The acid-catalysed reaction of thiols with alkyl 2,3-dideoxy-glyc-2-enopyranosides or glycals

The acid-catalysed (HCl or SnCl₄) reaction of alkyl 2,3 dideoxy-glyc-2-enopyranosides or 3,4,6-tri-O-acetyl-D-glycals with thiols leads to mixtures of alkyl 2,3-dideoxy-1-thio-glyc-2-enopyranosides and 3-S-alkyl-3-thioglycals. Under equilibrium conditions the latter are the preponderant products of the reaction. Cross experiments have confirmed the intermediacy of the allylic carbocation in the reaction. A rationalization of the substitution reactions of 1,2- and 2,3-unsaturated sugars based on HSAB principle has been proposed.     

Convergence of the perturbation theory expansion for spin-spin coupling constants

The convergence of the full perturbation theory expansion for the Fermi contact term in the spin-spin coupling constant of hydrogen fluoride has been studied. By examining the contribution of higher virtual states as the basis set is expanded to 18 atomic orbitals, it is shown that at this level of approximation the expansion does not converge. The need to also establish convergence before accepting values calculated from configuration interaction wavefunctions is discussed.The Puerto Rico Nuclear Center is operated by the University of Puerto Rico for the United States Atomic Energy Commission under Contract No. AT(40-1)-1833.     

THERMAL GENERATION OF ELECTRONICALLY EXCITED STATES BY α-PEROXYLACTONES: ACTIVATION PARAMETERS AND EXCITATION YIELDS AS A FUNCTION OF α-DEUTERATION*

Abstract— The kinetics and the direct, the 9,10-dibromo- (DBA) and the 9,10-diphenylanthracene- (DPA) enhanced chemiluminescence of tert-butyl α-peroxylactone 1, with and without α-deuteration, was investigated in order to probe the mechanism of direct chemiexcitation by these hyperenergetic molecules. The small secondary isotope effect on the rates and activation parameters suggests that a diradical mechanism is obtained. The partitioning of the diradical intermediate into excited vs ground state product and the yield of triplet excited product are moderately (ca. 5-fold) increased by α-deuteration, but the singlet yield is unaffected. A convenient and useful chemiluminescence method for the determination of fluorescence quantum yields has been developed.     

Energy decay and control for a system of strings elastically connected in parallel

In the present paper, we study energy decay and exact boundary controllability for a system of n one-dimensional linear wave equations coupled in parallel. The control obtained is a square integrable of the Neuman type for initial data with finite energy. The controllability time is near optimal value. We treat the case of control in the whole boundary and also in part of it.     

Nonlinear dynamical behavior of surface electromyographical signals of biceps muscle under two simulated static work postures

The primary objective of this study was to explore nonlinear characteristics, if any, of the surface myoelectrical (EMG) activity of the biceps muscle during simulated maximum voluntary contraction at a reference arm posture (MVCRP), and at an elevated static arm work posture (EAWP) observed on the manufacturing assembly line. The results showed that positive Lyapunov exponents existed for each of the trials in both work postures. The statistical test of significance showed that surface EMG of the biceps brachii was more chaotic under the MVCRP condition as compared to the EWAP condition. Based on the Kaplan-Yorke dimension, it was observed that the recorded two types of time series EMG data were similar in complexity. The study also found a significant difference in fractal dimensions between the two postures. It was concluded that the nonlinear dynamical EMG properties expressed in terms of the Lyapunov exponents, Kaplan-Yorke dimension, and fractal dimensions could be used as sensitive indices for evaluation of the level of muscular efforts under static work conditions.     

Scattering of small solutions to generalized benjamin-bona-mahony equation in several space dimensions

We show that the supremum norm of solutions with small initial data of the generalized Benjamin-Bona-Mahony equation u_t-△u_t=(b,▽u)+u^p(a,▽u)in x?Rⁿ,n≥2, with integer p≥3 , decays to zero like t^-2/3 if n=2 and like t^-1+6, for any δ0, if n≥3, when t tends to infinity. The proofs of these results are based on an analysis of the linear equation u_t-△=(b,▽u)) and the associated oscillatory integral which may have nonisolated stationary points of the phase function.     

Protection‐Group‐Free Synthesis of Sequence‐Defined Macromolecules via Precision λ‐Orthogonal Photochemistry

An advanced light‐induced avenue to monodisperse sequence‐defined linear macromolecules via a unique photochemical protocol is presented that does not require any protection‐group chemistry. Starting from a symmetrical core unit, precision macromolecules with molecular weights up to 6257.10 g mol^?1 are obtained via a two‐monomer system: a monomer unit carrying a pyrene functionalized visible light responsive tetrazole and a photo‐caged UV responsive diene, enabling an iterative approach for chain growth; and a monomer unit equipped with a carboxylic acid and a fumarate. Both light‐induced chain growth reactions are carried out in a λ‐orthogonal fashion, exciting the respective photosensitive group selectively and thus avoiding protecting chemistry. Characterization of each sequence‐defined chain (size‐exclusion chromatography (SEC), high‐resolution electrospray ionization mass spectrometry (ESI‐MS), and NMR spectroscopy), confirms the precision nature of the macromolecules.     

High peak power Nd:YAG laser pumped by 600-W diode laser stack

The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M²≈2.6) output energy of about 42 mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained for near TEM₀₀ output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM₀₀ mode.