mm I.D. preparative packed column, followed by the comparison of the enantiomeric purity vs. production curves for both the measured and the simulated systems.

Study of the interaction between two overloaded bands injected successively in non-linear chromatography

Band interaction in non-linear chromatography was studied using a delayed injection procedure which permits the easy adjustment of the degree of interference regardless of the real selectivity between the two compounds studied. This work represents an investigation of the mechanism of band interference, independent of the separation process. Special attention was given to the converse displacement effect, which corresponds to the displacement of the more strongly retained compound by the less retained one. Experimental evidence of this converse displacement is shown using two different systems. The intensity of this displacement is discussed in relationship to the concentration of the displacer and the waiting time between the two injections. Calculated band profiles, obtained with the semi-ideal model of chromatography and competitive Langmuir isotherms, are compared with experimental profiles. With one system good agreement between experiments and simulations was found whereas the other system studied showed significant deviations from the competitive Langmuir isotherm model.

Infra-red bandshapes of methylene-d 2 bending vibrations in n-hexatriacontane-n-hexatriacontane-d 74

Infra-red spectra in the CD 2 bending vibration region (1080–1100 cm −1) have been analysed for mixtures of deuterated and hydrogenated hexatriacontane. The i.r. data analyses are based on lattice dynamical calculations of guest deuterated molecules in the host n-C 36H 74 and infrared intensities calculated using the electro-optical parameter method. The calculated band profiles as a function of the deuterated molecule concentration compare favourably to experimental spectra taken at 80K. The high resolution, low temperature spectra reveal features heretofore only observed at much higher concentrations of deuterated species. Self deconvolution procedures were used to further resolve the spectra. Excellent agreement was found between calculated and experimental ratios of the i.r. intensity of certain dimer arrangements to that of singlet molecules. This intensity ratio was found to be a better measure of deuterated species concentration than the halfwidth of the CD 2 bending vibration band that had been previously used.