The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.     

1,2-Oxopalladation versus pi-allyl palladium route. A regioconvergent approach to a key intermediate for cyclopentanoids synthesis. New insights into the Pd(II)-catalyzed lactonization reaction

Regioconvergent synthesis of the key lactone 1 from an equimolar mixture of the two olefins 4 and 5 was achieved by unique Pd(II) chemistry. The synthetic versatility of lactone 1 has been demonstrated in the synthesis of iridoids and of the endo-Corey lactone 2, which is a key intermediate for the F(2)-isoprostane synthesis. Upon exposure of the sodium salts of 4 and 5 to a catalytic amount of Pd(OAc)(2) under oxygen, in the presence of AcOH, an isomeric lactone 12 was obtained in addition to the title compound 1. The Pd(II) lactonization was optimized by fine-tuning all the factors participating in the catalytic cycle: solvent, oxidant, co-oxidant, and Pd(II) source. The Hosokawa's heterobimetallic couple emerged as the catalyst of choice. With a Cu(II)-Pd(II) couple, the redox process was transferred to copper, and the formal oxidation state of palladium remained constant during the reaction. By virtue of this new methodology, lactone 1 was obtained in a rewarding 60% yield, along with isomeric lactone 12 in 30% yield. A detailed mechanistic study was carried out in order to elucidate the formation of lactones 1 and 12. Lactone 1 was formed from either olefin 8 or olefin 10; on the other hand, lactone 12 was formed exclusively from olefin 10. An intramolecular 1,2-acyloxypalladiation was invoked for the transformation of 8 into 1, whereas the pi-allyl complexes 13 and 11 were involved in the transformation of olefin 10 into 12 and 1, respectively.     

Theoretical study of the variations in the β-protons hyperfine splitting parameter of hydrocarbon radicals

The proton hyperfine splitting constants of a large number of positive, negative, and neutral radicals, have been examined in term of the Heller-McConnell relation a _H=B_c cos² whose validity is discussed. B is taken as a function of the energy of the singly occupied orbital and values are calculated by first order perturbation theory for the cases of a methyl, methylene, and dimethylene group attached to the system. Substantial agreement is found between theory and experiment indicating the correctness of the postulated cause of the B behaviour.

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Zusammenfassung Die Konstante der Hyperfeinaufspaltung für -Protonen einer großen Anzahl positiver, negativer und neutraler -Radikale wurde mit der Beziehung von Heller und McConnell als Grundlage untersucht; die Gültigkeit dieser Beziehung wird diskutiert. B wird als von der Energie des einfach besetzten Orbitals abhängig angenommen und seine Werte mit Hilfe der Störungstheorie erster Ordnung für die Fälle von Methyl-, Methylen- und Dimethylengruppe am -System berechnet. Aus der guten Übereinstimmung zwischen Theorie und Experiment wird auf die Richtigkeit der postulierten Ursachen für das Verhalten von B geschlossen.

     

Ultra-fast essential oil characterization by capillary GC on a 50 microm ID column

A 5 m x 50 microm capillary column with 0.05 microm stationary phase film thickness, with a calculated efficiency of almost 20,000 plates per metre (under optimum conditions), was used for very fasthigh resolution GC analysis of lime essential oil. The total analysis time of this volatile essential oil was less than 90 s. Fast GC is shown to be appropriate for essential oil quality assurance analysis, and quantitative results of key components are comparable with those obtained by using conventional GC analysis. The fast GC analysis is approximately 33 times faster than the conventional GC method.     

Copper(II) complexes of N-terminal protected tri- and tetra-peptides containing histidine residues

Copper(II) complexes of peptides containing two or three histidyl residues (Ac-HisGlyHis-OH, Ac-HisGlyHis-NHMe, Ac-HisHisGlyHis-OH and Ac-HisHisGlyHis-NHMe) have been studied by potentiometric, UV-Vis, EPR and CD spectroscopic measurements. The imidazole nitrogen atoms are described as the primary metal binding sites of all ligands resulting in the formation of various macrochelates in the pH range 4 to 7. The (Nim, N-, Nim)-co-ordinated [CuH-1L]0+ complexes were mainly detected in samples containing free carboxylates at the C-termini, whilst the [CuH-2L]-(0) complexes were the predominant species in slightly alkaline solution and their binding modes were described via 4N-co-ordination (Nim, N-, N-, Nim) in (7,5,6)-membered fused chelate rings. Deprotonation and co-ordination of the third amide nitrogens were detected above pH approximately 9 in all cases.     

NMR stereospecific long-range coupling and preferred conformations in some (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl] acrylic acids

NMR data have been used to assign the stereochemistry to some new (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl]acrylic acids. The (E)-molecules are biased in the s-cis conformation showing NMR spectroscopic features strictly depending on the conformation.The analysis of the NMR spectra reveals that the (Z)-isomers exist also in the s-cis conformation.     

Crystal structure and reactivity of 2-chloro-3,5-dinitrothiophene and of 2-phenylsulphonyl-3,5-dinitrothiophene with nucleophiles

The geometry obtained by a crystal structure determination of the title compounds, as compared with that of the analogous benzene derivatives, along with results of ab initio calculations, is used to interpret the different S_NAr reactivities in some thiophene and benzene compounds. The smaller rotation of the nitro groups with respect to the aromatic rings observed in thiophene derivatives should be considered a relevant factor in the higher reactivity in the thiophene series.     

Chiral 1,4-benzodiazepines. XII. Conformation in a solution of 7-chloro-5-phenyl-3(S)methyl-1,3-dihydro-2H-1,4-benzodiazepine

Deuterium labeled congeners of 7-chloro-5-phenyl-3(S)-methyl-1,3-dihydro-2H-1,4-benzodiazepine ( 8 ), i.e., compounds 9 and 16-18 were prepared and their lis-nmr spectra run. For computational studies compounds 9 and 16 were chosen. The results of lis measurements revealed that 16 is present in more than 97% in the boat-like conformation I (Scheme 3).     

A stable Nyström interpolant for some Mellin convolution equations

We consider the numerical solution of second kind integral equations of the form $$u(y) - \int\limits_0^1 {k(y/x)\frac{{u(x)}}{x}dx = f(y), 0 \le y \le 1,} $$ for some given kernelk(t). These equations, usually indicated as of Mellin type, arise in a variety of applications. In particular, we examine a Nyström interpolant based on the following product quadrature rule: $$\int\limits_0^1 {k(y/x)\frac{{u(x)}}{x}dx \approx \sum\limits_{i = 0}^n {w_{ni} (y)u(x_{mi} ).} } $$ This rule is obtained by interpolatingu(x) by the Lagrange polynomial associated with the set of Gauss-Radau nodes {x _ni}. Under certain assumptions on the kernelk(t), we are able to prove the stability of our interpolant and derive convergence estimates.     

Lelong numbers of positive plurisubharmonic currents

A (k,k)-current T on an open subset of C^N is plurisubharmonic if is positive. Positive plurisubharmonic currents admit Lelong numbers; we prove here that they are independent on the coordinates system. Moreover, if Y is an analytic subset of pure dimension, then the Lelong numbers of T on Y are given by a non negative weakly plurisubharmonic function.