Infrared study on niobium pentahalides and oxytrihalides trapped in cryogenic matrices

The i.r. spectra of Ar and N₂ matrices trapping vapours coming from solid NbX₅, NbOX₃ and mixtures NbX₅/Nb₂O₅ (X = Cl, Br, I) have been investigated. Differences in the spectra of NbCl₅ taken at different vaporization temperatures point to the presence of polymeric species in the vapour. The bands due to monomeric NbCl₅, at 443 cm⁻¹ (the E′ NbCl stretch) and at 395 cm⁻¹ (the A^″₂ NbCl stretch), can be recognized since they are more intense for matrices trapping vapours taken at higher furnace temperature. Annealing experiments also help in assigning monomer absorptions since they decrease in intensity under diffusion. It is remarkable, however, that the polymeric species formed in the matrix on annealing are different from that present in the vapour. The furnace temperature does not affect significantly the spectrum of NbBr₅ and the only two observed absorptions, at 315 cm⁻¹ (the E′ NbBr stretch) and at 288 cm⁻¹ (the A^″₂ NbBr stretch) are referred to the monomer. Here, also, the assignment is supported by the decrease in intensity undertaken by these two absorptions on annealing the matrix.Changes in furnace temperature do not give rise to detectable changes in the spectrum of matrices trapping Nb oxytrihalides, NbOX₃. In this case, the absorptions of monomers have been recognized on the grounds of the tendency to decrease under annealing. The A₁ NbO stretch is observed at 993 (X = Cl), 989 (X = Br) and 977 (X = I) cm⁻¹. The E NbX stretch is at 443 (X = Cl) and 339 (X = Br) cm⁻¹. The other A₁ NbX stretch has never been observed. The spectrum is complicated by band multiplicity probably due to multiple site trapping.     

Infrared spectroscopic studies of hydrogen-bonded complexes in cryogenic sulutions

We have measured the ν_s(OH) band parameters of the IR absorption spectra for a wide variety of hydrogen-bonded (HB) complexes of CH₃OH(D), CF₃CH₂OH, and (CF₃)₃COH(D) with some simplest representatives of various classes of bases in Xe and Kr in the temperature range 120–270 K. The ν_s(OH) absorption bands of the HB complexes in solution in atomic solvents have been demonstrated to be narrower by a factor of 2 to 4 than in molecular solvents at the same temperature. The fact that the ν_s(OH) bandwidths in Xe and in the gas phase at similar temperatures are practically the same indicates that these bandwidths are in both cases governed mainly by the contribution of “hot transitions” from a sequence of excited levels of the ν_β low-frequency bending mode of the hydrogen bond. The other characteristic features revealed for the complexes under study in liquid Xe and Kr at ν_s(OH) frequency shifts up to 500 cm⁻¹ include: (1) slight temperature dependence of the ν_s(OH) bandwidth (0.1–0.3 cm⁻¹/K), (2) almost “normal” isotope frequency ratio ν_s(OH)/ν_s(OD) (1.34–1.36) and (3) low ν_s(OH) temperature shift values (0.1–0.4 cm⁻¹/K).     

Infrared studies of cryogenic transitions in poly(ethylene terephthalate)

Measurements of infrared absorbance as a function of temperature down to 77°K have been made on poly(ethylene terephthalate). Transitions observed by other techniques such as torsion pendulum measurements are reflected in the infrared measurements by an abrupt change in the slope of the absorbance versus temperature. The infrared measurements are made on bands of known structural origin so the structures contributing to each transition can be determined. The β transition is shown by infrared to be a composite of two peaks with the lower-temperature portion arising from the amorphous regions and the higher-temperature portion from the crystalline regions.     

Results of two European interlaboratory studies for the determination of beta-agonists in various matrices.

Two Beta-Agonist Interlaboratory Studies, 5/96 and 11/97, were designed and evaluated according to the International Harmonised Protocol for the Proficiency Testing of (Chemical) Analytical Laboratories (ISO/REMCO N 280) jointly elaborated by ISO, IUPAC and AOAC. In the sense of proficiency testing, interlaboratory studies are part of external quality assurance systems. The Beta-Agonist Interlaboratory Studies 5/96 and 11/97 were intended to offer laboratories in the EU Member States an opportunity to check objectively their routine methods for the detection of beta-agonists and to demonstrate the reliability of their analytical results; 48 and 44 laboratories, respectively, all involved in official residue control, participated in the studies. Irrespective of the large number of satisfactory results achieved at higher concentration levels according to the z-score evaluation, the number of false negative results is still unsatisfactory. The results of both Beta-Agonist Interlaboratory Studies confirmed that comparability of analytical results is not yet ensured within the European residue control system.     

Infrared absorption and electron paramagnetic resonance studies of vinyl radical in noble-gas matrices

Vinyl radicals produced by annealing-induced reaction of mobilized hydrogen atoms with acetylene molecules in solid noble-gas matrices (Ar, Kr, and Xe) were characterized by Fourier transform infrared and electron paramagnetic resonance (EPR) spectroscopies. The hydrogen atoms were generated from acetylene by UV photolysis or fast electron irradiation. Two vibrational modes of the vinyl radical (nu7 and nu5) were assigned in IR absorption studies. The assignment is based on data for various isotopic substitutions (D and 13C) and confirmed by comparison with the EPR measurements and density-functional theory calculations. The data on the nu7 mode is in agreement with previous experimental and theoretical results whereas the nu5 frequency agrees well with the computational data but conflicts with the gas-phase IR emission results.     

Synthesis and Biological Evaluation of 14-Alkoxymorphinans. Part 10. 14-O-Methyl derivatives of 5-methylnaltrexone and 5-methylnaloxone

In several steps, 5, 14-O-dimethylnaltrexone ( 3 ) and 5, 14-O-dimethylnaloxone ( 4 ) were prepared starting from 5, 14-O-dimethyloxycodone ( 5 ). Compound 3 exhibited opioid agonism in vitro (guinea-pig ileum and mouse vas deferens preparations) and antagonism in vivo (AcOH-writhing test in mice), while compound 4 was found to be an agonist in vitro and in vivo.     

Steady state and time-resolved spectroscopic studies of 7-hydroxyquinoline in various polymeric matrices

The photophysical behavior of 7-hydroxyquinoline (7HQ) is studied in four polymer matrices, viz. polymethyl methacrylate (PMMA), cellulose acetate (CA), polyvinyl alcohol (PVA) and Nafion-117, at ambient temperature using both steady state and time-domain measurements. The study reveals both ground as well as excited state tautomerization in the case of PVA. In PMMA and CA, the intermolecular hydrogen bond to the host polymer results in proton transfer. Edge excitation red shift (EERS) and excitation and emission wavelength dependent decays are observed for normal emission in PMMA and CA matrices. These results are attributed to the trapping of a normal molecule in different geometries. In Nafion, the results show the presence of 7HQ cationic species trapped in the polymer.     

Luminescence of 1,4-naphthoquinone and the vitamin K system in Shpolskii matrices at 4 K

This work investigates the high-resolution phosphorescence spectra of 1,4-naphthoquinone and the vitamin K system in Shpolskii solvents at 4 K. The quasi-linear vibronic bands are discussed with regard to spectral assignments and polarization data. The effect of non-totally symmetric vibrations is also discussed.     

Photochemistry of cyclohexene oxide radical cations in freonic matrices at 77 K

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Thermodynamics of the isopropanol/n-hexane and isopropanol/n-heptane systems. Part I. Infrared studies

The association equilibria for different conglomerates are calculated for the isopropanol/n-Hexane, isopropanol/n-Heptane systems from the infrared spectra.

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Zur Thermodynamik des Isopropanol/n-Hexan und Isopropanol/n-Heptan Systems, I. Untersuchungen mittels IR-Spektroskopie

Zusammenfassung Es werden die Assoziationsgleichgewichte für verschiedene Konglomerate im Isopropanol/n-Hexan- bzw.-/n-Heptan-System aus den IR-Spektren abgeleitet.

     

Infrared matrix isolation evidence for the formation of (η5-cyclopentadienyl)tricarbonyl-molybdenum and -tungsten radicals in carbon monoxide matrices at 12 K

Infrared spectroscopic evidence including ¹³CO labelling and energy-factored force-field fitting is presented for the first time to show that the radicals (η⁵-C₅H₅)-M(CO)₃^? (M = MO, W) and HCO are produced on photolysis of (η⁵-C₅H₅)M(CO)₃H complexes in CO matrices at 12 K.     

Infrared studies of fluoroalcohol-base complexes in the gas phase,carbon tetrachloride solutions and argon matrices

Hydrogen bonded complexes of fluoroalcohols with oxygen-containing bases like dimethyl ether and alkanols have been investigated in argon matrices, in CCl₄ solutions and in the gas phase. The increase of the proton donor capacities with succesive substitution of fluorine atoms into the alkyl group of alcohols is indicated by a strong increase of the shift Δν of the OH stretching frequency in the complexes. For alkanols as acceptors, an increase of Δν is observed along the series CH₃OH, CH₃CH₂OH, (CH₃)₂CHOH, and (CH₃)₃CHOH. The frequency shifts Δν increase as one passes from the gas phase to CCl₄ solutions and argon matrices.     

Chromatographic analysis of alpha-tocopherol and related compounds in various matrices.

Tocopherols and tocotrienols (Vitamin E) are part of a group of "minor components" of main interest, present in the unsaponifiable fraction of many samples. Their importance in biological, metabolical and nutritional studies makes determination of tocopherols and related compounds of major interest. Present work critically reviews the different ways to perform sample pre-treatment and analysis of these compounds, related to the matrices, other analytes to be measured, sensitivity, and simplicity. The review includes well referenced tables that provide in-depth summaries of methodology for the chromatographic analysis of alpha-tocopherol and related compounds in foods, pharmaceuticals, plants, animal tissues and other matrices.     

Photochemical transformations of 1,3,5-trioxane radical cations in freonic matrices at 77 K

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.     

Photochemical reactions of radical cations of dimethylformamide in freon matrices at 77 K

Spectral characteristics of the radical cations (RC) of DMF (λ_max = 415 nm, ε_max = (2.6±0.8)·10³ L mol^?2 cm^?2) stabilized in an irradiated glassy freon mixture (CFCl₃ and CF₂BrCF₂Br) at 77 K were determined. Amide type radicals and RC of the matrix were shown to be formed by irradiation (λ=365–436 nm) of the radical cations of DMF in freon matrices using ESR and UV spectroscopy. The quantum yields of photoconversion of the DMF radical cations are independent of the wavelength of exciting light. It was found that the matrix structure affects the processes stabilizing the products of photoconversion of the DMF radical cations.     

Photochemical transformations of cyclic acetal radical cations in Freon matrices at 77 K

The efficiency of photochemical reactions of radical cations of cyclic acetals (1,3-dioxolane, 1,3-dioxane) is measured in different Freon matrices at 77 K and the influence of the latter on the reaction path is discovered. The possible nature of the paramagnetic complexes that form in photochemical reactions of cyclic acetal radical cations in Freon-11 is suggested.     

Conventional fluorescence spectrometry of polynuclear aromatic hydrocarbons in Shpol'skii matrices at 77 K

A simple fluorimeter assembled from commercial components and its use for the study and application of the Shpol'skii effect on polynuclear aromatic hydrocarbons (PAHs) in n-alkane matrices at 77 K are described. The correlation between the dimensions and geometries of PAHs and their corresponding Shpol'skii solvents is considered. Analytical figures of merit have been evaluated, and the power of the Shpol'skii technique with a conventional fluorimeter in the direct qualitative and quantitative determination of 11 PAHs in mixture is demonstrated. Comparisons with conventional room-temperature fluorescence and laser-excited Shpol'skii spectrometry are also commented upon.     

Triplet energy transfer to polymer matrices—I. Transfer at 77 K in a series of copolymers

The life-time of phosphorescence emission from triphenylene and coronene suspended in a range of styrene/methyl methacrylate copolymers has been measured at 77 K. It has been concluded that the polymer matrix quenches the triplet state of the polycyclic aromatics, presumably by an energy transfer mechanism.     

Conformation of biphenyl in a cyclohexane host at 10 K

The site-selected fluorescence, phosphorescence, and luminescence excitation spectra of biphenyl in a cyclohexane Shpol'skii matrix have been measured. There are three principal sites. The relative population of the sites depends on the freezing protocol. The S₁ ↔ S₀ 0&.sbnd;0 band, a forbidden gg transition in D_2h planar biphenyl, is weakly observed for one of the sites. For this site the zero-phonon lines in fluorescence and phosphorescence are accompanied by prominent sidebands but such sidebands are nearly completely absent in the excitation spectrum. The other two sites have negligibly weak 00 bands in fluorescence and minimal sidebands. We attribute the sidebands to translational and/or librational motion of biphenyl in the cyclohexane cage.