Vibrational spectra of the trans-trans and trans-cis conformers of dimethyl fumarate

Infrared and Raman spectra of dimethyl fumarate (DMFU) have been recorded in the temperature range 12–390 K. The solid state spectra are consistent with the trans-trans conformation. However, in the liquid phase (melt or solution in CCl₄) and in the vapour phase, additional infrared and Raman bands were observed due to the presence of the trans-cis conformer. For the trans-trans conformer the observed spectra have been assigned on the basis of a C_2h molecular symmetry. A normal coordinate analysis of trans-trans DMFU has been carried out using a modified Urey- Bradley force field to assist in the assignment. Observed frequencies for the trans-cis conformer have been assigned on the basis of a structure of C_s symmetry.     

Conformational analysis of methylboronic acid and its acyclic esters

With the use of RHF/6-31G(d) and MP2/6-31G(d)//RHF/6-31G(d) semi-empirical (AMI) and non-empirical quantum chemical approximations conformational isomerization of methylboronic acid and its methyl, isopropyl, tert-butyl, and phenyl esters is studied. With the exception of the tert-butyl analogue, the potential energy surface of molecules in these compounds, is shown to contain three minima that correspond to planar and near-planar conformers: cis-cis, trans-trans, and cis-trans, with the latter being the main one. The minima are separated by two conformational isomerization barriers corresponding to the orthogonal arrangement of one of the OH(OR) groups.     

Photochemical- and pH-switching Properties of a New Photoelastic Ligand Based Upon Azobenzene. Basicity and Anion Binding

Abstract

The synthesis of a new cylindrical macrotricyclic ligand (L) based upon two 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane rings connected via two azobenzene pillars is described. The ligands displays photoelastic properties: light absorption at 366 nm produces trans→cis isomerization of the azobenzene moieties with formation of trans-cis (L _E-Z ) and cis-cis (L _Z-Z ) isomers. Recovery of the thermodynamically more stable trans-trans (L _E-E ) isomer can be obtained by thermal back-isomerization or visible light (430 nm) absorption. The isomeric composition of L solution depends upon pH: for pH < 5.5 the L _E-E form is the unique species in solution, while increasing amounts of Z species are formed at increasing pH. The protonation constants of L _E-E have been determined in 0.1 mol dm^?3 Me₄NNO₃ solution at 298.1 ± 0.1 K. The protonated species of L form stable inclusion complexes with anionic species such as [Co(CN)₆]^3?.     

Structures and energies of seleno derivatives of biuret.Ab Initio comparative studies of diselenobiuret, selenobiuret, and selenothiobiuret

Ab initio studies (LCAO-MO method) on conformers of three seleno derivatives of the biuret molecules diselenobiuret [I], selenobiuret [II], and selenothiobiuret [III] were carried out at the Hartree-Fock (HF) and MP2 levels. The molecular geometries of these species were fully optimized at the HF level and characterized by analysis of the harmonic vibrational frequencies using a split-valence triple-zeta basis set augmented by a set ofd polarization functions on heavy atoms andp polarization functions on hydrogen atoms [TZP(d, p)]. The total energies of the HF-optimized structures were calculated at the MP2 (frozen core) level using a larger TZP (2df, 2pd) basis set. The potential energy searches revealed a total of 11 minimum-energy conformers (assigned astrans-trans, trans-cis, cis-trans, andcis-cis) and seven transition-state species for the title molecules. The two predicted conformers for diselenobiuret (Ia=trans-trans andIc=cis-cis) are characterized byC ₂ and the third byC _s symmetry. For selenothiobiuret two forms (IIIa=trans-trans andIIId=cis-cis) possessC ₁ and two (IIIb=trans-cis andIIIc=cis-trans) possessC _s symmetries, respectively. For selenobiuret, four formsIIa=trans-trans (C₁),IIb=trans-cis (C _s),IIc=cis-trans (C ₁), andIId=cis-cis (C₁), were obtained as a result of gradient optimization. Comparison of the relative energies for the considered species indicated that thecis-trans forms are the most stable conformations for all three systems at both the HF and MP2 levels of theory.     

Effect of temperature on the absorption spectra of the solvated electron in 1-propanol and 2-propanol: Pulse radiolysis and laser photolysis studies at temperatures up to supercritical condition

The absorption spectra of solvated electrons in 1-propanol and 2-propanol have been investigated from 22 to 270 °C at a fixed pressure of 7 MPa, by using both nanosecond pulse radiolysis and laser photolysis techniques. The results show that, even up to supercritical conditions, it is still possible to measure unambiguously the absorption spectra of solvated electron in these two propanols. The peak positions of the absorption spectra show a red-shift (shifts to longer wavelengths) as temperature increases, similar to water and other alcohols, but the temperature efficiency, dE_max/dT, of 1-propanol is larger than that of 2-propanol. In addition, in clear contrast to that of pulse radiolysis, in laser photolysis experiments, an increase in the maximum absorbance of the absorption spectra of solvated electron with temperature up to ∼200 °C is attributed to the increase of absorbance (CTTS absorption band) of I⁻ anion at 248 nm with increasing temperature.     

Mechanisms of antioxidant action: A photochemical study of transition metal dibutyl dithiocarbamates (and their photo-stabilizing role in a polyolefin matrix)

Solution photochemistry of some (N,N-di-n-butyldithiocarbamato) complexes and of the metal-free dithiocarbamate ligands has been used to investigate their role in the photo-oxidation of polyolefins. Transient absorption spectra were recorded for tetramethyl thiuram disulphide at around 600 and 400 nm, and for tetramethyl thiuram monosulphide which also shows an additional band at 350 nm. Kinetic analysis of the decay profiles showed the presence of two different species with first and second order kinetics for the 400 and 600 nm transients respectively. Tentatively, the 400 nm absorption was assigned to a triplet-triplet transition, while the 600 and 350 nm absorption regions were assigned to the dithiocarbamoyl and thiocarbamoyl radicals respectively. Spectrophotometric monitoring has shown that tetramethyl thiuram disulphide is formed during the process of photobleaching of the Fe(III) complex.     

An MNDO SCF MO investigation of the electronic structures and conformational preferences of model polycarbonate systems

Theoretical calculations within the MNDO SCF MO formalism have been carried out to investigate the relative heats of formation, conformation preferences and excitation energies of model polycarbonate systems. It is shown that the trans-trans and cis-trans configurations of diphenyl carbonate (DPC) are of comparable energy and have a strong energetic preference for orthogonal forms. The energy barrier for the interconversion of trans-trans and cis-trans orthogonal forms is small. Comparison of the electronic structure of DPC with simple carbonyl, carboxy ester and carbonate systems indicates that the nature of the first excited singlet of polycarbonate is of $\text{π → π}{}^1$ character in contrast to the $\text{n → π}{}^1$ assignment which has been described in the literature.     

Green Synthesis of Silver Nanoparticles Using Acacia farnesiana (Sweet Acacia) Seed Extract Under Microwave Irradiation and Their Biological Assessment

In the present study, Acacia farnesiana (Sweet acacia) seed extract is used to reduce Ag⁺ → Ag⁰ under microwave irradiation. The formation of silver nanoparticles (AgNPs) is monitored by recording the UV–Vis absorption spectra for surface plasmon resonance (SPR) peak at ~450 nm. The absorbance of SPR increases linearly with increasing temperature of the reaction mixture. Rapid reduction of silver ions occurred to form AgNPs, 80–90 % yield in about 150 s. A marginal decrease in pH and increase in solution potential (E) of the reaction mixture during the formation of AgNPs are in agreement with the proposed mechanism. XRD pattern of the AgNPs agree with the fcc structure of Ag metal, and the calculated crystallite size is ~17 nm. FT-IR and solid-state ¹³C NMR spectra indicate the functional groups of flavonones and terpenoids (biomolecules from plant extract) which are adsorbed on AgNPs, thereby the present method led to in situ biofunctionalization/bio-capping of AgNPs. TG analysis shows the thermal decomposition of these plant residues present on AgNPs at about 250 °C. The spherical shape of the particles with a diameter (?) in the range of ~15–20 nm is evident from FE-SEM image. Elemental analysis by EDX analysis confirms the presence of Ag as the only major element. The in vitro antibacterial screening of AgNPs shows that these bio-capped AgNPs have higher inhibitory action for E. coli and S. aureus followed by B. subtilis and P. aeruginosa. In addition, AgNPs show very good antioxidant property.     

Quantum-chemical study of thermodynamics of hydrogen-bonded methylamine-methanol complexes reaction with dimethyl carbonate

Thermodynamic parameters of the reactions of dimethyl carbonate cis-cis and cis-trans conformers with methylamine, methylamine dimer, and methylamine complexes involving linear methanol associates have been computed with the B3LYP and WB97XD quantum-chemical methods. The both methods have given similar results. Thermodynamically, reactions of the cis-trans conformer are preferred over the analogous reactions of the cis-cis conformer, and the reactions with methylamine dimer and methylamine-methanol trimer complex are preferred over the reactions with methylamine monomer. The acid-base properties of the hydrogen-bonded methanol complexes are significantly enhanced with increasing degree of association. Stability of the methylamine complexes with methanol clusters is increased with more of the alcohol molecules involved.     

The synthesis,X-ray crystal structure and optical properties of novel 1,3,5-triaryl pyrazoline derivatives

A series of novel 1,3,5-triaryl pyrazoline derivatives has been synthesized by the reaction of chalcone and 3-chloro-6-hydrazinylpyridazine in 47–82% yields. The structures of compounds obtained were determined by IR, ¹H NMR and HRMS spectra. Representatively, the spatial structure of compound 3d was determined by using X-ray diffraction analysis. Absorption and fluorescence spectral characteristics of the compounds were investigated in CHCl₃ by UV–vis absorption and emission spectra. The results showed that the absorption maxima of the compounds vary from 332 to 342 nm depending on the group bonded to benzene rings. The maximum emission spectra of compounds in CHCl₃ are dependent on groups in benzene ring in which a strong electron-donating group in benzene ring such as methoxyl group on C3 position of pyrazoline made the emission wavelength of 3e, 3f and 3g red shifted than that of compounds 3b, 3c and 3d with chlorine group. The intensity of absorption and fluorescence was also correlated with substituent on two aryl rings. In addition, the absorption spectra of these compounds change very little with increasing solvent polarity.     

Optical and other physical characteristics of amorphous Ge15Te85−xCux alloys

Composition dependencies of the optical properties of as- deposited amorphous Ge₁₅Te_85−xCu_x (x = 2, 3, 4, 5 and 6 at %) prepared by thermal evaporation have been studied. The optical transmission and reflection spectra are measured in the wavelength range (200–1200 nm). The optical absorption coefficient are studied for as- deposited samples. The optical absorption edge shift to higher energy range, as the copper content, x, increases in the film. Tauc's relation for the allowed direct transition is successfully describing the mechanism of the optical absorption. The refractive index, n has been determined from the transmission spectra measured at normal incidence. The absorption coefficient, α, therefore extinction coefficient, k, have been determined from the transmission spectra at the strong absorption region. The dispersion of refractive index is discussed in terms of the Single-oscillator Wimple–DiDomenico model. The optical dielectric constant is also estimated. The average heat of atomization and related parameters such as the average coordination number were also calculated from the heat of atomization and coordination number of the used elements. These obtained results were discussed in terms of chemically ordered network model and constraints theory.     

The transformation of inhomogeneously broadened spectra due to frequency migration

The motion-induced transformation of static spectra has been calculated for multipole and exchange interactions of randomly distributed centres. The non-monotonic dependence of the spectrum width on the modulation rate has been obtained under the assumption that the motion results in the Markoffian modulation of frequency. The difference in shape of experimental Raman spectra [J. Schoederet al., Mol. Phys.34, 1501 (1977); J. chem. Phys.66, 3215 (1977)], can be attributed to the fact that the spectra are motion-broadened in the former case and motion-narrowed in the latter case.     

The vacuum ultraviolet spectra of methylated silanes

The vacuum ultraviolet spectra of monomethylsilane, monomethylsilane-d₃, dimethylsilane, dimethylsilane-d₂, trimethylsilane, trimethylsilane-d₁, trimethylmonofluorosilane and tetramethylsilane have been recorded in the range 135 nm to 190 nm.     

Observation of hydrated electron, (SCN)2 .- and CO3 .- radical in high temperature and supercritical water

Transient radicals (hydrated electron, (SCN)₂ ^.- and CO₃ ^.-) formed in supercritical water have been observed by the pulse radiolysis technique. The change of spectra of these radicals with temperature has been measured. It was found that the spectra and absorption coefficients of the radicals, e^- _aq and (SCN)₂ ^.-, are strongly dependent on the temperature of the water. Since it was found that the absorption spectrum and molar absorption coefficient of CO₃ ^.- radical seem to be almost independent of temperature, G-values of OH and e^- _aq could be derived. Then, the absolute values of the absorption coefficients for the radicals could be calculated. The G-values of the radical products in water radiolysis tend to increase with increasing temperature up to 400°C. Based on the above observation, radiolysis of supercritical water is discussed.     

Comparison of background gamma-ray spectra between Los Alamos, New Mexico and Austin, Texas

Background counts in gamma-ray spectrometry are caused by a variety of sources. Among these are naturally occurring radioactive materials (NORM) in the environment, interactions from cosmic radiation, and contamination within the laboratory. High-purity germanium detectors were used to acquire long background spectra in Los Alamos, NM (elevation ~7,300 feet) and Austin, TX (elevation ~500 feet). This difference in elevation has a sizeable effect on background spectra due to cosmic interactions, such as (n,n′) and (n,γ). Los Alamos also has a fairly high NORM concentration in the soil relative to Austin, and this gives way to various spectral interferences. When analyzing nuclear forensics samples, these background sources can have non-trivial effects on detection limits of low-level fission products. By accurately determining the influence that elevation and environment have on background spectra, interferences within various laboratory environments can be more accurately characterized.     

The excited states of cycloporane. MCD spectrum,and CD spectrum of an optically active derivative

The MCD spectrum of gas phase cyclopropane and the CD spectrum of trans 1,2-dimethyl cyclopropane were measured in the spectral region 210–140 nm. The absorption spectra are also reported. Comparison of CD and absorption spectra of dimethyl cyclopropane and consideration of the anisotropy factors g = Δε/ε as a guide to the assignment of magnetic dipole allowed transitions led to the ordering of the states of the first excited configuration as A′₁, A′₂, E′ in order of increasing energy. The magnetic moments of the two observed allowed excited states of cyclopropane were determined from the MCD spectrum. LCAO MO level calculations of the MCD parameters $\text{A}$₁/$\text{D}$₀ for the lowest three excited E′ states were carried out and the results were used to discuss the assignment of the allowed transitions. It is concluded that configuration interaction is of considerable importance in the low energy excited states of cyclopropane.     

The magnetic circular dichroism spectrum of matrix-isolated TaO

TaO has been matrix-isolated in an argon matrix at 14 K and 24 K and studied spectroscopically in the visible region (300–850 nm). Both adsorption and magnetic circular dichroism (MCD) spectra have been recorded and analyzed. A determination of the total angular momentum quantum numbers (ω) for fourteen excited electronic states has been made. The g factors for the ground ²Δ_{$\text{3}\text{2}$} and excited ²φ_{$\text{5}\text{2}$} states have been determined from a moment analysis of the MCD and absorption spectra of the 450.3 nm band. The present study indicates the power of the combination of magnetic circular dichroism and matrix isolation for the assignment of excited electronic states of high temperature molecules.     

Influence of conformational structure on the vibrational spectra of the different isomers of 2,4,6,8-tetraphenylnonane,a polystyrene model molecule

The influence of the chain configurations and conformations on the infrared and Raman spectra of 2,4,6,8-tetraphenylnonane has been investigated. The out-of-plane, skeletal, 16b and 10b normal modes of the benzene rings, and the methyl rocking are particularly sensitive to conformational effect. The results permit evaluation of the trans-trans defect ratio in isotactic crystalline polystyrene and suggest that atactic polystyrene is better described by prevailing syndiotactic sequences of at least eight aliphatic bonds in the trans conformation.     

Thermodynamic and conformational investigation of the influence of CdTe QDs size on the toxic interaction with BSA

Water-soluble fluorescent colloidal quantum dots (QDs) have been widely used in some biological and biomedical fields, so the interaction of QDs with biomolecules recently attracts increasing attention. In this study, the fluorescence (FL) quenching method, circular dichroism (CD) technique, attenuated total reflection-Fourier transform infrared (ATR-FTIR) and UV-vis absorption spectra were used to investigate systematically the influence of CdTe QDs size on the toxic interaction with bovine serum albumin (BSA). Three size CdTe QDs with maximum emission of 543 nm (green-emitting QDs, GQDs), 579 nm (yellow-emitting QDs, YQDs) and 647 nm (red-emitting QDs, RQDs) were tested. The Stern-Volmer quenching constant (K_sv) at different temperatures, corresponding thermodynamic parameters (ΔH, ΔG and ΔS), and information of the structural features of BSA were gained. The FL results indicated that QDs can effectively quench the FL of BSA in a size-dependent manner, electrostatic interactions play a major role in the binding reaction, and the nature of quenching is static, resulting in forming QDs-BSA complexes. The CD and ATR-FTIR spectra showed that the secondary structure of BSA was changed by QDs, indicating the toxic on protein.     

The Raman and near ultraviolet spectra of thioanisole

The liquid Raman and vapour phase u.v. absorption spectra of thioanisole have been recorded and vibrational analyses performed. The vibrational structure accompanying the π* ← π electronic transition is similar to that in related monosubstituted benzenes and shows activity in the in-plane ring vibrations 1, 7a, 6a, 6b, 12 and 18b.