An Improved Anharmonic Force Field of Difluoromethanimine, F2C NH

The anharmonic force field of difluoromethanimine, F₂C NH, has been reinvestigated theoretically using a coupled-cluster singles and doubles approach, augmented for structural optimization and harmonic force field by a contribution of connected triple excitations, CCSD(T). The cubic and quartic force constants have been obtained by numerical derivatives computed from analytical quadratic force constants calculated by second-order Møller-Plesset perturbation theory, MP2. The quadratic force constants and the equilibrium structure of F₂C NH have then been scaled by a global least-squares fitting procedure to the spectroscopic data and parameters experimentally determined for this molecule. This force field, obtained in the internal coordinates space and therefore valid for all isotopomers of difluoromethanimine, yields a complete set of spectroscopic molecular constants providing a critical assessment of the experimental rotational and centrifugal distortion constants, fundamentals, overtones, and combination bands determined so far for F₂C NH. In addition, the final force field can be used to make predictions of all important vibrational and rotational parameters which should be accurate and useful for new spectroscopic investigations.     

The emprical general harmonic force field of ethane

A total of 175 spectroscopic data, accumulated from 10 isotopic species of ethane, are used to define all 22 parameters of the harmonic potential function within narrow limits. Before calculation, numerous Fermi resonances have been identified and quantified through infrared and Raman spectroscopic studies of CH₃CD₃ and its ¹³C isotopic species. This is an essential prerequisite to such an investigation, without which a self-consistent empirical data set cannot be achieved from which to determine physically meaningful force constants. Comparison of the empirical force constants with those predicted by scaled ab initio calculations shows an excellent degree of correspondence in all force constants, and confirms that both approaches can lead to essentially identical results. Calculated values of spectroscopic data of reliable quality are listed. These should be of value to future spectroscopic investigation of isotopic ethanes and for resolving the many resonance perturbations which are present.     

Charge distribution and covalency effects on mössbauer parameters in KFeS2

In this paper we report the effects of covalency on the Mössbauer Hyperfine parameters in KFeS₂. We emphasize the use of simple bonding schemes and models based on isomer shift calibration, point charge calculations and electronegativity differences in order to determine the ionic character of the atoms in the crystal. The previous Mossbauer spectroscopic work is also extended to liquid nitrogen temperature and it has been shown how the simple concept of electronegativity difference can also account for the order of magnitude of the measured saturation value of magnetic field.     

Understanding CeO2‐Based Nanostructures through Advanced Electron Microscopy in 2D and 3D

Engineering morphology and size of CeO₂‐based nanostructures on a (sub)nanometer scale will greatly influence their performance; this is because of their high oxygen storage capacity and unique redox properties, which allow faster switching of the oxidation state between Ce⁴⁺ and Ce³⁺. Although tremendous research has been carried out on the shape‐controlled synthesis of CeO₂, the characterization of these nanostructures at the atomic scale remains a major challenge and the origin of debate. The rapid developments of aberration‐corrected transmission electron microscopy (AC‐TEM) have pushed the resolution below 1 Å, both in TEM and in scanning transmission electron microscopy (STEM) mode. At present, not only morphology and structure, but also composition and electronic structure can be analyzed at an atomic scale, even in 3D. This review summarizes recent significant achievements using TEM/STEM and associated spectroscopic techniques to study CeO₂‐based nanostructures and related catalytic phenomena. Recent results have shed light on the understanding of the different mechanisms. The potential and limitations, including future needs of various techniques, are discussed with recommendations to facilitate further developments of new and highly efficient CeO₂‐based nanostructures.     

A new concept for sensitive in situ stable isotope ratio infrared spectroscopy based on sample modulation

Diode-laser absorption spectroscopy finds increasing applications in the emerging field of stable isotope research. To meet the requirements of the water isotopes measurement challenge in environmental research, ways have to be found to cope with the present limitations of spectroscopic systems. In this article, we discuss an approach based on the Stark effect in molecular spectra to reduce the influence of time-dependent, unwanted background structures generally superimposed on the desired signal from the spectral feature under investigation. A road map to high-sensitivity isotopic ratio measurements of water isotopes is presented. On the basis of an Allan Variance analysis of measured data, the detection limits have been calculated as a function of the integration time. To achieve the required optical density of about 6×10^?7 for H₂ ¹⁷O measurements, the duty cycle has to be optimized and the implementation of a sample modulation within an optical multipass cell is a promising approach to increase the stability of spectroscopic instrumentation required for ecosystem research and airborne atmospheric platforms.     

The combined use of SCF-MO calculations and frequency data in the evaluation of general harmonic force fields for molecules containing third row elements

The general harmonic force fields of several small molecules containing third row elements have been evaluated using a semiempirical method which combines SCF-MO calculations and limited frequency data. MOCIC (molecular orbital constraint using interaction coordinates) potential fields have been generated using the SCF-MO MNDO method which has recently been parameterized for third row elements. The general harmonic force field (GHFF) of molecules studied here (SO₂, NOCl, PF₃, PH₃, CCl₄, SiCl₄, SiF₄, SiH₄, and SF₆) are all well defined from spectroscopic data. The MOCIC functions are in excellent agreement with the spectroscopic GHFFs. Comparisons of the MNDO and MOCIC functions show substantial improvement in both primary and interaction valence potential constants when the MOCIC constraint procedure is used.     

Updated database plus software for line-mixing in CO2 infrared spectra and their test using laboratory spectra in the 1.5-2.3 μm region

In a previous series of papers, a model for the calculation of CO₂-air absorption coefficients taking line-mixing into account and the corresponding database/software package were described and widely tested. In this study, we present an update of this package, based on the 2008 version of HITRAN, the latest currently available. The spectroscopic data for the seven most-abundant isotopologues are taken from HITRAN. When the HITRAN data are not complete up to J″=70, the data files are augmented with spectroscopic parameters from the CDSD-296 database and the high-temperature CDSD-1000 if necessary. Previously missing spectroscopic parameters, the air-induced pressure shifts and CO₂ line broadening coefficients with H₂O, have been added. The quality of this new database is demonstrated by comparisons of calculated absorptions and measurements using CO₂ high-pressure laboratory spectra in the 1.5-2.3 μm region. The influence of the imperfections and inaccuracies of the spectroscopic parameters from the 2000 version of HITRAN is clearly shown as a big improvement of the residuals is observed by using the new database. The very good agreements between calculated and measured absorption coefficients confirm the necessity of the update presented here and further demonstrate the importance of line-mixing effects, especially for the high pressures investigated here. The application of the updated database/software package to atmospheric spectra should result in an increased accuracy in the retrieval of CO₂ atmospheric amounts. This opens improved perspectives for the space-borne detection of carbon dioxide sources and sinks.     

Approximate radiative properties of methane at high temperature

Band model parameters for high-temperature methane have been generated up to 2000 K from an extended spectroscopic database. Part of the spectroscopic data are issued from calculations made in the framework of the effective Hamiltonian approach. These data have been completed by a statistical extrapolation. The calculations of global radiative properties such as band absorbtances and total emissivities are in good agreement with the available experimental data showing that the contribution of the hot bands is correctly taken into account. Finally, the degree of correlation between CH₄, CO₂ and H₂O spectra in typical conditions of combustion applications is discussed. Band model parameters are available upon request.     

A combined experimental and computational studies of 3,3,6,6-Tetramethyl-9-(4-Methoxyphenyl)3,4,6,7,9,10 hexahydroacridine-1,8-dione

ABSTRACT

In this work, we have recorded the Fourier Transform Infrared (FTIR) and Ultra-Violet Visible (UV–Vis) spectra of 3,3,6,6-Tetramethyl-9-(4-Methoxyphenyl)3,4,6,7,9,10 hexahydroacridine-1,8-dione (C₂₄H₂₉NO₃) in the spectral range 4000–400?cm^?1 and 190–1400?nm, respectively. The thermo gravimetric (TG) analysis of the compound has been performed to check the thermal stability of the compound. The molecular geometry and complete vibrational spectra in the ground state are calculated by Hartree Fock (HF) and Density Functional Theory (DFT) using6-311G(d,p) basis set. The calculated vibrational harmonic frequencies are scaled using a proper scale factor, yielding a good agreement with the experimental data. Stability of the molecule arising from hyperconjugative interactions, charge delocalisation has been studied using natural bond orbital analysis (NBO). Mulliken charges, MEP mapping and temperature dependence on the thermodynamic properties in the optimised ground state have been calculated. UV–Visible spectrum of the molecule was calculated by using TD-DFT approach and the results were compared with the experimental one. We have calculated the several molecular parameters like ionisation potential, electron affinity, global hardness, electron chemical potential, electronegativity and global electrophilicity based on HOMO and LUMO energy values calculated at B3LYP/6-311G(d,p) level of theory. The calculated optimised structural parameters and vibrational wavenumbers are found to be in good agreement with the experimental results.     

The molecular structure and the analytical potential energy function of S2^- and S3^-

In this paper, the equilibrium geometry, harmonic frequency and dissociation energy of S2^- and S3^- have been calculated at QCISD/6-311＋＋G（3d2f） and B3P86/6-311＋＋G（3d2f） level. The S2^- ground state is of 2IIg, the S3^- ground state is of 2B1 and S3^- has a bent （C2v） structure with an angle of 115.65° The results are in good agreement with these reported in other literature. For S3^- ion, the vibration frequencies and the force constants have also been calculated. Base on the general principles of microscopic reversibility, the dissociation limits has been deduced. The Murrell-Sorbie potential energy function for S2^- has been derived according to the ab initio data through the least- squares fitting. The force constants and spectroscopic data for S2^- have been calculated, then compared with other theoretical data. The analytical potential energy function of S3^- have been obtained based on the many-body expansion theory. The structure and energy can correctly reappear on the potential surface.     

Calculation of the cubic symmetry force constants of methyl bromide and methyl chloride

The cubic symmetry force constants have been calculated for methyl bromide and methyl chloride through the method developed by Hoy et al. (Mol. Phys. 24, 1265–1290 (1972)). The spectroscopic constants recently reported in the literature have been utilized for the present analysis. Out of a total of 38 independent cubic force constants, 6 and 10 constants were constrained to zero for a methyl bromide and methyl chloride, respectively, in which a diagonal force constant of F₆₆₆ was included for both molecules. However, it is noted that those force constants which are only concerned with the A₁ symmetry coordinates have been determined independent of the constraint for both of these molecules.     

Ab initio calculation of force constants and equilibrium geometries

Force constants of the molecules HF, NH₃, CH₄ and BH₄ ^- have been calculated ab initio by the force method with a 73/3 + 1 gaussian lobe basis set. The results, including a former calculation on H₂O, agree well with experiment: the average relative error is 12 per cent for the diagonal force constants and the average absolute error is 0·06 mdyn/Å for the off-diagonal ones. The trends are also correctly reproduced. It is concluded that ab initio calculations of this accuracy can help to solve a number of spectroscopic problems. Force constants of BH₄ ^- have been determined from a combination of spectroscopic and ab initio information. Geometries have been obtained with little computing work and show good agreement with experiment.     

Phonon dispersion curves and densities of lithium-intercalated iron phosphorus trisulfide

Summary The lattice dynamics of Li-intercalated FePS₃ has been studied by means of a force constant model generated by a set of short-range two-body potentials. The intercalated phases have been investigated for the three stoichiometric compositions: Li_0.5FePS₃, LiFePS₃, Li_1.5FePS₃, with the aim of analysing the evolution of the host lattice normal modes as a function of the concentration, and of finding the dispersion of the new phonon branches induced by lithium. The above special values of lithium concentration have been chosen because the size of the unit cell keeps the same as in the host material. The force constants are fitted to the infrared data and the phonon dispersion curves and the phonon energy densities have been calculated. A spectroscopic method for monitoring lithium migration in the host material is proposed.     

Heats of formation of interstitial compounds and their alloys

The bonding of compounds of the form RY in the NaCl structure is predominantly ionic, as shown by spectroscopic studies of charge transfer. A formula for the heats of formation of these compounds (R=Ti, Zr, Hf, V, Nb, Ta and Y=C, N, O) is constructed based on a modification of Pauling's electronegativity table. The formula yields good agreement with experiment, and is refined to yield more accurate values of the R electronegativities. The heat of formation of NbN is anomalously small, and this may be correlated with its high superconducting transition temperature. Miscibility gaps in the alloy systems R′_xR″_1?xC are discussed.     

A Mulliken electronegativity scale and the structural stability of simple compounds

A new electronegativity scale is derived, in the spirit of Mulliken's original scheme, in terms of neutral atom spectroscopic data. The effect of changing the atomic configuration for C from covalent (sp³) to metallic (s²p²) is considered. Structural stability maps are presented for non-octet non-transition metal compounds, and for two classes of transition metal-non-transition-metal compounds.     

Emission spectra of pyrotechnic mixtures of heat flux simulators

Comprehensive optical spectroscopic studies of the combustion process of solid-state pyrotechnic mixtures based on Mg and Sr(NO₃)₂ have been carried out. Emission spectra of the mixtures in the ultraviolet, visible, and infrared wavelength regions have been studied under various atmospheric conditions taking into account radiation transfer in air along an optical path of observation up to 5 km long.     

The anharmonic force field of cis-1-chloro-2-fluoroethylene

A comprehensive anharmonic vibrational analysis of cis-1-chloro-2-fluoroethylene and its isotopomers has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles approach, augmented for structural optimization and harmonic force field by a contribution of connected triple excitations (CCSD(T)). The theoretical force field was scaled by global least-squares fitting to all spectroscopic data and parameters experimentally determined for this molecule. This final force field, employing standard perturbation theory, yields a complete set of spectroscopic molecular constants providing a critical assessment of experimental rotational and centrifugal distortion constants, fundamentals, overtones, and combination bands determined over many years. Effects of Fermi and Darling-Dennison resonances were included by matrix diagonalization.