Synthesis and Diels-Alder Reactions of a New Kind of Chiral Dienophiles: Cyclic Vinyl-p-tolylsulfilimines

A new kind of chiral dienophiles, cyclic vinyl-p-tolylsulfilimines (2a and 2b), were obtained from the corresponding (Z)-sulfinylacrylonitriles with HBF(4) and methanol. The asymmetric Diels-Alder reaction of optically pure 2a with cyclopentadiene under mild thermal or catalyzed conditions afforded only the endo-4a adduct with complete endo and pi-facial selectivities. The ability of the sulfilimine moiety to enhance the dienophilic reactivity of the double bond is similar to that of the sulfinyl group.     

Steady state hot atom kinetic theory model calculations. Time dependent rate coefficients for the nonthermal 18F + H2 reaction

This paper briefly summarizes the methodology and results for a new type of hot atom kinetic theory model calculation. The microscopic time dependence for the nuclear recoil ¹⁸F + H₂ reaction is characterized using the steady state kinetic theory of hot atom reactions. The relaxing hot atom laboratory momentum distribution is represented using a local equilibrium model. The calculated results illustrate the nature and significance of quasi steady state reaction rate coefficients and of the dynamical coupling between reactive heating and nonreactive cooling phenomena. Under nuclear recoil conditions the nonthermal ¹⁸F + H₂ reaction, which exhibits a total hot yield of 100%, fails to establish a high temperature steady state.     

Proton stopping powers: Binary encounter calculations based on accurate speed distributions for target electrons

Atomic and molecular electronic stopping powers for medium energy protons (≈ 10 keV-10 MeV) have been calculated using the binary-encounter approximation in conjunction with (1) either an energy or maximum impact parameter cut-off based on minimum excitation energies; and (2) ab initio electronic speed distributions. The maximum impact parameter approach yields good agreement with experiment for inert gases and closed-shell polyatomic molecules comprised of first-row atoms.     

An improved synthesis of 2-oxa-7-azaspiro[3,5]nonane and analogs as novel reagents in medicinal chemistry

A detailed synthesis of novel spirocyclic oxetane analogs is described for the first time.     

F-to-HF reactions. IX. Nonthermal complications for the CH4/C3F6/C2F6 moderated nuclear recoil system

Results are reported from moderated nuclear recoil ¹⁸F experiments with the CH₄/C₃F₆/C₂F₆ mixture system. At a 99.5% confidence level measurement precision of ±3.4%, non-thermal F-to-HF reactions are phenomenologically suppressed at C₂F₆ moderator concentrations in the range of 95.0–99.95 mol-%. Effectively equilibrium reaction conditions can be established in well-designed experiments of this type.     

Ultrafast changes of molecular crystal structure induced by dipole solvation

Femtosecond photoexcitation of organic chromophores in a molecular crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer as is evident from transient vibrational spectra. The structural response of the crystal to the dipole change is mapped directly for the first time by ultrafast x-ray diffraction or diffuse scattering. Changes of diffracted and transmitted x-ray intensity demonstrate an angular rearrangement of molecules around excited dipoles following the 10 ps kinetics of charge transfer and leaving lattice plane spacings unchanged. Transient x-ray scattering is governed by solvation, masking changes of the chromophore molecular structure.