Conformation of radicals in a γ- irradiated linear perfluoroalkane at 77 K

The principal geometrical parameters of the carbon skeleton of ~CF₂CFCF₂~ and ~CF₂CF₂ radicals at 77 K show good agreement with spiral configuration of a perfluoroalkane molecule in crystal. The conformations of radicals resulting from fluorine atom abstraction or C-C bond cleavage are determined by minimum of bond rotation potential energy in a linear perfluoroalkane helix. The analysis of the spectrum of ~CF₂CFCF₂~ and ~CF₂CF₂ radicals formed in γ-irradiated linear n-perfluorododecane and polytetrafluoroethylene was performed. The conformational angles and hyperfine coupling constants were determined for α- and Β-fluorine atoms at 77 K.     

Excess enthalpies of 2-propanol + cyclohexane and 2-propanol + benzene + cyclohexane at 298.15 K

With an isothermal dilution calorimeter excess enthalpies have been determined at 298.15 K for 2-propanol + cyclohexane and 2-propanol + benzene + cyclohexane mixtures. The results are fitted with an associated-solution model. Predicted excess enthalpies for the ternary mixture agree well with the experimental results.     

Translational diffusion of polystyrene in cyclohexane at infinite dilution from 283 to 348 K

The translational diffusion coefficients are reported for polystyrenes with molecular weights 2000, 4000, and 9000 in cyclohexane at infinite dilution from 283.2 to 348.2 K. The results suggest that the hydrodynamic radii stay constant to within ±3% and that the real chain for molecular weight 9000 at 308.2 K is well represented by a Gaussian or Monte Carlo model involving the idea of statistical steps. Furthermore, the success of Hildebrand's free-volume empiricism has led to a correlation between Dμ₀/T and M which represents the observed diffusivities to within ±2%.     

Crystal Structure and Conformation of a 10-Thiabilirubin

 A crystal structure determination of a bilirubin analog with a sulfur instead of a C(10)–CH₂ linking the two dipyrrinones is reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those obtained from molecular dynamics calculations as well as to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like other bilirubins, the component dipyrrinones of the analog are present in the bis-lactam form with (Z)-configurated double bonds at C(4) and C(15). Despite the large differences in bond lengths and angles at –S–vs.–CH₂–, the crystal structure shows considerable similarity to bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds – although the interplanar angle of the ridge-tile conformation of the title compound is smaller (∼ 86°) than that of bilirubin (∼ 98°). The collective data indicate that even with long C–S bond lengths and a smaller C–S–C bond angle at the pivot point on the ridge-tile seam, intramolecular hydrogen bonding persists.     

Crystal Structure and Conformation of a 10-Thiabilirubin

Summary.  A crystal structure determination of a bilirubin analog with a sulfur instead of a C(10)–CH₂ linking the two dipyrrinones is reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those obtained from molecular dynamics calculations as well as to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like other bilirubins, the component dipyrrinones of the analog are present in the bis-lactam form with (Z)-configurated double bonds at C(4) and C(15). Despite the large differences in bond lengths and angles at –S–vs.–CH₂–, the crystal structure shows considerable similarity to bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds – although the interplanar angle of the ridge-tile conformation of the title compound is smaller (∼ 86°) than that of bilirubin (∼ 98°). The collective data indicate that even with long C–S bond lengths and a smaller C–S–C bond angle at the pivot point on the ridge-tile seam, intramolecular hydrogen bonding persists. Received August 16, 2001. Accepted September 12, 2001     

Excess thermodynamics properties of propyl propanoate+hexane+cyclohexane at 298.15 K

Summary In this paper we present excess molar volumes and excess molar enthalpies of binary and ternary mixtures containing propyl propanoate, hexane and cyclohexane as components at 298.15 K. Excess molar volumes were calculated from the density of the pure liquids and mixtures. The density was measured using an Anton Paar DMA 60/602 vibrating-tube densimeter. Excess molar enthalpies were obtained using a Calvet microcalorimeter     

Antiparallel β-Sheet Conformation in Cyclopeptides Containing a Pseudo-amino Acid with a biphenyl moiety

The biphenyl-containing pseudo-amino acids 2′-(aminomethyl)biphenyl-2-carboxylic acid (Abc; 1 ) and 2′-(aminomethyl)biphenyl-2-acetic acid (Aba; 2 ) are used as rigid spacers in the backbone of the cyclic peptides cyclo (-Abc-Ala-Phe-Gly-)₂ ( 5 ), cyclo(-Abc-Ala-Val-Gly-)₂ ( 6 ), cyclo(-Aba-Gly-Phe-Ala-)₂ ( 7 ), and cyclo(-Aba-Ala-Phe-Gly-)₂( 8 ). Three different interconverting diastereoisomers are found in solutions of each of these cyclopeptides due to the atropisomerism of the biphenyl units. NMR Techniques and molecular-dynamics calculations allow to conclude that the major diastereoisomer of 5 (and 6 ) in (D₆)DMSO adopts a β-sheet conformation. It is proposed that the pseudo-amino acid 1 of (R)-chirality forms, with attached L -amino acids, a H-bonding pattern comparable to a β-turn (see D in Fig. 4 and F ).     

Excess molar volumes and viscosities of binary mixtures of cyclohexane and n-alkane at 298.15 K

Awwad, A.M. and Salman, M.A., 1986. Excess molar volumes and viscosities of binary mixtures of cyclohexane and n-alkane at 298.15 K. Fluid Phase Equilibria, 25: 195-208.Excess molar volumes, viscosities, excess molar viscosities, and excess molar activation energies of viscous flow were determined for binary mixtures of cyclohexane + n-pentane, + n-hexane, + n-heptane, + n-octane, + n-nonane, + n-decane, + n-dodecane, + n-tetradecane and + n-hexadecane at 298.15 K. The effect of orientational order of n-alkane on solution molar volumes and viscosities is investigated as well as the adequacy of the Flory theory and free volume theories used to predict solution molar volumes and viscosities. For longer n-alkanes V^E, η^E and ΔG*^E are positive and associated with the orientational order.     

Excess molar enthalpies at 298.15 K of (cyclohexane + a methylquinoline) and of (benzene + a methylquinoline)

Excess molar enthalpies of (cyclohexane or benzene + 2-methylquinoline, +4-methylquinoline, + 6-methylquinoline, + quinoline, + 1-methylnaphthalene, and + 2-methylnaphthalene) were measured at 298.15 K (the mixtures with 2-methylnaphthalene were studied at 309.15 and 318.15 K). From the results for (cyclohexane + a methylquinoline), all of which are strongly positive, it is possible to show that CH₃N interactions follow the expected order in accordance with the distance between the groups in the molecule. Measurements on (benzene + a methylquinoline) cannot be satisfactorily correlated with the dipole moment of the methylquinoline. The different activity of the methyl group in accordance with its position in the molecule accounts for the calorimetric behaviour of most of the mixtures.     

Polarized electronic spectra of tetrahedral nickel(II) ions in a Cs2ZnCl4 host lattice at temperatures between 300 and 10 K

The polarized visible spectra of tetrahedral nickel(II) ions in a Cs₂ZnCl₄ lattice at temperatures between 300 and 10 K is reported. Cs₂ZnCl₄ is orthorhombic and the polarization results confirm that the electric dipole process is operative. The degree of distortion from tetrahedral symmetry of the NiCl₄²⁻ moiety along each principal axis of the crystal is a major factor in determining the intensity of the spectrum taken with the incident-beam polarization in that direction. The spectra are unusual in that the double peak characteristic of tetrahedral nickel at room temperature is retained at 10 K along the two crystallographic axes where the distortion from tetrahedral symmetry is most marked. In common with most previous studies of tetrahedral nickel spectra at low temperature, the spectrum taken at 10 K with the incident-beam polarization along the third axis produces a quite different spectrum than at room temperature. The change occurs gradually between 200 and 100 K. A qualitative model to explain these observations based on the concept that a dynamic Jahn-Teller effect is operative at room temperature is proposed.     

Further evidence for phase transitions in biphenyl near 40 K and 16 K

Phase transitions have been detected in biphenyl at about 42 and 17 K by noting changes in the transmitted light intensity when the crystal is placed at extinction between crossed polaroids and the crystal temperature slowly varied. The phase transition at 42 K is gradual (i.e. it occurs over a range of temperature) while the 17 K transition is abrupt. The effect is observed only for light propagating normal to an a*b section which implies that the atomic displacements at the phase changes are largely restricted to this plane. An analogous behaviour is observed for biphenyl-d₁₀ except that the onset of the gradual transition on cooling is at 38 K and the abrupt transition is at 24 K.     

High-resolution spectroscopy of bacteriochlorin in n-alkane host crystals at 4.2 K

Site-selection spectroscopy of free-base bacteriochlorin (7,8,17,18-tetrahydroporphin) in n-hexane, n-octane and n-decane at 4.2 K are reported. The vibrational frequencies of the ground state, and first and second excited singlet states were determined from the fluorescence and excitation spectra. Low-frequency modes were observed, from which the angle of reorientation of the Molecule upon excitation is estimated (< 1.5°).     

Enthalpies of mixing of binary and ternary mixtures containing diisopropylether,benzene and cyclohexane at 303.15 K

Enthalpies of mixing H have been measured for liquid binary mixtures of diisopropylether (DIPE)+benzene or cyclohexane and for liquid ternary mixtures diisopropylether+benzene+cyclohexane at 303.15 K and constant pressure using a C80 calorimeter. A Redlich-Kister type equation was used to correlate experimental results.     

Perturbed sites and host—guest—host exciton cascade in the biphenyl isotopic mixed crystal phosphorescence

An interesting energy cascade is observed in the phosphorescence spectra of 1% biphenyl-h₁₀ in biphenyl-d₁₀ (2–15 K); strongly perturbed host sites, with energy levels below that of the protonated guest, quench the guest sites at higher temperatures (11–15 K). The identification of the perturbed sites is based on vibrational characteristics (both intensity and frequency), obtained with the help of phosphorescence spectra of biphenyl-h₁₀ and biphenyl-d₁₀ in an argon matrix, indicating an isotope dependent vibronic structure. A partial vibrational analysis is presented, resulting in confirmation of the first triplet state of biphenyl as orbitally ungerade. The dynamics of the triplet excitation are discussed, including several possible mechanisms explaining the non-Boltzmann nature of the low-temperature steady state.