Hyper-Rayleigh scattering spectrum of liquid nitromethane

The vertical vertical (VV), horizontal vertical (HV), and vertical horizontal (VH) hyper-Rayleigh scattering (HRS) spectra were measured for liquid CH3NO2 at T= 300 K. The main HRS spectral component has a width upsilon1=1.28 +/- 0.04 cm(-1), which gives an orientation relaxation time pi=4.1 +/- 0.1 ps in good agreement with other experiments. However, the VH spectrum also contains a previously unobserved strong narrow peak at zero-frequency shift, absent from the VV and HV spectra, which is due to a slowly relaxing longitudinal orientation mode. The upper bound on the width of this peak is 5 MHz, which corresponds to a relaxation time pi > 30 ns.     

Eutectic phase polymerization of activated ribonucleotide mixtures yields quasi-equimolar incorporation of purine and pyrimidine nucleobases

The RNA world hypothesis requires a plausible mechanism by which RNA itself (or precursor RNA-like polymers) can be synthesized nonenzymatically from the corresponding building blocks. Simulation experiments have exploited chemically reactive mononucleotides as monomers. Solutions of such monomers in the prebiotic environment were likely to be very dilute, but in experimental simulations of polymerization reactions dilute solutions of activated mononucleotides in the millimolar range hydrolyze extensively, and only trace amounts of dimers and trimers are formed. We report here that random medium-size RNA analogues with mixed sequences (5- to 17-mers with traces of longer products) can be synthesized in ice eutectic phases that are produced when dilute solutions of activated monomers and catalysts (Mg(II) and Pb(II)) are frozen and maintained at -18 degrees C for periods up to 38 days. Under these conditions, the monomers are concentrated as eutectics in an ice matrix. Hydrolysis of the activated mononucleotides was suppressed at low-temperature ranges, and polymerization was enhanced with yields up to 90%. Analysis of the mixed oligomers established that incorporation of both purine and pyrimidine bases proceeded at comparable rates and yields. These results suggest that ice deposits on the early Earth could have facilitated the synthesis of short- and medium-size random sequence RNA analogues and thereby provided a microenvironment suitable for the formation of biopolymers or their precursors.     

The different steric environments of the 5-exo- and 5-endo- substituent for a pair of isomeric tricarbonyl(methoxycyclohexa-1,-diene)irons

A shift reagent has been employed to demonstrate the sterically-hindered environment of the methoxy group in tricarbonyl(5-endo-methoxycyclohexa-1,-diene)iron relative to that of the methoxy group in the 5-exo analogue.     

Aldehydes from alkyllithiums with 2-carbon homologation

1-Phenylthio-trimethylsilylalkanes, uhich are readily converted to aldehydes, are prepared by the addition of alkyllithiums to phenylthioethene, trimethylsilylethene and 1-phenylthio-1-trimethylallylethene.     

Molecular orbital calculations of iron(II) complexes of diimine ligands

Summary Molecular orbital calculations using-, the INDO method have been carried out fm the [FeL(CN)₄]² and [FeLH(CN)₄] complexes, as well as for the tree ligands, with L 2 × pyridine, 1,10-phenanthroline.2,2-bipyridyl. 2,2-rimidine, 2, 3,3-bipyridazine and 4.4-bipyrimidine. Calculations of residual charge at carbon atoms in the ligand rings. correlating with relative nucleophilicity of the compounds, corresponds with observed differences of rate of reaction of the complex [FeI;]² With nucleophiles.     

Rare earth niobate systems and the phosphor GdNbO4:Bi

An investigation to determine the compounds present in the Ln₂O₃Nb₂O₅ systems (Ln = La, Gd, and Y) and attempts to activate such compounds with Ti, In, Sb, Bi, Eu, and Tb revealed that only the orthoniobate (LnNbO₄) compound was a suitable host lattice and LnNbO₄:Bi under 2537 Å excitation produced the best phosphors. GdNbO₄:Bi is a brighter phosphor than the La and Y analogs, emitting at slightly higher energies. The position of the GdNbO₄:Bi peak emission at 4500 Å is independent of the activator concentration whereas peak emissions for LaNbO₄:Bi and YNbO₄:Bi move to lower energies with increasing Bi concentration.     

A study of the reaction of bicyclo[5.1.0]octa-2,5-diene with sulphur dioxide

Thermally-induced reaction between bicyclo[5.1.0]octa-2,5-diene (I) and sulphur dioxide under dry conditions is toluene-d₈ as solvent leads to the unexpected formation of the hitherto unknown diene sulphone 7-thiabicyclo[4.2.1]nona-2,4-diene 7,7-dioxide (II).     

Atomistic molecular dynamics simulations of chemical force microscopy

Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.     

A heterocyclic peptide nanotube

An open-ended hollow tubular structure is designed based on hydrogen-bond-directed self-assembly of a chimeric cyclic peptide subunit comprised of alternating alpha- and epsilon-amino acids. The design features a novel 1,4-disubstituted-1,2,3-triazole epsilon-amino acid and its utility as a peptide backbone substitute. The N-Fmoc-protected epsilon-amino acid was synthesized in high yield and optical purity in three steps from readily available starting materials and was employed in solid-phase peptide synthesis to afford the desired cyclic peptide structure. The cyclic peptide self-assembly has been studied in solution by (1)H NMR and mass spectrometry and the resulting tubular ensemble characterized in the solid state by X-ray crystallography.     

Rapid activation analysis of trace vanadium in tissue using 3.8-minute vanadium-52

Submicrogram amounts of vanadium in rat liver tissue have been analyzed by rapid activation analysis. A 5-min radiochemical separation coupled with γ-ray spectrometry permitted utilization of the 3.8-min vanadium-52 radioisotope. With this procedure the lower limit of detection at a thermal neutron flux of 10¹² n/cm²/sec was about 3·10^-9 g of vanadium.