Studies of magnetic resonance phenomena in polymers. I. The effects of free volume and segmental mobility on the motion of nitroxide spin probes and labels in poly(methyl methacrylate)

Thin films of spin-probed and spin-labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) at X-band frequency (9.2 GH_z) and at various temperatures. Direct spectral evidence is presented to indicate that the composite ESR spectra observed in a certain temperature range originate from two states of distinctly different mobility, one with slow motions corresponding to a glassy state and the other with fast motions corresponding to a liquidlike state. The coexistence of these two states at temperatures considerably below the glass transition temperature can be explained as a result of the effect of free volume in a solid polymeric glass.     

NMR Studies on the Effect of Solvents on the Dimerization of Benzoic Acid

The chemical shifts of carboxyl proton in benzoic acid have been measured in a variety of solvents. The characteristic monomer and dimer chemical shifts have been evaluated at 23°. A detailed discussion is given to illustrate the solvent shifts of carboxyl proton, from which the dielectric constant of solvent is considered as the main factor. The plot of carboxyl proton shift against (∈—1)/(∈+1) reveals a roughly linear dependence, indicating that a high field shift will be brought about by an increasing dielectric constant of solvent. Another linear relationship between log (δ—δ_D) and log ∈ suggests that an increase of ∈ will destroy the cyclic dimer, leading to an increase of the monomeric population and therefore to a higher field of the carboxyl proton signal.     

Syntheses of New Isocephems and Isodethiaoxacephems as Antimicrobial Agents

The cis-configurated isocephem 26 as well as isodethiaoxacephems 21 and 33 were synthesized (Schemes 4 and 5). The key step involves chlorination of the corresponding carbanions of 23, 18 , and 31 with CF₃SO₂Cl followed by internal alkylation. β-Lactams 3, 21, 26 , and 33 were found to possess biological activity against several pathogenic microorganisms in vitro. Electronic activation of the lactam moiety in isodethiaoxacephem 33 remarkably enhanced its biological activity.     

Thermal decomposition of N2O near 900 K studied by FTIR spectrometry: Comparison of experimental and theoretical O(3P) formation kinetics

The spin-forbidden dissociation reaction of the N₂O(X¹Σ⁺) ground state has been investigated by both quantum calculations and experiments. Ab initio prediction at the CCSD(T)/CBS(TQ5)//CCSD(T)/aug-cc-pVTZ+d level of theory gave the crossing point (MSX) energy at 60.1 kcal/mol for the N₂O(X¹Σ⁺) → N₂() + O(³P) transition, in good agreement with published data. The T- and P-dependent rate coefficients, k₁(T,P), for the nonadiabatic thermal dissociation predicted by nonadiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree very well with literature data. The rate constants at the high- and low-pressure limits, k₁^∞ = 10^11.90 exp (−61.54 kcal mol⁻¹/RT) s⁻¹ and k₁^o = 10^14.97 exp(−60.05 kcal mol⁻¹/RT) cm³ mol⁻¹ s⁻¹, for example, agree closely with the extrapolated results of Röhrig et al. at both pressure limits. The second-order rate constant (k₁^o) is also in excellent agreement with our result measured by FTIR spectrometry in the present study for the temperature range of 860-1023 K as well as with many existing high-temperature data obtained primarily by shock-wave heating up to 3340 K. Kinetic modeling of the NO product yields measured at long reaction times in the present work also allowed us to reliably estimate the rate constant for reaction (3), O + N₂O → N₂ + O₂, based on its strong competition with the NO formation from reaction (2) which has been better established. The modeled values of k₃ confirmed the previous finding by Davidson et al. with significantly smaller values of A-factor and activation energy than the accepted ones. A least-squares analysis of both sets of data gave k₃ = 10^{12.22 ± 0.04} exp[− (11.65 ± 0.24 kcal mol⁻¹/RT)] cm³ mol⁻¹ s⁻¹, covering the wide temperature range of 988-3340 K.     

Synthesis of 3-aminopyrazinone mediated by 2-pyridylthioimidate-ZnCl2 complexes. Development of an efficient route to a thrombin inhibitor

A six-step preparation of thrombin inhibitor drug candidate 1 from pyrazinone 7 in 47% overall yield is described. The problem of low reactivity between weak amine nucleophile 4 and poor electrophile 3-bromopyrazinone 17 was overcome with the use of pyridinylthioimidate 27 in the presence of ZnCl(2) to afford adduct 3 in high yield. Several zinc complexes were characterized by solution and solid-state NMR and X-ray crystallographic analyses, and provided insight into the reaction mechanism. Preparation of pyridine N-oxide amine 4 was accomplished via a selective oxidation of the corresponding pyridinylamine 6. Pyridinylthioimidate 27 was prepared from pyrazinone 7 via a two-step one-pot process in near quantitative yield. Chlorination of the pyrazinone ring in 3 followed by hydrolysis and amide coupling completed the synthesis of 1. This chromatography-free synthesis was used successfully to prepare multikilogram quantities of the drug with reproducibility and high purity.     

Eine carben—metallbindung in [3.3.0]-nickelabicyclen

The complex (Ph₃P)Ni(PhNCNPh) (II) is formed from (Ph₃P)₂Ni(PhNCNPh) (I) in toluene at 60°C. II can also be prepared directly from (COD)₂Ni, Ph₃P and PhNCNPh (45% yield). (COD)₂Ni reacts with dairylcarbodiimides in THF to give (ArNCNAr)₃Ni(THF) (VIII). The structure of VIIIa (Ar  phenyl) has been determined by X-ray crystallography. The molecule is shown to contain a [3.3.0]nickela bicyclic group in which the carbodiimide molecules are linked to form a 2,5,8-h³-carbene ligand. Some properties and reactions of II and VIII are described.