Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

Raman study NH4Cl near the tricritical point

The Raman spectrum of NH₄Cl has been studied as a function of temperature and pressure close to the multicritical point at which the λ-transition changes from first to second order. Variations in the intensities of appropriate bands have been used to calculate the critical exponents for the order parameter. By varying the temperature at fixed pressures of zero, 3.0 k bar and 1.5 k bar, exponent values of β = 0.154 ± 0.012, β = 0.33 ± 0.06 and β = 0.26 ± 0.06 have been obtained for the first-order, second-order and multicritical regions respectively.Our first-order exponent value is close to that observed by previous workers, our second-order value is close to that predicted by the three dimensional Ising model, and our multicritical value is close to that predicted by mean field theory.     

Raman study of ammonium halides treated as an ising pseudospin-phonon coupled system

Raman spectroscopy has been used to study the λ-transition regions of NH₄Cl and NH₄Br. An Ising pseudospin-phonon coupled model has been fitted to the observed Raman data. In particular we report here the results of fitting the model to the frequencies and damping constants of the non-soft TO component of the lattice modes of NH₄Cl and NH₄Br. Both crystals have the orientations of their NH⁺₄ ions disordered at temperatures above their λ transitions, and have (orientation-ignored) O_h structures (β) containing one formula unit per unit cell. At temperature just below the λ transitions, NH₄Cl shows a ferro-ordered T_d structure (gS) with one formula unit per unit cell whereas NH₄Br shows an antiferro-ordered D_4h structure (γ) with two formula units per unit cell. The disorder at high temperature allows all modes to appear in the Raman spectrum, but the spectra are dominated by Γ and M point phonons. The bands studied here are due to the TO component of the F_lu β-phase lattice mode. For NH₄Cl the Γ point phonons are studied, whereas for NH₄Br M point phonons of the β-phase which become Γ point phonons of the γ phase are used. Close fitting of the model to the experimental results is obtained for the ordered (γ and Σ) phases and meaningful critical exponents are obtained.     

Correlations of the thermodynamic quantities with the frequency shifts of a lattice mode close to the I–II transition in NH4Br

We correlate here the specific heat C_p with the frequency shifts (1/V) (?V/?T)_p and the thermal expansivity α_p with the (1/ν) (?V/?P)_T close to the I–II transition in NH₄Br. This correlation is performed for the Raman mode of ν_s (140 cm^?1) using the molar volume data for NH₄Br. It is shown here that spectroscopic modifications of the Pippard relations are applied satisfactorily to the I–II phase transition by using a lattice mode of NH₄Br.     

Experimente und Modellrechnungen zum chemischen Transport von V2O5 mit NH4Cl

Experiments and Calculations for the Chemical Transport of V₂O₅ with NH₄Cl For the chemical transport of V₂O₅ in a temperature gradient (T₂ – T₁ = ΔT = 100 K) the influence of temperature (T₂ = 770 K to 970 K) on the transport rate n′(V₂O₅) using an admixture of 2 to 8 mg NH₄Cl (2,3 to 9,2 × 10^?3 mmol/cm³) has been investigated. Also the dependence of n′ on the admixture of the transport agent has been examined from 2 to 52 mg NH₄Cl (T₂ = 850 K, T₁ = 750 K). We observed that n′ increases with increasing temperature and increasing admixture of NH₄Cl. The model calculations show the opposite tendency of the dependence on temperature; for all experiments the value of n′ was lower by a factor of 10 to 320 than the calculated one. These deviations indicate, that our knowledge on the gas phase of this system is incomplete.     

Conformational analysis of asymmetric ethanes : Extraction of acyclic conformational ligand constants from time-averaged geminal fluorine anisochronism

The syntheses and ambient-temperature ¹⁹F NMR data are reported for 27 asymmetric ethanes of the general formula RCF₂CXYZ, including a complete series of 10 compounds with R = Cl and all combinations of the 5 ligands H, F, Cl, Br and Ph. Within the theoretical framework of a previously proposed heuristic mathematical model the geminal ¹⁹F chemical shift differences are fitted to chirality functions X = ?R (λx - λy) (λy - λz) (λz - λx) to yield acyclic conformational ligand constants λ and substituent parameters ?. It is demonstrated that the λ constants already reported for an analogous series of 10 compounds with R = Br are transferable to the Cl series with ?_Cl = 0.63 ± 0.07. Crude first approximations are also reported for the normalised (according to ?_Br = 1) ligand constants λ_Ch3, λ_OH, λ_OCH3 and λ₁ and for the substituent parameter λ_H. It is argued that the λ values thus ext play a role in the conformational analysis of asymmetric ethanes that is conceptually analogous to the conformational free energies in monosubstituted cyclohexanes.     

1.5th- and 2.5th-order derivative adsorption voltammetry of the cobalt(II)—2-(5′-bromo-2′-pyridylazo)-diethylaminophenol system

On the basis of the adsorptive characteristics of the cobalt(II) complex with 2-(5′-bromo-2′-pyridylazo)diethylaminophenol at a hanging mercury drop electrode in NH₃NH₄Cl, a more sensitive method for the determination of cobalt based on 1.5th- and 2.5th-order derivative adsorption voltammetry has been developed. The detection limit is one order of magnitude lower than that of normal linear-sweep adsorption voltammetry. The equations of the 1.5th- and 2.5th-order derivative of current with respect to time, peak potential and distance from peak to peak are discussed.     

Resonance in the rotational dependence of the isotope-selective reaction between IiCl(A 3Π1) and acetylene

The rotational dependence of the isotope selective reaction between IⁱCl(A ³Π_l, υ′, J′) and acetylene (C₂H₂, C₂D₂) forming the photoaddition product 1,2-iodo-chloro-ethyene was studied for ν′ = 19 and ν′ = 20. The enrichment factor in the photoproduct shows resonances for J′ = 32, ν′ = 19 and J′ = 6, υ′ = 20.     

Étude des systémes ternaires polymère-mélanges de solvants: relation entre solvatation préférentielle et viscosité

The preferential solvation of ternary systems of polymer with mixed solvents is characterized by the λ′ parameter which depends on the thermodynamic properties of the system and also on some molecular parameters of the polymer (molecular weight, index of polydispersity, index of branching etc.). The dependence of λ′ on molecular parameters can be illustrated by a unique relation between the λ′ parameter and the intrinsic viscosity [η]:λ′[η] = λ′_∝ [η] + a′ +. This representation is verified for polystyrene and polymethyl methacrylate in several mixed solvents.     

Einfluss intramolekularer wasserstoffbrücken auf die carbonylbandenlage von substituierten thiophen-carbonsäureestern

In this series of molecules it is possible to describe the carbonyl wave number ν′_CO by the bond order p_CO, calculated with HMO, also in case of great shifts, when considering the intramolecular H-bonding within the parameters. This choice of parameters is justified by PPP-calculations. The different effects of the intramolecular H-bonding on the π- as well as on the σ-system for which the wave numbers ν′_CO or ν′_NH respectively are a kind of probe will be discussed on the basis of PCILO-calculations.     

Quadrupole interactions in mixed-valency states of indium chloride

Quadrupole interaction parameters have been determined for InCl_1.5, InCl_1.8 and InCl₂ by using ¹¹¹In as a probe for perturbed λ-λ angular correlation measurements. For InCl_1.5 a broadened spectrum is obtained with estimated quadrupole interaction frequency ν_q ≈ 110 MHz. Three interactions, comprising one static and two time-dependent components, are required to fit the data for InCl_1.8 and InCl₂. In each case the interactions are identical, but with different population coefficients. The static interaction has a narrow width (δ = 4%) and frequency ν_q = 135.0 ± 1.1 MHz.     

Vibrational spectra in the ν(SiH) and ν(SiD) regions of chloro and bromodisilanes and ab initio geometry studies of C2H5Cl,Si2H5Cl and 1,1-Si2H4Cl2

Infrared measurements in the gas phase are reported for the ν(SiH) and ν(SiD) regions of Si₂H₅X, Si₂D₅X, 1,1-Si₂H₄X₂ and 1,1-Si₂D₄X₂ species where X = Cl, Br. Incomplete Raman data have also been obtained. All three possible isolated SiH stretching frequencies are observed in the spectra of the Si₂D₄X₂ samples, but only two from the Si₂D₅X ones. The missing ν^is(SiH) values are obtained by use of the frequency sum rule, and by harmonic local mode force field treatments of all the available ν(SiH) and ν(SiD) data, using a procedure previously tested on disilane.Ab initio calculations of the geometries of C₂H₅Cl, Si₂H₅Cl and 1,1-Si₂H₄Cl₂ using the 6-31G* basis set are reported. Trends in r_e(CH) or r_e(SiH) values reflect trends in ν^is(CH) or ν^is(SiH) ones. The alpha, trans and gauche effects of halogen are similar in CH and SiH compounds, although smaller in the latter. In both cases, ab initio calculations predict larger effects than are observed in the spectra, especially for the α effect of halogen.A kinetic isotope effect in the halogenation of disilane may occur. Reassignment of earlier spectra of disilyl iodide species is proposed.     

A molecular beam study of a van der Waals complex; vibration-inversion transitions in NH3-Ar

Three ν₂ vibration-inversion transitions of NH₃-Ar have been found with linewidths smaller than 2 MHz, yielding a minimum predissociation lifetime of 80 ns. While one of these lines was previously identified as a low frequency inversion doublet transition, the other two lines belong to the high frequency component of the ν₂ vibration-inversion doublet.     

Errata

The π → π* absorption spectrum of thioformaldehyde has been recorded at relatively high pressures and path lengths. The system is quite extensive and in H₂CS displays a progression of bands in an interval of 476cm^?1 which can be followed out to ν′ = 12. This is assigned to the ν′₃ CS stretching mode. A second weaker set of bands is assigned to 2ν′₄ the out-of-plane bending mode. The 725 cm^?1 interval observed here compared to the 711 cm^?1 value of the ã state leads us to the conclusion that the barrier to inversion is less than 50 cm^?1.     

Selective site deuteration on the sugar ring as an efficient marker of conformation in nucleosides: The CD stretching mode of the (2′-R)-[2′-2H]-2′-deoxyuridine and its 3′,5′-O-(1,1,3,3-tetraisopropyldisiloxan-1,3-diyl)-derivative

A uridine specifically deuterated on the deoxyribose ring at the C2′, and the same compound in a more rigid form, due to a chemically fused ring (between C3′ and C5′), have been synthesized. By NMR, the coupling constants J_1′-2′ and J_3′-4′ have been determined and the populations of the C2′-endo and C3′-endo conformers have been deduced for the two compounds. Comparison of these results with an FTIR and Raman study of the stretching mode νCD on the C2′ site allows a specific assignment of each observed band to one of the two classes of conformers. This enables us to consider the further step: the direct recognition, by IR and Raman spectroscopies, of the local conformation at a specific site of a synthetic oligonucleotide, using the νCD signals as marker bands.     

Vibrational relaxation of phenol in benzonitrile and benzene solutions

The Raman bandwidths and the frequency shifts of the ν₁₂(A₁) mode and the IR ν₃(OH) stretching mode of phenol and phenol-OD have been measured as a function of concentration in benzonitrile and benzene solutions. Opposite isotope effects of deuterium substitution in the hydroxyl group of phenol on the bandwidths of the ν₁₂ and the ν₃(OH) modes have been found. The experimental bandwidths are discussed in terms of available theoretical models for dephasing and other mechanisms of broadening. The isolated binary collision dephasing model of Fischer-Laubereau, the Knapp-Fischer concentration-fluctuation model and the Robertson-Yarwood model have been tested. It has been stated that the purely repulsive potential is responsible for vibrational dephasing of the ν₁₂ mode of phenol in benzene while the concentration-fluctuation model reproduces the experimental data for that mode in benzonitrile. The coupling between the ν₃(OH) and ν₂(OH…N) modes is the dominant mechanism for broadening of the ν₃(OH) mode of phenol in benzonitrile.     

Double complex salts [M(NH3)5Cl][M′Br4] (M = Rh, Ir, Co, Cr, Ru; M′ = Pt, Pd): Synthesis, x-ray diffraction characterization, and thermal properties

Double complex salts (DCSs) with [M(NH₃)₅Cl]²⁺ (M = Rh, Ir, Co, Cr, Ru) cations and [PtBr₄]^2? anions were prepared in high yields. The salts were two-phase mixtures of the anhydrous and monohydro DCSs. Anhydrous analogues containing [PdBr₄]^2? anions with M = Cr or Ru were synthesized. All the compounds were characterized using a set of physicochemical methods. The crystal structure of chloropentaamminechromium(III) tetrabromopalladate(II) was solved: space group Pnma, Z = 4, a = 17.068(2) Å, b = 8.315(12) Å, c = 9.653(14) Å. The [M(NH₃)₅Cl][M′X₄] (M = Rh, Ir, Co, Cr, Ru; M′ = Pd, Pt; X = Cl, Br) compounds were shown to be isostructural. The [M(NH₃)₅Cl][PtBr₄] · H₂O monohydrates are isostructural to the [M(NH₃)₅Cl][PdCl₄] · H₂O monohydrates (space group P2₁/c, z = 4). The properties of the compounds were comparatively analyzed. The tendencies of the thermal stability of the complexes were elucidated. The thermolysis products of the double complex salts obtained under a helium or hydrogen atmosphere were studied.     

A re-examination of the infrared and ultraviolet spectroscopy of trifluoroacetyl fluoride and trifluoroacetyl chloride: An experimental and theoretical study

The fundamental IR vibrational modes of trifluoroacetyl fluoride CF₃C(O)F and trifluoroacetyl chloride CF₃C(O)Cl have been re-examined by ab initio molecular orbital calculations and compared with literature assignments. Several bands of the IR spectrum are reassigned. The Q-branch and integrated absorption cross-sections have been measured for ν₁, ν₃, ν₄ and ν₁₁ fundamental bands for both pressurized and unpressurized samples on each molecule. The UV absorption spectra of CF₃C(O)F and CF₃C(O)Cl show a structureless continuum with a maximum at 21Onm (σ_max=3.20±0.02 × 10⁻²⁰ cm² molecule⁻¹) and 255 nm (σ_max=7.66±0.26 × 10⁻²⁰ cm² molecule⁻¹), respectively. The nature of the electronic transition giving rise to the UV absorption spectrum for CF₃C(O)F and CF₃C(O)Cl has been examined by ab initio molecular orbital calculations. It is attributed to the A¹A″←X¹A′ electronic transition.     

Synthesis,physicochemical characterization and biological activity of nano complexes of iron(III) of 3(2'‐hydroxyphenyl)‐5‐(4′‐substituted phenyl) pyrazolines and aspartic acid

Mixed ligand complexes of Iron(III) with aspartic acid and 3(2′‐hydroxy phenyl)‐5‐(4′‐substituted phenyl) pyrazolines of type [Fe(C₄O₄NH₆)₂(C₁₅H₁₂N₂OX)] and [Fe(C₄O₄NH₆)(C₁₅H₁₂N₂OX)₂], where (C₄O₄NH₆) = aspartate, (C₁₅H₁₂N₂OX) = deprotonated 3(2′‐hydroxyphenyl)‐5‐(4′‐substituted phenyl) pyrazolines (X = H, CH₃, OCH₃, Cl), have been synthesized. These newly synthesized derivatives have been physicochemically characterized by elemental analysis (C, H, N, Cl and Fe), magnetic moment data, thermogravimetric analysis, molar conductance, cyclic voltammetry, spectral analysis (UV–visible, IR, far IR and fast atom bombardment mass spectrometry). Scanning electron microscopy, transmission electron microscopy and X‐ray powder diffraction studies have been carried out for powdered samples, which show nanometric particles of these derivatives. Antibacterial and antifungal potential of free pyrazoline and some iron(III) complexes have been evaluated. Copyright © 2012 John Wiley & Sons, Ltd.