Unusually high pyramidal geometry of the bicyclic amide nitrogen in a complex 7-azabicyclo[2.2.1]heptane derivative: Theoretical analysis using a bottom-up strategy

The high pyramidalization of the bicyclic amide nitrogen found in the crystal structure of a dipeptide incorporating (1S,2S,4R)-N-benzoyl-2-phenyl-7-azabicyclo[2.2.1]heptane-1-carboxylic acid has been investigated using quantum mechanical calculations. More specifically, a bottom-up strategy based on the study of model molecules of progressive complexity has been used. First, an appropriate quantum mechanical method has been selected by examining the distortion of the amide bond in three simple model molecules. Next, the amide distortion induced by the norbornane ring has been evaluated by considering three different 7-azabicyclo[2.2.1]heptane amides. After this, the suitability of quantum mechanical calculations to predict the effect of the substituents on the pyramidalization of the bicyclic amide nitrogen has been investigated by comparing experimental and theoretical parameters for a number of compounds. Finally, the factors responsible for amide distortion in the (1S,2S,4R)-N-benzoyl-2-phenyl-7-azabicyclo[2.2.1]heptane-1-carboxylic acid derivative have been elucidated using a hierarchical approach. For this purpose, several derivatives were generated by removing or modifying the substituents attached to the 7-azanorbornane system. Results have been discussed in terms of intramolecular specific interactions.     

Highly enantioselective synthesis of fluorinated gamma-amino alcohols through proline-catalyzed cross-Mannich reaction

A new, simple route for the synthesis of fluorinated beta-alkyl gamma-amino alcohols in optically pure form in only two steps and featuring proline catalysis from inexpensive and readily available starting materials is described. The applied strategy allows for the introduction of diversity into both the beta-fluoroalkyl and alpha-alkyl groups of these compounds. [reaction: see text]     

Bubble oscillation and inertial cavitation in viscoelastic fluids

Non-linear acoustic oscillations of gas bubbles immersed in viscoelastic fluids are theoretically studied. The problem is formulated by considering a constitutive equation of differential type with an interpolated time derivative. With the aid of this rheological model, fluid elasticity, shear thinning viscosity and extensional viscosity effects may be taken into account. Bubble radius evolution in time is analyzed and it is found that the amplitude of the bubble oscillations grows drastically as the Deborah number (the ratio between the relaxation time of the fluid and the characteristic time of the flow) increases, so that, even for moderate values of the external pressure amplitude, the behavior may become chaotic. The quantitative influence of the rheological fluid properties on the pressure thresholds for inertial cavitation is investigated. Pressure thresholds values in terms of the Deborah number for systems of interest in ultrasonic biomedical applications, are provided. It is found that these critical pressure amplitudes are clearly reduced as the Deborah number is increased.     

Memory effects in a non-uniform flow: A study of the behaviour of a tubular film of viscoelastic fluid

Summary Formal use of constitutive equations such as that ofOldroyd in the mathematical model of a flow leads, in general, to a higher order differential equation than is obtained for a purely viscous fluid, and so we expect to need more boundary conditions in order to specify the problem completely. (These extra boundary conditions may be thought of as arising from the need to specify what the fluid remembers of the flow outside the region of interest.) In flows which are uniform spatially, or uniform with time for a material element, the uniformity will provide the extra information and so no extra conditions are needed. Similarly for confined flows, where no new fluid enters the region of interest, no information about flow outside this region is needed.Here the steady flow of a tubular film of a viscoelastic fluid is studied with the particular aim of examining the effect of these extra boundary conditions in a situation where they may be expected to have some significant influence on the flow as a whole. The flow, while being geometrically complex, is essentially an elongational free-surface flow involving the biaxial stretching of a thin axisymmetric tubular film. Features of the constitutive equations studied are the presence of a non-zero relaxation time and the possibility of a variable viscosity. One effect of the non-zero relaxation time is that a tube of constant radius (possible but unstable for aNewtonian fluid) is not dynamically possible. Preliminary computational results suggest that the effect of the extra upstream boundary conditions is not large, and also have failed to show any major difference between the two generalisations of theMaxwell model which have been used.With 1 figure     

Imidazolinone and triazine herbicides in soils in relation to the complexes formed with Cu(II) ions

Imidazolinone and triazine herbicides are used in many countries and may have a great impact on metal biocycles in soil. This article deals with the dynamics of imidazolinone and triazine herbicides in soils related to the formation of complexes with Cu(II) ions, which can be very stable. The stability constants of the complexes formed by five imidazolinone herbicides and ten triazine herbicides with Cu(II) ions are determined by means of fast, easy, and inexpensive measurements performed by ultraviolet–visible spectroscopy, for imidazolinones, and voltammetry (cyclic and differential pulse), for triazines. Because of the occurrence of dissociation reactions, the determinations were performed at three pH values for imidazolinones and at one pH value for triazines. In aqueous solutions of 5 < pH < 10 (corresponding to the majority of soils of agricultural use), the herbicides form very stable complexes with the Cu(II) ions, the complexes being integrated by two ligands (herbicides) and one copper ion. In conclusion, crops treated with such herbicides in conjunction with Cu(II) salts experience a decrease in its persistence and effectiveness. In addition, the herbicides and the copper ions may pass to the phreatic layer of the soil, increasing the chance of pollution.     

Solubility and preferential solvation of benzocaine in {methanol (1) + water (2)} mixtures at 298.15 K

The equilibrium solubility of benzocaine (BZC) in several {methanol (1) + water (2)} mixtures at 298.15 K was determined. Solubility values are expressed in mole fraction and molarity and were calculated with the Jouyban–Acree model. Preferential solvation parameters of BZC by methanol (δx_1,3) were derived from their thermodynamic solution properties using the inverse Kirkwood–Buff integrals method. δx_1,3 values are negative in water-rich mixtures (0.00 < x₁ < 0.32) but positive in the other mixtures (0.32 < x₁ < 1.00). To explain the preferential solvation by water in the former case, it is conjecturable that the hydrophobic hydration around non-polar groups of BZC plays a relevant role in the solvation. Moreover, the higher solvation by methanol in mixtures of similar cosolvent compositions and methanol-rich mixtures could be explained in terms of the higher basic behaviour of methanol regarding water.     

Analytical assessment of the oscillating chemical reactions by use chemiluminescence detection

This paper introduces the chemiluminescence (CL) detection in oscillating reaction-based determinations using the analyte pulse perturbation technique, a straightforward and expeditious approach to deriving quantitative analytical information from oscillating chemical reactions. The behavior of the H(2)O(2)-KSCN-CuSO(4)-NaOH oscillating system in the presence of luminol was examined by using the proposed detection method and the classical potentiometric technique. Some analytical and practical aspects of both detection systems are discussed. A new analytical method for the determination of vitamin B(6) based on the sequential perturbation produced by different amounts of this substance on the oscillating chemical system was also developed in order to assess the potential of CL detection for routine analyses. The calibration curve thus obtained was linear over the range 0.5-2.0 mumol of vitamin B(6), and the precision and throughput were also quite good (3.04% as RSD and nine samples h(-1), respectively). The proposed method was validated by determining the vitamin in pharmaceutical preparations.     

On feasible sets defined through Chebyshev approximation

LetZ be a compact set of the real space with at leastn + 2 points;f,h1,h2:Z continuous functions,h1,h2 strictly positive andP(x,z),x(x ₀,...,x _n ) ⁿ⁺¹,z , a polynomial of degree at mostn. Consider a feasible setM {x ⁿ⁺¹z Z, –h ₂(z) P(x, z)–f(z)h ₁(z)}. Here it is proved the null vector 0 of ⁿ⁺¹ belongs to the compact convex hull of the gradients ± (1,z,...,z ⁿ ), wherez Z are the index points in which the constraint functions are active for a givenx* M, if and only ifM is a singleton.This work was partially supported by CONACYT-MEXICO.     

Crystal structure of the Λ-[4M] enantiomer of cis-diaqua-bis(1,3-dimethylpteridine-2,4(1H,3H)-dione-N5,O4) copper(II) perchlorate monohydrate

Single-crystal X-ray diffraction studies have been performed on the complex cis-diaqua-bis(1,3-dimethylpteridine-2,4(1H,3H)-dione-O⁴,N⁵) copper(II) perchlorate monohydrate, [Cu(DLM)₂(H₂O)₂](ClO₄)₂ · H₂O. This compound crystallizes in the orthorhombic system, P2₁2₁2₁ space group, Z = 4 with a = 8.749(1) Å, b = 14.213(1) Å, c = 21.200(2) Å, and V = 2636.2(4) ų. The metal ion is surrounded in octahedral geometry by two mutually quasi-perpendicular N5,O4-coordinated lumazine chelators with the O4 atoms trans and the N5 atoms cis. The polyhedron is completed by two cis-coordinated water molecules. The absolute configuration of the complex can be described, following Bailar's nomenclature, as the enantiomer of the stereoisomer 4M. A three-dimensional hydrogen-bond network is defined by water molecules, perchlorate anions, and the O2 of the pteridine A.