Cation sitting in aromatic cages: ab initio computational studies on tetramethylammonium–(benzene)n (n=3–4) complexes

Quantum chemistry study was performed on interaction between tetramethylammonium (TMA) and aromatic cages by means of the MP2 method to show how TMA sits in an aromatic cage that is composed of benzenes. The MP2 calculations on TMA–(benzene)_n complexes demonstrate that the more the benzene molecules in the aromatic cage, the stronger the binding strength between the cage and TMA. In details, the structure of TMA–(benzene)_n (n = 1–4) complexes can be easily constructed by superimposing n TMA‐benzene complexes via TMA, and the binding energies of the TMA–(benzene)_n complexes are the sum of the n corresponding TMA‐benzene systems. For instance, the distances between the N of TMA and the plane of the benzene ring are 4.238, 4.252, 4.264 ,and 4.276 Å, respectively, for TMA–(benzene)_n (n = 1–4) complexes, and the BSSE corrected binding energies at MP2/6‐311++G** level are ?8.8, ?17.3, ?25.8 and ?34.3 kcal/mol, respectively, for TMA– (benzene)_n (n = 1–4) complexes. Thus, this study provides us useful information on how a cation interacts with an aromatic cage in terms of complex geometry and binding strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational properties of ibuprofen–cyclodextrin inclusion complexes investigated by Raman scattering and numerical simulation

The modifications to the vibrational spectra produced by inclusion into cyclodextrins on the vibrational spectra of of the non‐steroidal anti–inflammatory drug ibuprofen, by inclusion into cyclodextrins have been investigated by means of Raman scattering and numerical simulation. These changes are discussed and explained by comparison with the theoretical vibrational wavenumbersfrequencies and Raman intensities obtained by quantum and classical numerical simulations, disentangling the effects directly related to the complexation process, from those to be ascribed to non‐covalent dimerization of ibuprofen due to hydrogen bonding. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis,UV–Vis and CV properties of a structurally related series of bis(Arylimino)acenaphthenes (Ar‐BIANs)

Herein, we report the synthesis and electrochemical and spectroscopic characterization for a series of six bis(arylimino)acenaphthene, compounds commonly used as ligands in catalysis, polymerization and materials. Low energy absorption bands are the result of an intraligand charge transfer between the aryl groups and the acenaphthene core, and their energies obey a Hammett relationship with respect to substitution on the aryl. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis,spatial structure and spectral properties of pyrylo‐4 (thio) squaraines variously substituted in cyclobutene moiety

Synthetic routes have been developed to a number of (thio) squaraine dyes containing the residues of CH‐acids at the central cyclobutene ring. The electronic and spatial structure as well as the chemical conversions and optical behaviour of the compounds obtained have been studied both theoretically and by X‐ray diffraction analysis, ¹H NMR and electronic spectroscopy. As shown, the electronic nature and sterical characteristics of the central ring substituents give rise to some general conformational features and crystal packing regularities and also govern the spectral position of the first π–π^* absorption band. The structure–property relationships established in the study provide guidance for the purposeful design of deeply coloured (thio) squaraines. Copyright © 2015 John Wiley & Sons, Ltd.     

Radiative properties of cirrus clouds in the infrared (8–13 μm) spectral region

Atmospheric radiation in the infrared (IR) 8–13 μm spectral region contains a wealth of information that is very useful for the retrieval of ice cloud properties from aircraft or space-borne measurements. To provide the scattering and absorption properties of nonspherical ice crystals that are fundamental to the IR retrieval implementation, we use the finite-difference time-domain (FDTD) method to solve for the extinction efficiency, single-scattering albedo, and the asymmetry parameter of the phase function for ice crystals smaller than 40 μm. For particles larger than this size, the improved geometric optics method (IGOM) can be employed to calculate the asymmetry parameter with an acceptable accuracy, provided that we properly account for the inhomogeneity of the refracted wave due to strong absorption inside the ice particle. A combination of the results computed from the two methods provides the asymmetry parameter for the entire practical range of particle sizes between 1 and 10,000 μm over the wavelengths ranging from 8 to 13 μm. For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), and the anomalous diffraction theory (ADT) can lead to a substantial discontinuity in comparison with the FDTD solutions for particle sizes on the order of 40 μm. To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM). For the IR 8–13 μm spectral region, we show that SSPM is a more accurate approximation than ADT, MSFES, and IGOM. The SSPM solution can be further refined numerically. Through a combination of the FDTD and SSPM, the extinction and absorption efficiencies are computed for hexagonal ice crystals with sizes ranging from 1 to 10,000 μm at 12 wavelengths between 8 and 13 μm.

Calculations of the cirrus bulk scattering and absorption properties are performed for 30 size distributions obtained from various field campaigns for midlatitude and tropical cirrus cloud systems. Ice crystals are assumed to be hexagonal columns randomly oriented in space. The bulk scattering properties are parameterized through the use of second-order polynomial functions for the extinction efficiency and the single-scattering albedo and a power-law expression for the asymmetry parameter. We note that the volume-normalized extinction coefficient can be separated into two parts: one is inversely proportional to effective size and is independent of wavelength, and the other is the wavelength-dependent effective extinction efficiency. Unlike conventional parameterization efforts, the present parameterization scheme is more accurate because only the latter part of the volume-normalized extinction coefficient is approximated in terms of an analytical expression. After averaging over size distribution, the single-scattering albedo is shown to decrease with an increase in effective size for wavelengths shorter than 10.0 μm whereas the opposite behavior is observed for longer wavelengths. The variation of the asymmetry parameter as a function of effective size is substantial when the effective size is smaller than 50 μm. For effective sizes larger than 100 μm, the asymmetry parameter approaches its asymptotic value. The results derived in this study can be useful to remote sensing studies of ice clouds involving IR window bands.     

Solvent effects on chemical processes: new solvents designed on the basis of the molecular–microscopic properties of (molecular solvent + 1,3‐dialkylimidazolium) binary mixtures

The purpose of this work was to analyze the microscopic feature of binary solvent systems formed by a molecular solvent (acetonitrile or dimethylformamide or methanol) and an ionic liquid (IL) cosolvent [1‐(1‐butyl)‐3‐methylimidazolium tetrafluoroborate or 1‐(1‐butyl)‐3‐methylimidazolium hexafluorophosphate]. The empirical solvatochromic solvent parameters E_T(30), π*, α, and β were determined from the solvatochromic shifts of adequate indicators. The behavior of the solvent systems was analyzed according to their deviation from ideality. The study focused on the identification of solvent mixtures with relevant solvating properties in order to select mixed solvents with particular characteristics. The comparison of the molecular–microscopic solvent parameters corresponding to the selected binary mixtures with both ILs considered at similar mixed‐solvent composition revealed that the difference is centered on the basic character of them. A kinetic study of a nucleophilic aromatic substitution reaction between 1‐fluoro‐2,4‐dinitrobenzene (FDNB) and 1‐butylamine (BU) developed in (acetonitrile or dimethylformamide + IL) solvent mixtures is presented in order to investigate and compare the solvent effects on a chemical process. For the explored reactive systems the solvation behavior is dominated by both the dipolarity/polarizability and the basicity of the media, contributing these solvent properties to accelerating the chemical process. Copyright © 2007 John Wiley & Sons, Ltd.     

Near‐forward/high‐pressure‐backward Raman study of Zn1 − xBexSe (x ~ 0.5) – evidence for percolation behavior of the long (Zn―Se) bond

The apparently universal 1‐bond → 2‐mode percolation behavior in the Raman spectra of zincblende semiconductor alloys is generally observed for the short bond only, and not for the long one. In this work we perform a combined high‐pressure‐backward/near‐forward Raman study of the leading percolation‐type (Zn,Be)Se alloy (~50 at.% Be), which exhibits a distinct percolation doublet in the spectral range of its short Be―Se bond, in search of a Zn―Se analogue. The high‐pressure‐backward insight is not conclusive per se, but clarifies the perspective behind the near‐forward Raman study. The latter reveals an unique Zn―Se phonon–polariton. Its fair contour modeling depending on the scattering angle is achieved within the linear dielectric approach, based on ellipsometry measurement of the ZnBeSe refractive index. Somewhat surprisingly this reveals that the phonon–polariton in question is a ‘fractional’ one in that it carries only half of the available Zn―Se oscillator strength, as ideally expected in case of a BeSe‐like bimodal Raman behavior of the long Zn―Se bond. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.     

Theoretical study of keto–enol tautomerism by quantum mechanical calculations (the QM/MC/FEP method)

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i‐butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gas‐phase calculation was Becke, three‐parameter, Lee–Yang–Parr/6‐311G(d,p)//Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d). The free energies of solvation were included in the calculation by using the free‐energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free‐energy perturbation method. Three different models, incorporating no‐water, one‐water, and two‐waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (ΔG^?_gas) to decrease by half relative to the water‐free mechanism, but there was no effect on the relative difference in free energy, ΔG_gas. The solvation effects (ΔG_sol), based on quantum mechanical/Monte Carlo/free‐energy perturbation calculations, were added to those of the gas‐phase results of the one‐water and two‐waters models. The two‐waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (ΔG_rxn) were less than 1.0 kcal mol^–1. In summary, the inclusion of solvent molecules in gas‐phase calculations plays a very important role in producing results consistent with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.     

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

The optimized geometry and structural features of the most prospective electro‐optic crystal 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST), and the vibrational spectral investigations have been comprehensively described with the near infrared Fourier transform (NIR FT) Raman and Fourier transform infrared (FT‐IR) spectra supported by the density functional theoretical (DFT) computations to elucidate the contribution of vibrational modes to the linear electro‐optic (LEO) effect. Mulliken population analysis and natural bond orbital (NBO) analysis have also been carried out to analyze the effects of intramolecular charge transfer (ICT), intramolecular hydrogen bonding and hyperconjugative interactions on the geometries. The influence of CT interaction between the phenyl ring and the dimethylamino group of the nonlinear optical (NLO) chromophore on the endocyclic and exocyclic angles, and the electronic effects such as hyperconjugation and back‐donation on the methyl hydrogen atoms have been examined. The concurrent intense activation of Raman and IR activities of the effective conjugation vibrational coordinate, which significantly contributes to the LEO effect resulting from the strong electron–phonon (e/ph) coupling, has been analyzed in detail. The effects of frontier orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), transition of electron density (ED) transfer and the influence of planarity in the stilbazolium ring on the first hyperpolarizability are also discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural and optical properties of Ba(Ti1−x,Nix)O3 thin films prepared by sol–gel process

Ba(Ti_1−x,Ni_x)O₃ thin films were prepared on fused quartz substrates by a sol–gel process. X-ray diffraction and Raman scattering measurements showed that the films are of pseudo-cubic perovskite structure with random orientation and the change of lattice constant caused by Ni-doping with different concentrations is very small. Optical transmittance spectra indicated that Ni-doping has an obvious effect on the energy band structure. The energy gap of Ba(Ti_1−x,Ni_x)O₃ decreased linearly with the increase of Ni concentration. It indicates that the adjusting of band gap can be achieved by controlling the Ni-doping content accurately in Ba(Ti_1−x,Ni_x)O₃ thin films. This has potential application in devices based on ferroelectric thin films.