摘要:

AbstractIrradiation of 1‐(p‐nitrophenyl)‐5,5‐dimethyl‐1,3‐hexadiyne (NDHD) in aqueous sulfuric acid yields conjugated allenyl ketones (1 and 2) and β‐dicarbonyl compounds (3 and 4). The products 1 and 2 are primary photoproducts while 3 and 4 are secondary products formed by thermal hydration of the primary photoproducts showing the maximum quantum yield at H0= −1·0. A mechanism involving a synchronous addition of hydronium ion (H3O+) to a triplet state of

ISSN:0894-3230    

DOI:10.1002/poc.610081102

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electron‐transfer quenching of Ru(bpy) 32+(tris(2,2′‐bipyridine)ruthenium(II) excited state by 2,4‐dichlorophenol (2,4‐DCP), 2,5‐dichlorophenol (2,5‐DCP) and 2,6‐dichlorophenol (2,6‐DCP), in their anionic forms, was studied in de‐aerated solutions of neat methanol and 25, 50 and 75% v/v aqueous methanol at 30°C. The quenching rate constants (kq) were determined and their values range from 1 × 107to 8 × 109M−1s−1. In all the mixtures methanol : water the kqvalues for 2,5‐DCP were lower than for 2,4‐DCP and 2,6‐DCP. This result was discussed in terms of the effect of chloro atom on the quencher oxidation reaction. The photoredox product Ru(bpy) 3+was observed by laser flash photolysis and its quantum yield (ΦRu(+)) calculated by using a comparative method with the formation of zinctetraphenylporphyrin triplet state. The yields of cage escape (Yce) for the different methanol/water mixtures were calculated from the obtained values of ΦRu(+)). Yceincreases from 0·1–0·2 in 25% v/v aqueous methanol to 0·5–0·6 in neat methanol. This change of Ycewith the solvent composition was discussed in terms of combined effects of viscosity and solvent cage on the rate constant of cage escape (k−d) and the rate constant for back electron‐transfer to the ground state from geminate pair (kbc), respectively. The constants k−dwere calculated with the Eigen equation. The constants kbcwere estimated from Yceand k‐dby applying a simplified reaction scheme.

ISSN:0894-3230    

DOI:10.1002/poc.610081103

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractProton transfer processes in both the ground and excited states in anils of aromatic α‐hydroxyaldehydes (salicylaldehyde, 2‐hydroxynaphthalene‐1‐carbaldehyde and the novel 10‐hydroxyphenanthrene‐9‐carbaldehyde) have been studied by a combination of spectroscopic techniques. Solution1H and13C NMR is used to establish the position of the tautomeric equilibria. UV‐visible absorption and fluorescence spectral data help to characterize the existence, in all cases, of excited‐state intramolecular proton transfer (ESIPT) pheonmena. Semi‐empirical calculations involving full geometry optimization and calculation of heats of formation for the ground state (AM1) and vertical excitation energies and oscillator strengths (INDO/S) are in agreement with the exper

ISSN:0894-3230    

DOI:10.1002/poc.610081104

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe equilibrium constant,K an(H 2CO), for the formation of α‐aminoacetonitrile from formaldehyde, ammonia and hydrogen cyanide was evaluated at 25°C. A first estimation ofK an(H 2CO)was obtained from extrathermodynamic relationships of the type logK′ vs ∑σ*. The final value was then obtained from a comparison of the experimental and calculated pH dependences of α‐hydroxy‐ and α‐aminoacetonitrile concentrations. From these results, it appears that, after equilibrium, the ratio between the concentrations of the two precursors glycine and hydroxyethanoic acid, is a linear function of the concentration of free ammonia, i.e. [CH2(OH)CN] [CH2(NH

ISSN:0894-3230    

DOI:10.1002/poc.610081105

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe synthesis of symmetrical blocked pyrylium derivatives by reaction of a cyclic aromatic ketone with ethyl orthoformate goes through an intermediate which can be isolated under certain conditions. This intermediate is unstable when starting from 1‐tetralone derivatives but, surprisingly, it can be isolated easily when the synthesis is performed with 1‐indanone derivatives. Further, in the latter case, this intermediate cyclizes hardly into the expected blocked pyrylium derivative. Its ionic structure was demonstrated by mass spectrometry and by 500 MHz1H and13C NMR techniques including heteronuclear H–C COSY and DEPT polarization transfer experiments. Hydrolysis of this intermediate in aqueous ethanol solution forms, in acidic or basic media, a pseudo‐base, which in the latter case presents strong similarities with that obtained by the base‐catalysed ring‐opening of a blocked pyrylium ion. In both acidic and basic solutions, the pseudo‐base obtained by the two sources shows identical UV–visible absorption spectra. The difficulty of the intermediate (g) originating from 1‐indanone derivatives in cyclizing into the blocked pyrylium structure, in contrast to the intermediate formed from 1‐tetralone derivatives, is te

ISSN:0894-3230    

DOI:10.1002/poc.610081106

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe equilibrium structures ofcis‐ andtrans‐1,3‐diphosphetane were evaluated at the RHF/6–31g** level of optimization, with MP2 electron correlation correction. Both structures are puckered, thetransisomer being slightly lower in energy than thecisisomer. The inversion barrier in 1,3‐diphosphetane is lower in energy than in ph

ISSN:0894-3230    

DOI:10.1002/poc.610081107

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe nucleophilic ring opening of propylene oxide by the lithio derivative of the trioxabicyclic orthoester 1 leads, after acidic treatment, to the title lactones 3a and 3b with no evidence of δ‐lactones 3c and 3d. The stereochemistry of these two lactones has been established by X‐ray diffraction analysis. The ring conformations observed in the solid phase are similar in both lactones and are compared with those observed in related molecules. However, the NMR spectroscopic analysis shows that this similarity does not exist in solution; lactone 3a adopts a different conforma

ISSN:0894-3230    

DOI:10.1002/poc.610081108

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe utility of thiophene as a diene for Diels–Alder reactions was studied byab initiomethods. The reactivity was evaluated with three dienophiles, vinyl alcohol, ethylene and acrylonitrile. Qualitatively the reactivity was determined by comparison of the reactant's frontier orbital energies, bond orders and charge transfer in the transition states. Quantitatively the reactivity was determined by estimation of activation energy with RHF/6‐31G*, MP2/6‐31G*, MP3/6‐31G*//MP2/6‐31G*, and MP4/6‐31G//MP2/6‐31G* ab initio calculations. The results of the calculations predicts that the reactivity of thiophene with acrylonitrile is similar to butadiene and ethylene with a preferred formation of th

ISSN:0894-3230    

DOI:10.1002/poc.610081109

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610081101

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY