Conformational properties of cyclic pentaalkoxy phosphoranes: Apical–equatorial attachment and nonchair conformation of the phosphorus-containing six-membered ring

A series of phosphoranes with pentacovalent phosphorus contained in a 1,3,2-dioxaphosphorinane ring have been studied by ¹H NMR. One compound was investigated by low-temperature ¹³C NMR and another by X-ray crystallography. Although the ¹H NMR parameters observed are time-averaged, the coupling constants can be accounted for if the phosphoranes have the six-membered ring attached apical–equatorial to phosphorus and occupy in solution rapidly isomerizing boat or slightly twisted boat conformations similar to that found in the X-ray study.     

Molecular orientation in channel flows of main-chain thermotropic liquid crystalline polymers

 In situ wide angle X-ray scattering is employed for quantitative measurements of flow-induced molecular orientation in channel flows of thermotropic hydroxypropyl-cellulose (HPC). An extrusion die, constructed to allow X-ray access, generates slit flow, slit flow with superimposed contractions and expansions in cross-section, and slit flow past an obstruction. In slit flow, weak molecular orientation develops slowly with downstream position. Superimposed extension associated with contraction flows leads to a strong enhancement in orientation, which persists with distance further downstream of the contraction. Conversely, transverse extension present in expansion flows generates a bimodal orientation state and substantial reductions in average molecular orientation. These results are compared to earlier measurements on a commercial fully aromatic thermotrope. HPC is found to respond more strongly to superimposed extension, and more weakly to the prevailing inhomogeneous shear flow than the commercial material. Received: 22 October 1999/Accepted: 13 January 2000     

Facile preparation of the individual diastereomers of thymidine 3′,5′-cyclic phosphorothioate (cTMPS)

A facile synthesis of the individual Sp and Rp 3,5-cyclic thymidine phosphorothioates is reported. The key intermediate is a methyl phosphite triester which is readily converted under free-radical conditions to the diastereomeric thiophosphates which are separated on SiO₂ and then cleanly dealkylated by $\text{t}$-BuNH₂, to the ami ne salts of title compounds.     

Spectrum of certain non-self-adjoint operators and solutions of Langevin equations with complex drift

As part of a program to evaluate expectations in complex distributions by longterm averages of solutions to Langevin equations with complex dirft, a simple one-dimensional example is examined in some detail. The validity and rate of convergence of this scheme depends on the spectrum of an associated non-selfadjoint Hamiltonian which is found numerically. In the regime where the stochastic evaluation should be accurate numerical solution of the Langevin equation shows this to be the case.     

O2AIBN,free-radical method for the stereospecific oxidation of nucleoside trialkyl phosphites: new preparation of the individual,diastereomeric, 18O-labeled thymidine 3′, 5′-cyclic monophosphates.

An experimentally easy method for reqio- and stereospecific introduction of isotopic ¹⁸O or ¹⁷O) oxygen into phosphorus di- and triesters of biological interest is described and applied to ¹⁸O-labelinq of cTMP's.     

Two-Dimensional Turbulence with Rigid Circular Walls

We report numerical computations of decaying two-dimensional Navier--Stokes turbulence inside a circular rigid boundary. We summarize previously reported calculations involving no-slip boundary conditions and present results with higher spatial resolution than achieved before (with, however, no qualitative changes in the observed behavior). We then report new results with stress-free boundary conditions (for a viscous fluid, but bounded by a perfectly slippery wall). The method used is spectral, involving expansions of the fields in orthonormal sets of functions which obey two boundary conditions (circular analogues of the Chandrasekhar–Reid functions). The computation takes place entirely in the spectral space. Large-scale Reynolds numbers are typically less than a thousand. Interest focuses on the role played by angular momentum, in determining the decay of the turbulence with no-slip boundary conditions, and the role of possible other ideal invariants in the stress-free case. Received 30 September 1996 and accepted 5 February 1997     

Oneway Sagnac device to measure absolute velocity

The difference in the intensity of light produced by two independent beams passing in opposite directions through a oneway Sagnac device may be used to measure the absolute velocity of the device and, thus, the solar system.     

A study of the spectra and acidity of 4,5-dihydro-1H-benz[g]indazol-7-ol and 2,3a,4,5-tetrahydro-7-hydroxy-3a-methyl-3H-benz[g]indazol-3-one

The synthesis of 2,3a,4,5-tetrahydro-7-hydroxy-3H-benz[g]indazol-3-one is recorded for the first time. Infrared, pmr, and uv spectral analysis of this pyrazolone and of 4,5-dihydro-1H-benz[g]indazol-7-ol strongly support the presence of one tautomer in each case. Measurements of pK_a values are in agreement with the proposed tautomeric structures based upon comparisons with simpler model systems. The data are taken in aqueous solution since the compounds are soluble at 10^?5 M and the results may more closely approximate that for the structure under physiological conditions.     

Li+ cation coordination in [Li2(CF3SO3)2(diglyme)] and [Li3(C2F3O2)3(diglyme)]

The title compounds, poly­[[[bis(2‐methoxy­ethyl) ether]­lithium(I)]‐di‐μ₃‐tri­fluoro­methanesulfonato‐lithium(I)], [Li₂(CF₃SO₃)₂(C₆H₁₄O₃)]_n, and poly­[[[bis(2‐methoxy­ethyl) ether]­lithium(I)]‐di‐μ₃‐tri­fluoro­acetato‐dilithium(I)‐μ₃‐tri­fluoro­acetato], [Li₃(C₂F₃O₂)₃(C₆H₁₄O₃)]_n, consist of one‐dimensional polymer chains. Both structures contain five‐coordinate Li⁺ cations coordinated by a tridentate diglyme [bis(2‐methoxy­ethyl) ether] mol­ecule and two O atoms, each from separate anions. In both structures, the [Li(diglyme)X₂]^? (X is CF₃SO₃ or CF₃CO₂) fragments are further connected by other Li⁺ cations and anions, creating one‐dimensional chains. These connecting Li⁺ cations are coordinated by four separate anions in both compounds. The CF₃SO₃^? and CF₃CO₂^? anions, however, adopt different forms of cation coordination, resulting in differences in the connectivity of the structures and solvate stoichiometries.