New fir laser lines and frequency measurements from optically pumped13CD3OH3

We report the discovery of 57 new fir laser lines from¹³CD₃OH molecule optically pumped by a waveguide CO₂ laser of 300 MHz tunability. For all lines, precise frequency offset measurements between the CO₂ line center and the center of the absorbing¹³CD₃OH line were performed using the transferred Lamb-Dip technique. We have also measured directly the frequency of seven FIR laser lines by heterodyning with already known laser lines. We present a complete list of all known laser lines (134) and frequency measurements (24) for this molecule.Work supported by CNPq, FAPESP, FAEP-Brasil, and CNR-Italia     

Effect of Finite Spectral Width on the Modulational Instability of Langmuir Waves described by a Nonlinear Schrödinger Equation

By using the two-point space correlation function an equation for the power spectral density for a random Langmuir field has been derived. The dispersion relation for a monochromatic wave is regained for a delta spectrum. For a Gaussian spectrum, the maximum growth rate is less than that for a monochromatic wave. For a “meander spectrum”, the growth rate is increased with the width of the spectrum in the first stage then decreased for further increase of the width.     

The combined effects of evaporation and quench steps on asymmetric membrane formation by phase inversion

An analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition-dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation-period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor-phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 seconds.     

Systematic Errors Associated with the CPMG Pulse Sequence and Their Effect on Motional Analysis of Biomolecules

A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT₂values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τ_einternal correlation times being affected somewhat more thanS²order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions.     

Deciding Frattini is NP-complete

We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.This work was supported in part by Swiss NSF grant 20-32644.91.     

Inertia-preserving secant updates

Corrections to the last two equations of Ref. 1 are given.     

Intersegmental interaction and critical concentrations in PET–HFIP solutions

Spectroscopic determinations of the concentrations associated with the onset of intermolecular interaction in solutions of poly(ethylene terephthalate) (PET) are described. The intersegment forces perturb the electronic spectra and induce ground-state aggregation. These phenomena become observable at different chain densities, reflecting the influence of concentration on the coil dimensions, as well as the dynamical nature of the interactions. An estimate of the equilibrium constant for PET dimer formation in hexafluoroisopropanol is reported.     

Step-strain deformations for viscoelastic fluids: Formulation of a strain-coupling constitutive equation

An integral constitutive equation which includes strain-coupling effects is proposed. The constitutive equation is more general than the K-BKZ equation, and it is shown that this equation can explain various important aspects of double-step stress-relaxation data.     

A composite methodology for multistage degradation of polymers

Five different analytical schemes for examining isothermal and nonisothermal degradation of polymers were reviewed and found inadequate for describing multistage decomposition. The different schemes were experimentally tested using thermogravimetric analysis data for an ethylene-vinyl acetate (EVA) copolymer, which exhibited a well-behaved two-step decomposition process in a nitrogen environment. Based on these experimental and analytical findings, a generalized methodology was developed capable of describing the exhibited complex decomposition behavior of polymers by combining two or more kinetic mechanisms in a series or parallel arrangement. This composite of combinational methodology thus extends established analytical schemes for describing complex decomposition of polymers in a rational manner, incorporating both experimental and theoretical considerations.     

New synthesis of 2',3'-didehydro-2',3'-dideoxynucleosides.

The 3'-iodonucleoside 4 and the 3'-O-methylsulfonylthymidine 9 have been synthesized by condensation of silylated uracils 2 with methyl 5-O-tert-butyldiphensilyl-2,3-dideoxy-3-iodo-D-threo-pentofuran oside (3) and methyl 5-O-tert-butyldiphenylsilyl-2-deoxy-3-methylsulfonyl-D-erythro- pentofuranoside (8), respectively. The nucleoside 4 and 9 produced the corresponding 2',3'-didehydro-2',3'-dideoxynucleosides 5 in an elimination reaction on treatment with sodium methoxide. The compounds 5b showed no antiviral activity against HIV-1.